The Sequence stratigraphic approach to the interpretation of the sedimentary section started in the mid 70's (Vail et al 1977). This and other papers from this group of Exxon geologists immediately prompted discussion on the geologic value of Sequence Stratigraphy and its inter-relationship with earlier approaches to stratigraphic interpretation. Influenced by these papers and other literature on the topic Ashton Embry and Octavian Catuneanu become prosletizers and critics of how best to apply Sequence Stratigraphy. Since then at least two groups of earth scientists developed with different perspectives on the formalization of Sequence Stratigraphy in terms of methodology, Surfaces, units and terminology.
Both groups are reviewing the Sequence stratigraphic literature and making recommendations regarding the methods and terminology of Sequence Stratigraphy. The ISSC Task Group plans to publish their results in a revised edition of the International Stratigraphic Guide (ISG). The North American Commission on Stratigraphic Nomenclature (NACSN) will evaluate feedback from both the IWGSS and the ISSC reports in preparation for an upgraded version of the North American Stratigraphic Code.
The first two posted pdf files above are circulated first drafts and outline the initial positions of the two groups and It would be misleading to suggest these are set in stone. The three pdf documents below represent the first comment from Octavian Catuneanu on the ISSC report, Ashton Emry's response and his short history of the orgin of the groups. Scientific critiques of the reports and subsequent documents from other Earth Scientists comments will be posted as text documents while longer responses and/or illustrations will be posted as pdf's. One objective of this page is that these comments should help the authors revise their manuscripts and positions.
Reply to ISSC Report Catuneanu
Comment on Reply Embry
"Shoehorning in Sequence Stratigraphy and its ConSequences" by Ashton Embry
ISSC's Position on Sequence Stratigraphy
Maria Cita, Emeritus Professor of Geology University of Milano, Honorary Fellow of GSA and Honorary Member of INQUA, chair of the Italian Commission on Stratigraphy and chair of the ISSC - the Subcommission of Stratigraphic Classification of the International Commission on Stratigraphy(IUGS) has or is developing guides to LithoStratigraphy, cycloStratigraphy, chemoStratigraphy, MagnetoStratigraphy, BioStratigraphy, and ChronoStratigraphy tied to the International Stratigraphic Guide. Maria Cita is the guiding hand behind the current International Subcommission on Stratigraphic Classification (ISSC) Task Group on Sequence Stratigraphy, and she has asked that this web site place a statement related to her position on the current debate currently ongoing on the USC web site tied to Sequence Stratigraphy. She has suggested that this statement be direct and diplomatic, while encouraging the protagonists to channel their energies into a constructive ending to the current confrontation. Maria recognizes that there is no unique approach to Sequence Stratigraphy that fits all settings or suits every stratigrapher. Her plan is that the ISSC produce a short annotated document that considers the major points of agreement and difference between the various Sequence stratigraphic schools. Her intent is that the ISSC document provides general guidelines (not rules) that review the simple, clear, reasonable concepts of Sequence Stratigraphy for the student and seasoned professional. She would like the document to include at least three examples, from different continents and different time slices, of real world application of the various concepts to Sequence Stratigraphy so the reader can see how to apply these schemes to their own stratigraphic setting. She would like this document to evolve into something the stratigraphic community will be able to stand by and use as a guide. Her intent is that the document would acknowledge that Sequence Stratigraphy has evolved but questions of "style" and interpretation still remain! Click the link that follows to view earlier & current ISSC Newsletters
The attached minutes are of the deliberations of the ISSC meeting at the 33rd International Congress, Oslo, Norway. They record progress in understanding among the various proponents but the issues and conclusions of the meeting participants still remain controversial and may even be incorrect (see Octavian Catuneanu's comments to these minutes below). The minutes are posted here with the intent that they might be considered as a working agenda for a discussion among the SEPM friends of Sequence Stratigraphy at the coming AAPG Denver. As indicated above Earth Scientists interested in standardizing these criteria for Sequence Stratigraphy are encouraged to a contribute constructive discourse based on the issues and conclusions of the Oslo meeting. Submissions should be sent to seqstrat@gmail.com as text or word files.
Debate - Dated and Chronological order
Zoran Kunštek wrote on Monday the 13 August at 4:48 am (See Zoran Kunštek's Image below the text box too!) Dear Colleagues, The constructive debate about sequence stratigraphy, towards its formalization is undoubtedly reasonable. It becomes very useful and practical method in geological interpretation, especially for subsurface. But, following the discussion between IWGSS and ISSC it becomes obvious that somebody ignored the most important statements of sequence stratigraphy: Posamentier et. al, 1985: ?Sequence stratigraphic concept should be applied as tool to approach rather than as template.? Anderton, 1985: ?Sequence stratigraphy should be considered as way of looking at and ordering geologic data, rather than end in itself.? For the first time we have a useful method - sequence stratigraphy - that includes several tools and types of data as seismic + well-logs + stratigraphy (and not seismic vs. well-logs vs. stratigraphy – from IWGSS ?Towards the Formalisation…?) All of these data are equally important, and their integration allows us to fill in the gaps and inadequacies of each tool individually. Sequence stratigraphy allows us to understand the interplay of sedimentation and erosion equally. Those are two opposite processes that act simultaneously. Base level changes (erosional base, Marjanac, 1996), on the other hand defines accommodation space, and their changes cause the shift of facies basinward or landward as well as changes in depositional trends. We have to accept that some parts of the land for certain time span ?never? come under water so erosion acts continuously, simultaneously some parts of the basin have continuum in sedimentation. For that reason subareal erosion poses greater time span landward than basinward (shown by Wheeler’s diagrams). Correlative conformity represents maximal basinward shift of base level at particular moment and it is connected with small portion of SU over the part of time that belongs to subaquateous erosion (RAV or RSmE), and sediments reworked and/or removed basinward belong to new portion of time. MRS (TS; CTS, FS, …) is first onlap on erosional surface representing change in depositional trend and shift of base level landward. So, C.C. should be placed immediately below first onlap visible on seismic section. MFS, on the other hand, represents maximal onlap of seismic reflector (strata) on certain erosional surface (unconformity), for that reason there should be no further onlap reflectors above it, reason being obvious – change in depositional trend caused by base level shift. I understand that sequence stratigraphy is defined as ?study of rocks relationship within chronostratigraphic framework (time table) of the repetitive, genetically related strata bounded by surfaces of erosion or non deposition, or their correlative conformity? (Van Wagoner et. al, 1988), but some could take it too seriously. We should admit that chronostratigraphic approach is rather rigid for such diachronous sequence stratigraphic surfaces, even if those surfaces have small diachronosity. Sequence and the unconformities that define it, as well as other important surfaces, posses certain time spans that need to be defined, hence geochronological approach (certain sediment package for amount of time) should be more appropriate for interpretation. Later, at the very end of interpretation we can put it in the rather rigid chronostratigraphic framework (certain time span for amount of sediments). At the end, Embry’s simple T-R sequence is more appropriate for analysis and interpretation, rather than others with three or even four system tracts. It gives the explorer more freedom for interpretation, for nature while simple still isn’t mathematics. Sincerely Yours Zoran Kunštek <Zoran.Kunstek@ina.hr> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Ashton Embry wrote on 2nd August 2007 at 1.02PM Dear Zoran, Thank you very much for your comments. It is always useful to hear from a practitioner with no axe to grind. I have only a few comments to make in reply to your statements “I have some doubts about usage of chronostratigraphy for sequence stratigraphic interpretation. Wouldn’t it be more appropriate to use geochronology for interpretation?” I could not agree more. The identification of specific time surfaces is always very difficult and subjective and is rarely reproducible. Our experiences over the last 100 years in lithostratigraphy and biostratigraphy have clearly shown us that chaos is the end result when you mix chronostratigraphy with material-based stratigraphy. The authors of the stratigraphic codes and guides purposefully separated chronostratigraphic practices from the material-based stratigraphic disciplines for very good reasons. Basically, almost all the problems and confusion currently associated with sequence stratigraphy are rooted in the attempted use of chronostratigraphic methods, surfaces and units in sequence stratigraphy. Unfortunately some of the strongest advocates of such a hybrid stratigraphic discipline (chrono-sequence stratigraphy) have little knowledge or understanding of the developmental struggles in stratigraphy which took place over the last 100 years. Then, why not use term ‘retrogradational system tract’ instead of transgressive system tract for time span when basin is starving of sediment. Also, ‘progradational system tract’ instead of regressive system tract for time span when accommodation space gets smaller because of great sediment income as well as base level fall. There are two reasons for preferring Transgressive ST over Retrogradational ST and Regressive ST over Progradational ST. One is that the name “transgressive systems tract” is entrenched in the literature and I don’t think it would be reasonable to change the name of this unit especially given that it is a well defined, material-based unit. Notably Galloway called the TST a Retrogradational ST unit but the name never caught on. Once you accept the name transgressive systems tract then name regressive systems tract becomes unavoidable. The real problem lies in claims that the regressive systems tract can be subdivided into three systems tracts on the basis of the conjecture that two time surfaces can be recognized within the RST. Despite the fact no one has ever been able to recognize these time surfaces on cross sections or in outcrop or offer any reasonable guidance on how one might recognize them, the advocates of chrono-sequence stratigraphy still base their entire sequence stratigraphic classification system on these abstract entities. To me this is a classic case of a model rather than data controlling classification. I would also note that a final phase of progradation of the shelf margin can occur during the initial phase of transgression and the early development of the TST. This does not happen often and it is often localized to areas of high supply. However, because it does happen, the start of progradation and the start of transgression do not always coincide. “ Further, Embry (2002) connects SU with MRS via RAV (SR-U / SR-N) instead of MRS-RSmE-SU connection, why, if we deal with maximum regression?” The main problem with an SU-RSME-MRS boundary is that the basinward end of the SU is not co-terminus with the landward end of the RSME. Thus these three surfaces do not form one continuous boundary, a key requirement of a sequence boundary. As discussed in the report, the SU-SR-U-MRS very often do form one continuous boundary and such a combination of surfaces represents one option for a depositional sequence boundary (defined as an SU plus correlative surfaces). Such a boundary basically separates strata deposited during regression below from overlying strata deposited during transgression and thus is a boundary between two different “genetic” deposits. "Embry, Ashton" <AEmbry@nrcan.gc.ca> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Kunštek Zoran wrote on Jul 19, 2007 at 3:01 AM Dear Colleagues, I have some doubts about usage of chronostratigraphy for sequence stratigraphic interpretation. Wouldn’t it be more appropriate to use geochronology for interpretation, according to no isochronous sequence surfaces? Later some can put it in to relatively rigid chronostratigraphic time span (time table). Next question is connected to Embry (2002) descriptive term, as he mentioned in papers, ‘maximum regressive surface’ (MRS) and two system tracts that Embry proposed TST and RST. As he mentioned MRS is appropriate descriptive term. Than, why not use term ‘retrogradational system tract’ instead of transgressive system tract for time span when basin is starving of sediment. Also, ‘progradational system tract’ instead of regressive system tract for time span when accommodation space gets smaller because of great sediment income as well as base level fall. Further, Embry (2002) connects SU with MRS via RAV (SR-U / SR-N) instead of MRS-RSmE-SU connection, why, if we deal with maximum regression? Sorry for my English, Sincerely Yours BSc Zoran Kunštek Senior Geologist INA d.d. SD Naftaplin Croatia, 10 000 Zagreb Šubiceva 29 Tell. 00385 1 4592632, Mob. 00385 98 1602965 e-mail: zoran.kunstek@ina.hr ++++++++++++++++++++++++++++++++++++++++++++++ Embry, Ashton wrote Jun 20, 2007 5:44 PM In reply to Alex MacNeil's Comments of June 12 on “Shoehorning Essay” It is nice to see someone commenting on my comments. That is what debate is all about. And it especially helpful to have Alex debating the subtleties of recognizing a BSFR and a CC because that is a key topic in this debate. If we can all agree how one can recognize BSFRs and CCs in our sections just like we do MFSs and MRSs we can all go away happy. Of course, if reasonable criteria are not established for recognizing and correlating these time surfaces, I suspect we will remain at a stalemate with the co-existence of a material-based approach as advocated by ISSC and a mixed time-based/material-based approach as advocated by IWGSS. I am confident that common sense will eventually win the day. MacNeil: “Dr. Embry’s paper on “shoehorning” in sequence stratigraphy focussed on two papers (MacNaughton et al. 2000; MacNeil and Jones 2006) that he considers to be good examples of scientists forcing their research into sequence stratigraphic models.” Embry: I just want to emphasize that the two examples discussed in the Shoehorning Essay were not chosen by me but were offered by the authors as good examples of studies in which BSFRs and CCs were recognized in well exposed strata. I simply commented on the problems associated with such interpretations. I do not know if these studies are representative or are “good” examples of such analysis. MacNeil: “I find his paper to be more an example of why grey literature should be taken with a “grain of salt” Embry: I don’t consider that Shoehorning essay as part of the literature, grey or any other colour. It is simply part of this online debate and I assume no one will ever formally reference anything on this webpage. MacNeil: “I do not know how Embry has come to conclude that deposits above the BSFR automatically correlate to the instantaneous switch from normal to forced regression at the shoreline, but it is an incorrect assumption.” Embry: Alex and I agree that the BSFR is a time surface that separates strata deposited during base level rise below from strata deposited during base level fall above. It is well accepted that the BSFR is a conformity and Catuneanu often refers to it as a “correlative conformity”. This means there is either a very small or no time gap across the surface. If an unconformable surface separates strata deposited during base level rise below from strata deposited during base level fall above then such a surface is referred to as the type of unconformity it is (e.g. MFS unconformity) and not as a BSFR. I would hope everyone accepts these rather straightforward constraints on a BSFR. Given the above, then the BSFR does indeed represent the time surface at the start of base level fall with the proviso there may be an immeasurable short time lag (days?) between the start of fall and the first particle of sediment to hit the sea floor at a given locality. As Posamentier and Morris (2000, p.39) state “this bounding surface (they are referring to the BSFR) has chronostratigraphic significance insofar as it represents the paleogeography at a moment in time” There is no doubt as to what moment in time they are referring to because on the same page they say the BSFR “represents the surface that exists at the time of initiation of sea level fall”. Notably Catuneanu also refers to the BSFR as the ‘paleo sea floor” at the start of forced regression as mentioned by Alex. I hope Alex will give some careful thought to what a BSFR is and what it represents. It is quite clear that, by definition, it forms at the moment of start base level fall. MacNeil: “the BSFR marks the base of sediments deposited during forced regression.” Embry: Once it is realized that the BSFR is a conformable surface then such a statement logically leads to the conclusion that the BSFR marks the start of base level fall at the shoreline (forced regression). MacNeil “With regards to our second BSFR and the falling stage systems tract at the top of the reef system, Embry states that a reasonable case could be made for the sediments atop of the MFS (that we assigned to the FSST) having been deposited during sea-level rise. No, this is precluded by the stratigraphic relationships, facies associations, and diagenetic fabrics, as explained on pages 349-352 in the paper and shown in Fig. 18,” Embry: There are two problems here. The main one is the surface referred to as a BSFR on the outer shelf is an unconformable MFS and not a BSFR. Marginward, MacNeil and Jones have interpreted a BSFR and show it as a dashed line downlapping onto the MFS (see their fig 18). They have not presented any evidence that at that precise horizon there is a change in sedimentary composition such that one could reasonably interpret the strata immediately below such a surface were deposited during base level rise and the strata immediately above were deposited during fall. Almost all of the arguments on pages 349-352 deal with the deposits on the outer shelf which directly overlie the MFS. These may well be all deposited during fall but the surface at their base is an MFS not a BSFR. The only bona fide attempt at identifying a BSFR is that dashed line on Fig 18 which is underlain by “HST” and its placement is more of a guess than a well supported interpretation. MacNeil: “This paragraph of Embry is completely incorrect – we do show onlapping and base level rise in our figure. If one looks at our figure and uses the water-table (WT) line as a datum, our sea level position in part C is LOWER than in part D. It is a subtle difference in position” Embry: I got out my magnifying glass and my calipers and indeed Alex is right, M+J do show a very small rise in sea level (a metre or two at best) between stage C and stage D. It is not easy to see on their diagram that the marine carbonate sediments are onlapping the basinward end of the unconformity which must be a SR-U, given that marine strata overlie it. I will accept their interpretation that some marine carbonates onlap the very end of the unconformity although I’d feel better if this contact was described in detail and it was demonstrated beyond a reasonable doubt that the strata underlying the onlapping strata had been subaerially exposed. Regardless, this interpretation has no bearing on their interpretation that the base of the Biohermal interval is a CC; that is, the time surface formed at the start of base level rise. Such a thin interval of onlapping strata could easily be equivalent to only the uppermost portion of the shallowing biohermal strata and could even be reasonably interpreted as being correlative with the TST interval at the top of the biohermal interval. The important consideration here is MacNeil and Jones’ interpretation that carbonate sedimentation greatly increased at the start of base level rise (started at the CC). This is not sedimentologically reasonable. At the start of sea level fall the outer shelf was rather deep and was not the site of much in situ carbonate deposition (thin biostromal unit). As sea level fell it eventually resulted in the outer shelf becoming shallow enough to become a carbonate factory and this is marked by the initiation of the biohermal unit. It is not reasonable to interpret, as M+J do, that sea level fell exactly to the critical water depth to initiate a carbonate factory on the outer shelf and then IMMEDIATELY started to rise. It is likely it kept falling during part or all of the biohermal growth and then eventually started to rise, probably very near the top of the biohermal unit. My experience with initial base level rise in carbonates is that it is often very fast and the start of base level rise coincides with transgression. Such a “standard” scenario is easily accommodated in the stratigraphic relationships illustrated by M+J by correlating the onlapping strata with the TST (see M+J fig 18 for how easily this can done). There is no justification for interpreting the base of the biostromal unit (or any other specific horizon within it) as start rise. A better (simpler) case can be made for start rise at start transgression but even this interpretation is somewhat speculative. The bottom line is it is impossible to identify the horizon that marks the base of base level rise with any semblance of objectivity and any attempt to do so is simply shoehorning. MacNeil: I maintain that MacNeil and Jones (2006) provides a case study of the BSFR in outcrop and core, significantly undermining arguments made in the ISSC paper. The “shoehorning” argued for by Embry is based on his own definitions, and as outlined above, misrepresentation of the material presented in MacNeil and Jones (2006). Embry: I would maintain that M+J provides a good example of shoehorning. Alex felt he had to identify a BSFR and CC (Alex was taught sequence stratigraphy by Catuneanu at U of A) and thus chose surfaces he could see and labeled them as a BSFR and a CC. Unfortunately, as discussed herein and in the Shoehorning essay, such surfaces are not reasonable representations of the time surfaces at start BL fall and start BL rise on the basis of either empirical or theoretical considerations. If he had not been instructed to find such surfaces in order to recognize a HST, FSST and LST, I doubt if he would have made such interpretations. Such interpretations are forced on the data by the interpreter thinking that he/she needs to follow the prevailing mixed time-based/material-based approach (trying to see the unseeable). If Alex had used the material-based approach, as is done in all other stratigraphic disciplines, he would not have felt compelled to label (shoehorn) various facies changes or unconformities as time surfaces (BSFRs and CC). "Embry, Ashton" <AEmbry@nrcan.gc.ca> +++++++++++++++++++++++++++++++++++++++++++++++++++ Embry, Ashton wrote Jun 20 2007 1:59 PM in reply to Mark Smith Dear Mark, I just read your comments on the debate website. I enjoyed your talk at the CSPG conference but did not get the chance to speak with you. I think we can profitably exchange views. I would be interested to hear more about the surface you have interpreted as a BSFR and a picture or two and even a diagrammatic X section would be helpful for a discussion. First of all you have decide if the recognized surface is an unconformity (significant time gap across it) or is conformable (little to no time gap across it). If the surface is conformable, you have to ask yourself "Is it reasonable to interpret this conformable facies change as a time surface which formed at the moment (and I stress moment) base level began to fall?". If the surface is an unconformity, you have to ask how such an unconformity was created and whether or not it is reasonable to interpret that all strata below the surface formed during base level rise and all strata above were deposited during base level fall. As long as you can defend your interpretations with data and solid theoretical reasoning then the sharks won't have a feeding frenzy on a BSFR interpretation . You are wise to try and get some clarification at this stage of research. There is a great deal of extraneous material and innumerable, very suspect interpretations in the sequence stratigraphic literature and it is not easy for students to know what is good and what is not good. Best regards, Ashton Embry "Embry, Ashton" <AEmbry@nrcan.gc.ca> ++++++++++++++++++++++++++++++++++++++++++++++++ On Jun 18, 2007 1:27 PM Mark D Smith wrote As a student who plans on publishing his work to the scientific community in the near future, I am finding the recent debate on sequence stratigraphy, specifically the basal surface of forced regression of great interest. Unfortunately, also being a student makes me a little hesitant to jump into the waters with the sharks and so I will keep this communication brief, and if people on either side are interested I can elaborate further. My research examines Neoproterozoic deep-water (slope and basin facies) sedimentary rocks (Old Fort Point Formation) in the southern Canadian Cordillera and I recently presented some of the material at the 2007 AAPG and CSPG. Although the MRS and MFS can be identified, they are difficult to pinpoint where the exact surface is in outcrop (more like zones rather than surfaces). However, the BSFR (as I have interpreted it) is a sharp contact, easily recognizable in the field, possesses different facies (members of the OFP) above and below the delineated surface and can be correlated across the Windermere basin (>35,000 km2). The surface empirically exists, whether a label is attached to it or not. For those in the Alberta, Canada region, this surface can be easily viewed at the Lake Louise Ski Hill or along the Yellowhead highway about 14 km west of Jasper, Alberta. I have numerous photos, sections and thoughts on how this surface is expressed in my research (not neccessarily sequence stratigraphy overall) and would be interested to know what people think prior to submitting my work for publication. Thank you for your time. Mark... Mark D. Smith Department of Earth Sciences Marion Hall, 140 Louis Pasteur University of Ottawa Ottawa, ON K1N 6N5 Ph: 613.562.5800 x6729 <msmit074@uottawa.ca> "Look at me, I'm a grad student. I'm 30 years old and made 600 dollars last year." - Bart Simpson ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On June 12, 2007 at 2:39 pm Alex MacNeil wrote Reply to “Shoehorning in Sequence Stratigraphy” Dr. Embry’s paper on “shoehorning” in sequence stratigraphy focussed on two papers (MacNaughton et al. 2000; MacNeil and Jones 2006) that he considers to be good examples of scientists forcing their research into sequence stratigraphic models. As the primary author of MacNeil and Jones (2006), I find his paper to be more an example of why grey literature should be taken with a “grain of salt”, and not generally referenced in primary research articles. I feel that he has misrepresented and ignored important parts of MacNeil and Jones (2006), and distorted definitions to create/justify his arguments. While I offer my perspectives, I encourage everyone concerned with these issues to read the references and draw their own conclusions. 1. In our paper, we identify two BSFR’s in a Late Devonian succession. All of Embry’s arguments challenging our recognition of the surface are based on his definition of a BSFR, which is not the definition found in the literature. The BSFR was defined by Hunt and Tucker (1992) as: “a chronostratigraphic surface separating older sediments of the preceding highstand systems tract, deposited during slowing rates of relative sea-level rise and stillstand, from younger sediments, deposited during the base-level fall” (pg. 5; my italics added). From additional discussion in Hunt and Tucker (1992) it is clear that recognition of a BSFR does not imply that overlying deposits were deposited the instant base level started to fall – the definition otherwise applied by Embry. Furthermore, Hunt and Tucker (1995) stated that the BSFR marks the “onset of falling sea-level as recorded in the rock record” (pg. 149). I do not know how Embry has come to conclude that deposits above the BSFR automatically correlate to the instantaneous switch from normal to forced regression at the shoreline, but it is an incorrect assumption. Catuneanu (2002) was referenced in our paper as providing the definition for the BSFR. Catuneanu (2002) states, “The basal surface of forced regression was introduced by Hunt and Tucker (1992) to define the base of all deposits that accumulate in the marine environment during the forced regression at the shoreline” (p. 16). He goes on to state that the surface “represents the paleo-sea floor at the onset of base level fall at the shoreline”. This statement does not imply or require that deposits above the BSFR correspond to the instant that base-level started to fall, and we do not accept this assumption being applied to our study. Indeed, on pg. 340 in our paper, we state that a lag period existed between exposure of Reef Complex #1 and initiation of deposits that form the FSST – a statement ignored by Embry. Simply stating the work of Hunt and Tucker and Catuneanu, the BSFR marks the base of sediments deposited during forced regression. Yes, the surface is diachronous, but its recognition allows any geologist to walk up to the outcrop and understand that rocks above the surface were deposited at some point during the base-level fall, and are temporally distinct from underlying rocks (of deeper water origin) that belong to the preceding highstand systems tract. This can have important implications for understanding controls on the carbonate sediments that accumulated and how the system evolved. 2. With regards to our second BSFR and the falling stage systems tract at the top of the reef system, Embry states that a reasonable case could be made for the sediments atop of the MFS (that we assigned to the FSST) having been deposited during sea-level rise. No, this is precluded by the stratigraphic relationships, facies associations, and diagenetic fabrics, as explained on pages 349-352 in the paper and shown in Fig. 18, which Embry fails to mention. This discussion includes reference to recent analogs in the Bahamas and Cayman Islands. 3. On page 7 of the “Shoehorning” paper, Embry states, “Notably there is NO indication of base level rise during the deposition of the biohermal deposits of the outer ramp. This is clearly indicated on the MacNeil and Jones diagram which shows NO onlap on the unconformity during the deposition of the biohermal sediments. Also note that their sea level position is at exactly the same level in the Phase D section as it is in Phase C section despite their claim of base level rise during this time. Thus, by way of their diagram, even MacNeil and Jones acknowledge that base level was not rising during the deposition of the biohermal sediment.” This paragraph of Embry is completely incorrect – we do show onlapping and base level rise in our figure. If one looks at our figure and uses the water-table (WT) line as a datum, our sea level position in part C is LOWER than in part D. It is a subtle difference in position only because 1) we placed its lower position using rather conservative field evidence, as discussed on pg. 335, 2) the large scale of the figure, and 3) the figure was originally drafted at a full tabloid size and scaled-down to letter size in the publication process. The yellow and light-yellow facies at the left hand side of Reef Complex #2 are onlapping reef-front and fore-slope deposits of Reef Complex #1. This was discussed in the paper and figured close-up in Figure 13, which was titled “Deposits of Reef Complex #2 onlapping margin of Reef Complex #1” – but this was not mentioned in Embry’s paper.... The reef-front of Reef Complex #1 is onlapped by restricted lagoon deposits of Reef Complex #2, resulting in a clear juxtaposition of shallow water facies against older, deep water facies. Even Esther Jamieson, in her original work at this locality for her PhD thesis in the 1960s, which was only based on weathered outcrop (not the nice roadcut that we had), noted (Fig. 9 in her thesis and page 30 in the text) that there are facies discrepancies at this locality that are not “normal”. Even though she failed to recognize the significance of the facies relationships, it was obvious that a significant stratigraphic feature is present. To state that we show “no onlap” is incorrect, and to not acknowledge the text discussions about the onlapping succession, or the corresponding, full-page figure that details the onlapping, blatantly misrepresents our work. I maintain that MacNeil and Jones (2006) provides a case study of the BSFR in outcrop and core, significantly undermining arguments made in the ISSC paper. The “shoehorning” argued for by Embry is based on his own definitions, and as outlined above, misrepresentation of the material presented in MacNeil and Jones (2006). Sincerely, Alex MacNeil <alex.j.macneil@esso.ca> +++++++++++++++++++++++++++++++++++++++++++++ On Jun 8, 2007 at 12.00 pm Ashton Embry wrote John, Thank you for your very thoughtful and clearly stated comments. Some of your points were emphasized in the ISSC report and I would say we are in agreement on most of the points you make. I think there is some misunderstanding on a few points and I thought it would be worthwhile if I clarified them so that everyone is clear on what was actually recommended in the ISSC Report. "Certainly insertion of definitions into the code that use time or genetic origin as an interpretive element in defining units or boundaries would indeed be a violation of the spirit of either the North American or International stratigraphic codes". In general this is correct with one caveat. Using time as the interpretive element is fine for chronostratigraphy. Chronostratigraphy was introduced specifically in the 1940s to take time concepts and time surfaces/units out of the material-based disciplines, which now would include sequence stratigraphy, and gather them up into one discipline. Chronostratigraphy, as you rightly note, depends on the integration of correlations of material-based horizons/boundaries/surfaces. Basically one evaluates the diachroneity of all available material-based boundaries from the five disciplines and makes the best guestimate of the location of time surfaces. You can't actually see a time surface (it has no physical characteristics), only infer its position. Notably, each of the five material-based disciplines includes surfaces with low diachroneity and surfaces with high diachroneity. The trick is to use only those with low diachroneity (and time barriers of sequence stratigraphy) for interpreting a local chronostratigraphic framework. The available biostrat boundaries allow one to correlate the local framework to the global time scale. "We are all well familiar with the idea that we are not really mapping time-lines but are actually mapping first and last appearance of fossils in the rock column." In this case one would be mapping biostratigraphic boundaries and not time surfaces. Such boundaries often have low diachroneity so are great for interpreting time relationships by means of compensating for the diachroneity. I think it is crucial to keep the concept of a time surface completely separate from a material-based surface which essentially is never a time surface no matter how much we would like it to be. "Similarly, we all know that maximum flooding surfaces, regressive-surfaces of erosion, flooding surfaces, etc. are not recording truly synchronous events, and thus do not record absolute time-lines. We do, however, know that each of these has some time-significance." You are right that various surfaces recognized in sequence stratigraphy are either (approximate) time barriers or have low diachroniety and, as emphasized in the ISSC Report, such surfaces are most valuable for correlation, framework building and unit definition "Likewise, it seems perfectly fair, and of broad practice across all fields of stratigraphy, to use time-based techniques to hone in on a physical bounding characteristics (e.g. using biostratigraphy to help trace a correlative conformity through units where the surface is otherwise difficult to find and distinguish physically from various surface candidates, choosing one potential unit over a similar unit for lithostratigraphic correlation because its age is clearly more consistent with correlation than the alternative unit, etc.) ". This is exactly what is done in sequence stratigraphy as was discussed in the ISSC Report. One MFS can look exactly like another and correlation lines from other material-based disciplines (bio, mag, chemo, litho) can be most useful in guiding the matching of specific sequence stratigraphic surfaces. I might point out here that it is not acceptable to extend a sequence stratigraphic surface (the boundary of a sequence stratigraphic unit) such as an MFS along a biostratigraphic or magnetostratigraphic boundary just as we can't extend a biostratigraphic boundary (e.g. a FAD) along an MFS. A specific MFS can only be extended by correlation to MFSs at other localities with boundaries from the other disciplines sometimes guiding such correlation. "What would likely be a stretch is to define boundaries by specific point in time, on the premise that this time records a specific event like maximum transgression, particularly in applying this where there is no physical criterion or discrete surface to map. This would not be against the rules either, but it would be chronostratigraphy rather than sequence stratigraphy, which is already codified. Why bother?" Exactly!! This is chronostratigraphy and is NOT sequence stratigraphy. Thus it would be against the rules in sequence stratigraphy but completely within the rules for chronostratigraphy. As the authors of the Code and Guide emphasized, it is most important to not mix the two very different stratigraphic methods/boundaries/units. This is the basic reason why the ISSC Report recommended NOT to use the BSFR and CC which are time surfaces, for which, in John's words, "there is no physical criterion or discrete surface to map.". We could not have expressed it better!! "Most all of us are actually mapping physical surfaces with physical criteria when doing sequence stratigraphy and deliberately choosing surfaces because we know they will have time and/or genetic significance. There isn’t a thing wrong with that approach." I agree there is absolutely nothing wrong with this approach and this is exactly what was recommended and emphasized in the ISSC Report. Mapping physical surface on the basis of their characteristic physical criteria is the empirical approach we have strenuously argued for. I would emphasize that John's critical phrase is "mapping physical surfaces with physical criteria ". The physical criteria that characterize a given surface allow it to be recognized at various localities and make it available for correlation. "It’s pretty clear that the masses do not particularly like the Ashton choices." I think everyone feels fine about the utility of the SU, SR, MRS, MFS and RSME which are the surfaces the ISSC TG has recognized as valid surfaces of sequence stratigraphy and recommend for usage in sequence stratigraphic unit building. What the masses don't seem to like is our "choice" to interpret that the BSFR and CC are NOT valid surfaces of sequence stratigraphy because no one has ever presented any physical criteria to allow them to be recognized in outcrop or core. Furthermore, given that they are defined as time surfaces which are independent of sedimentation (paleo-sea floors in the IWGSS report), such surfaces meet all the criteria for being chronostratigraphic surfaces. "Bottom line is most all the surfaces discussed so far are valid surfaces within the context of that which they are meant to define." Given that you have not discussed a BSFR and a CC, I would agree with this. However the debate/ disagreement between the ISSC TG and IWGSS boils down to whether or not the BSFR and CC are material-based surfaces of sequence stratigraphy or are time-based surfaces of chronostratigraphy. It simply comes down to, are there or are there not adequate physical criteria for recognizing such surfaces in well exposed strata in most geological settings. The five agreed upon surfaces meet this requirement. I have recognized and correlated thousands of such surfaces and published cross sections illustrating them. So has Octavian for that matter. MacNaughton et al and MacNeil and Jones (and thousands of other authors) have also had no problem with delineating and correlating acceptable MFSs, MRSs, SRs and SUs. However, as discussed in the Shoehorning essay, they could not delineate and correlate a single, acceptable (meets the definition of the surface) BSFR or CC . Notably no one else has ever accomplished such a feat including Octavian. Of course many have used high diachroniety facies surfaces as shoehorned compromises which lead to not-so-great, non-actualistic interpretations. The fact that no one has been able to recognize such surfaces in rocks underscores the lack of physical criteria required to have them accepted as bona fide, material-based surfaces of sequence stratigraphy as discussed in the ISSC Report and by John in his earlier comment. The best anyone has done is to "tag" a seismic reflector as such a surface. I would emphasize such seismic interpretations are highly equivocal and do not constitute proof of existence of a discrete, specific surface type. Such seismic tagging is simply another form of shoehorning and Octavian’s favourite seismic section, which is used repeatedly in his text book, is a classic example of this. "While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary." Actually you might do well to begrudge Henry such a sequence boundary simply because it has no physical characteristics which allow it to be recognized and correlated. Even Henry acknowledges such a lack of physical attributes. I think it is most important to understand that Henry Posamentier sees sequence stratigraphy as equivalent to chronostratigraphy. Here are a few quotes from Henry: “The essence of sequence stratigraphy involves the recognition of time synchronous surfaces”; “The correlative conformity represents a surface that exists at the time of initiation of sea-level fall”; “The correlative conformity may have no obvious diagnostic attributes”. Henry and Octavian represent the view that sequence stratigraphy includes chronostratigraphy and that is the basic differences between their mixed time-based/material-based approach and the material-based approach of the ISSC TG. Given that you appear to be on the material-based only side of the street, you might want to more closely examine what Henry proposes as a sequence boundary. "That said, the choice of sequence-bounding surface seem to be the most contentious. There are issues with choosing any of the three/four favorites in the larger debate over the best fundamental sequence-bounding surface. While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary, I would also say that forcing me to abide by this as the true sequence boundary would render sequence stratigraphy pretty much useless to me, owing to my work being mostly within continental-interior basins." Here is where I think there is some significant misunderstanding of the recommendations of the ISSC Report. The Report emphatically recommends that a sequence/ sequence boundary be completely generic and that workers are free (encouraged) to define as many specific types of sequence boundaries as their data allow. The generic definition offered is that a sequence boundary is a specific type of unconformity and its correlative (i.e. physically join together) surfaces. Doesn’t get any simpler or flexible than this! Two common types of sequence boundaries were discussed in the Report and the definitions of these two types were taken straight from the literature (Van Wagoner et al. 1988 and Galloway, 1989). One was a depositional sequence boundary defined as a subaerial unconformity and its correlative surfaces. The other was an R-T or genetic stratigraphic sequence boundary defined as an unconformable MFS and its correlative surfaces. It was further noted that other types of sequence boundaries could be defined on the basis of other specific types of unconformities and their correlative surfaces. Also, it is even possible to have different sub-types of a given type of sequence boundary. For example we noted that very often a MRS and SR-U are correlative (join together) with an SU and thus represent a one possible combination of surfaces for a bona fide depositional boundary. If other workers demonstrate that other types of material-based surfaces correlate to an SU, they could establish other combinations for a depositional sequence boundary. We believe it would be inflexible if only one specific combination of correlative surfaces were allowed for a given type of sequence boundary as advocated by Catuneanu (2006) and the IWGSS report. It is the specific unconformity type which defines a given type of sequence boundary NOT the correlative surfaces. We believe this is about as flexible as it gets and certainly matches/exceeds the flexibility of the IWGSS proposal. "Codifying any of these surfaces as the “true bounding surface” just means that most of the remaining sequence-stratigraphic community will simply ignore the code." No argument here and I hope you agree that it is worthwhile to try to reach consensus on specific names for specific types of boundaries and units to help communication. We suggested commonly used names for the two types of sequence boundaries most often employed in sequence stratigraphy. We leave it to others to name other types of sequence boundaries. "Likewise, the other forms of stratigraphy don’t demand such unique criteria for defining unit boundaries. There are at least seven types of fossil combinations that can be used to define biozones in the North American code." As was noted in the ISSC Report, we used the biozone concept as the guide to how a sequence can be effectively defined. In both cases the definitions are generic and modifiers allow specific types to be defined - specific types of fossil change (e.g. assemblage biozone) or unconformity types (depositional sequence). As mentioned earlier, this system allows new types of biozones/sequences to be defined as required. "The individual worker is given enormous latitude as to what can be lumped into a lithostratigraphic unit or with regard to which rock layer is to be picked as the last placed within the unit. Why are we choosing to be so picky with sequence stratigraphy?" I hope you can now see we are not being picky but are being extremely flexible. All we ask is that material-based surfaces be used as they are in every other stratigraphic discipline except chronostratigraphy. Unfortunately a few of our most vociferous detractors have completely misrepresented the ISSC recommendations in various posts so I can understand why John had a wrong and negative impression of our recommendations for defining sequence boundaries. "While it has difficulties too, a more inclusive approach like that favored by Octavian." I would note that Octavian's approach is less inclusive. It does not offer a generic definition for a sequence. It does not allow different sub-types for a given sequence boundary. The biggest problem of course is that Octavian encourages the use of featureless time surfaces for the definition of different types of sequence boundaries. This last problem of course leads to all the problems discussed in the Shoehorning essay and is the source of the “difficulties” that John alludes to. I would again reiterate it is hard to imagine how one could come up with a more flexible and pragmatic approach than that recommended in the ISSC Report. A worker can mix and match as he/she pleases with the only caveat being that all surfaces have to be material-based. " I can think of no processes that demands shoehorning more than codification." There is no doubt that guidance or a codification in an official publication of an international stratigraphic commission could potentially lead to major shoehorning IF the guidance/codification is based on a non-material entity. It is critical to note that main problem which leads to shoehorning is not where the recommendation is but the nature of the recommendation. Once you recommend a non-material surface for a boundary, people are going to have to use "compromise" material-based surfaces to compensate. Thus, even though neither NACSN or ISSC have not offered any recommendations on sequence stratigraphy, there is MAJOR shoehorning going on simply because a couple of abstract time surfaces have become entrenched in the literature and textbooks on sequence stratigraphy. Sedimentary geologists are pressured to identify these entrenched, abstract surfaces (peer pressure) and, as discussed in my recent essay, this has lead to a lot of most unfortunate shoehorning and non-actualistic interpretations of sedimentary processes and depositional histories. "Ulrich-ism" reigns supreme right now with many people interpreting diachronous facies contacts as time surfaces. "The moment we codify any form of sequence stratigraphy, we are going to be attempting to set the demand that everyone from that day forth shoehorn into the decided code." Not really. As long as we recommend, material-based surfaces to define units there will be no need to shoehorn. I would note that there is no noticeable shoehorning in the other material-based stratigraphic disciplines. When the time surfaces are eliminated, everyone will be able to recognize the accepted surfaces and, in most instances, there will be agreement on their placement. I am sure, if I took John, Octavian and Ron up one of my sections, we would all agree on the approximate placement of the MRS, MFS, SU and SR-U. We might haggle over a few tens of centimetres for the exact placements and such is interpretation. The same goes for a well log section although there may be some slightly larger variation in placement choices due to the lower resolution. However we could still have a good discussion as to which picked surface was the best one. Most importantly, none of us would be able to point to a BSFR or a CC in the best exposed section even though such time surfaces would be there in theory. Each of us might shoehorn such surfaces in at very different horizons and there would be no way to discuss which is a better shoehorned pick. And that is the basic difference between the empirical surfaces which need no shoehorning and the unrecognizable time surfaces which do. "By the way, I noticed Octavian’s e-mail has somehow fallen from the mailing list." Good catch. It would be nice if Octavian could supply a few examples of where he has identified and correlated BSFRs and CCs in rocks (outcrop, logs) so we get beyond the “do as I say, not as I do” problem. He certainly has published lots of cross sections with MFSs, MRSs. SRs and SUs on them. The fact that his book lacks a single stratigraphic cross section which illustrates the delineation and correlation of BSFRs and CCs is telling. Ashton Embry: AEmbry@nrcan.gc.ca +++++++++++++++++++++++++++++++++++++++++++++++++++ On Jun 8, 2007 at 1:07pm John M Holbrook wrote Chris, Please to feel free to post these comments and the Ashton response. My comment pertained mostly to the e-mail exchanges, but Ashton points out that other people are involved in the debate who are not on the e-mail listing. Please to shift these on over. Thanks, John Folks: I have been following the sequence-stratigraphic debate between Aston and what appears to be the sequence-stratigraphic world at large with great interest. I would like to humbly offer some comments regarding points that seem to have emerged from the debate. 1) Aston does have a valid point. The stratigraphic code is built on the core principle that stratigraphic units should be defined based solely on “objective” and “reproducible” descriptive criteria. Each form of stratigraphy is defined based on the criteria that are used to define boundaries/units. The main issue with codification of sequence stratigraphy from the beginning has been that we are accused of imposing interpretive criteria for defining key units and boundaries. Certainly insertion of definitions into the code that use time or genetic origin as an interpretive element in defining units or boundaries would indeed be a violation of the spirit of either the North American or International stratigraphic codes. 2) Time as the force behind defining criteria While it might be bad form to use a time-interpretation as criteria do define a boundary, it is perfectly legitimate to define a boundary based on observable criteria that has time significance. Case in point, biostratigraphy. We all recognize this as the most fundamental means of defining “time-rock” units, and commonly define time-lines in the rock record using this method. We are all well familiar with the idea that we are not really mapping time-lines but are actually mapping first and last appearance of fossils in the rock column. We also know enough about evolution to know that species dispersion and extinction are diachronus events. Yet, we have long felt comfortable basing our original concept of geologic time, and the fundamental process of mapping time-rock units, based on biostratigraphic criteria. Similarly, we all know that maximum flooding surfaces, regressive-surfaces of erosion, flooding surfaces, etc. are not recording truly synchronous events, and thus do not record absolute time-lines. We do, however, know that each of these has some time-significance. It is perfectly legitimate to deliberately choose to map these surfaces because of the expectation that they will yield time/event-significant information. This is indeed one of the more powerful aspects of sequence stratigraphy. Likewise, it seems perfectly fair, and of broad practice across all fields of stratigraphy, to use time-based techniques to hone in on a physical bounding characteristics (e.g. using biostratigraphy to help trace a correlative conformity through units where the surface is otherwise difficult to find and distinguish physically from various surface candidates, choosing one potential unit over a similar unit for lithostratigraphic correlation because its age is clearly more consistent with correlation than the alternative unit, etc.). What would likely be a stretch is to define boundaries by specific point in time, on the premise that this time records a specific event like maximum transgression, particularly in applying this where there is no physical criterian or discrete surface to map. This would not be against the rules either, but it would be chronostratigraphy rather than sequence stratigraphy, which is already codified. Why bother. In short, mixing time and physical criteria to define units is far from unprecedented; it is in fact the norm. What we are charged to do, however, is clearly define the physical criteria we use to define boundaries, and state the time significance of this choice separately. Though we commonly appear to “define” boundaries by such time/event criteria as first episode of relative sea-level drop, most all of us are actually defining boundaries by such physically mapable criteria as landwardmost surface of downstep in coastal onlap. While it isn’t so hard to find a person who violates from time to time, most all of us are actually mapping physical surfaces with physical criteria when doing sequence stratigraphy and deliberately choosing surfaces because we know they will have time and/or genetic significance. There isn’t a thing wrong with that approach. 3) Surfaces surfaces everywhere! Most of the disagreements seem to stem from the fact that we each prefer to map different surfaces from a wide selection of possible surfaces. This is usually either because these are the surfaces that are easiest to map given the data and settings in which we tend to work, or because these are the surfaces that yield the information we prefer to know. It’s pretty clear that the masses do not particularly like the Ashton choices. While I’m thinking I might find more support for my choices, I would hardly expect them to be universally accepted as the pure surfaces either. Bottom line is most all the surfaces discussed so far are valid surfaces within the context of that which they are meant to define. That’s why we like to use them. That said, the choice of sequence-bounding surface seem to be the most contentious. There are issues with choosing any of the three/four favorites in the larger debate over the best fundamental sequence-bounding surface. While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary, I would also say that forcing me to abide by this as the true sequence boundary would render sequence stratigraphy pretty much useless to me, owing to my work being mostly within continental-interior basins. Codifying any of these surfaces as the “true bounding surface” just means that most of the remaining sequence-stratigraphic community will simply ignore the code. This is destine to generate a lot of confusion. Likewise, the other forms of stratigraphy don’t demand such unique criteria for defining unit boundaries. There are at least seven types of fossil combinations that can be used to define biozones in the North American code. The individual worker is given enormous latitude as to what can be lumped into a lithostratigrahic unit or with regard to which rock layer is to be picked as the last placed within the unit. Why are we choosing to be so picky with sequence stratigraphy? While it has difficulties too, a more inclusive approach like that favored by Octavian makes a certain since if we are to bother with codification at all. 4) Shoehorning I can think of no processes that demands shoehorning more than codification. The moment we codify any form of sequence stratigraphy, we are going to be attempting to set the demand that everyone from that day forth shoehorn into the decided code. How would science have been served by codifying Vail, et al 1977? By the way, I noticed Octavian’s e-mail has somehow fallen from the mailing list. I added it back in my listing. Cheers, John "Holbrook, John M" <holbrook@uta.edu> +++++++++++++++++++++++++++++++++++++++++ On June 6th 2007 William Helland-Hansen wrote Guidelines for Sequence Stratigraphy – Comment to the ongoing debate Dear Sequence Stratigraphers There are a few points that I feel is important to communicate after having read most of the ongoing dialogue on the formalization of Sequence Stratigraphy: Without rephrasing the definition of sequence stratigraphy, this discipline basically deals with unconformities (with an unconformity defined as “a surface of erosion or nondeposition that separates younger strata from older rocks and represents a significant hiatus”, Mitchum, 1977) and the sedimentary deposits between the unconformities – all put together in a genetic and chronostratigraphic framework. Unconformities include levels of erosion, non-deposition and very slow deposition that can mark facies-belt jumps - or major environmental shifts - across surfaces or condenced intervals. They are caused by base-level changes and brought about most commonly by allogenic forcing mechanisms, but also by large scale autogenic forcing mechanisms (e.g. delta-lobe shifting). Deposits between unconformities consist of sediment and minor breaks (diastems) (e.g. common channel erosion, base of storm bed, base of turbidite etc) not caused by base-level changes (Embrys WTFC). These between-unconformity successions can be analyzed in terms of Walther’s law and are deposited in a response to a particular trend of environmental change – a migration trajectory of the depositional system. The systems tract concept, as currently applied, should be avoided in formalization attempts because it anticipates a succession in depositional trends. For example: 1) Vertically expanded successions of alternating regressive and transgressive strata without intervening base-level falls and 2) laterally expanded successions of alternating normal and forced regressions, without intervening transgressions, are both common in the stratigraphic record. These situations are not included in the proposed schemes. Furthermore, neither of the drafts from the two groups propose how the stratigraphic problems of sequences delineation in continental and deep-marine successions should be tackled. Formalization, if any, should be on the “tool box” level (cf. earlier comment by Ole Martinsen) – without any pre-conceived succession of sedimentary trends and tracts: What kind of breaks do we have, and what kind of trends do we have between the breaks? If we can really define all these as “concrete” – without forcing successions of events or trends into one universal template – then we may be a step further. Meanwhile, the concept of allostratigraphy plus the original definition by Mitchum 1977 (“.. a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities”) seem to work just fine. May June 6th 2007 William Helland-Hansen, University of Bergen E-mail <William.Helland-Hansen@geo.uib.no> ++++++++++++++++++++++++++++++++++++++ On June 3rd 2007 Ashton Embry wrote: Reply to Alex MacNeil It was a very nice surprise to see that our debate attracted comments from a member of Gen Y, the generation that represents the future of our discipline. It was also very encouraging that Alex addressed the main issue of the debate in his incisive comments and I thank him for reminding me of another example of an attempted application of the seven-surface/four systems tract approach to a real world succession. Such real world examples are completely lacking in Catuneanu (2006) and the draft IWGSS report and they are undoubtedly the acid tests for the acceptance or rejection of such an approach. Below are replies to some of Alex’s specific comments. “In several parts of the ISSC draft paper (e.g., page 44), the authors assert (rather strongly) that no one has ever presented any defining characteristics of a BSFR or demonstrated such a surface in core or outcrop. Before making such strong assertions, the authors should have taken the time to familiarize themselves with the recent literature on sequence stratigraphy. The MacNeil and Jones (2006) paper in Sedimentology documented the BSFR in outcrop and core, and undermines some of the key arguments made in the ISSC paper.” Actually we make the point that no one has ever presented any suitable/acceptable defining characteristics for a BSFR or a CC. Many people have used various types of real stratigraphic surfaces that have observable characteristics as “compromises” for the two time surfaces. However, in every case we looked at, such “compromise surfaces” were not time surfaces (paleo sea floors) or even reasonable approximations. I may be slowing down in regards to remaining current with the sequence stratigraphic literature but I try to read as many papers as I can. I hope Alex will be happy to hear that I read his paper soon after it was published. I cut my stratigraphic teeth on Devonian reefal carbonates almost 40 years ago you never forget your first love. I downloaded the MacNeil and Jones sequence stratigraphy paper as soon as I became aware of it. I was flattered that they referenced my very old papers from the early 70s and even used those less than euphonious, lithologic terms (e.g. bindstone) I dreamed up while lying in a hospital bed in 1969. As is discussed in the “Shoehorning and its Consequences” post, I found that the surfaces MacNeil and Jones labeled as BSFR (2 surfaces) and CC (1 surface) do NOT fit the definition of a BSFR and a CC. The compromise surfaces labeled as BSFR and CC are an unacceptable attempt to recognize such surfaces. I will not discuss the details of these surfaces in this reply and the reader is referred to ‘Shoehorning and its Consequences” for a detailed discussion of the MacNeil and Jones’ attempt to recognize a BSFR and a CC. “Likewise, the biotic composition of the biostrome is consistent with an intermediate water depth (ironically, I can reference some of Embry’s original work from the Arctic on suggested absolute water depths for Devonian carbonate facies to support the intermediate water depth) between deeper water, nearly-drowned carbonates of the TST and HST and shallow water, high-energy carbonates of the ensuing LST.” I am pleased my past work helped Alex in facies interpretation. There is no doubt MacNeil and Jones have identified a nice shallowing upward succession from the basal TST to the top of the high energy carbonates which, as interpreted, are capped by a well documented MRS. The question becomes how one can recognize the time surfaces at the start and end of base level fall in such a shallowing upward succession. MacNeil and Jones have used within trend facies boundaries to do so and, as discussed in the “Shoehorning” post, their rationalizations that such facies changes occur at the instant that base level starts to fall or starts to rise are not convincing. In fact they demand very unlikely and non-actualistic sedimentological processes and events. I can understand why they chose such facies contacts as their best guess as to the placement of the time surfaces because the facies contacts were the only recognizable surfaces present. In other words, these surfaces were the only “compromise surfaces” available to label as a BSFR and a CC. “In carbonate systems with ramp-type geometries, the Basal Surface of Forced Regression can be identified as the surface that separates depth-controlled biofacies, and biofacies controlled or influenced by trophic resources, if the change in base-level shifted nutrient regimes…. Note that sediment shed in the HST has a specific grain size range (for review see MacNeil and Jones 2006) and changes in grain size can be used to constrain the shift from normal to forced regression.” The basic, underlying assumption here is that such changes (e.g. in nutrient regimes, grain size, etc) happen over a large area at the instant when base level starts to fall. Thus it is assumed that such changes mark a paleo-seafloor (time surface) at the start of BL fall. This assumption of very significant changes occurring over the basin at the moment of start base level fall is not reasonable or actualistic. There is no doubt changes occur as base level falls but the preserved record of such changes (i.e. the recognized facies change) varies in time and space. Thus labeling such a facies/ material-based change as recognized at various localities as a time surface is not realistic or acceptable. “Delineation of the BSFR with these empirical criteria is consistent with the type of data that Embry et al. argue (e.g., last paragraph on page 40) can be used to define the MRS and MFS. As such, to argue that there is no empirical data that can be used to identify a BSFR invokes a double standard and is incorrect” This is fundamentally not so. The BSFR is defined as a time surface whereas the empirical surfaces such as the MRS and MRS are defined by observable characteristics. The difference between the two types of definitions is profound. MacNeil and Jones see a change in trend from a shallowing to a deepening in their shallow water carbonate rocks and, as described in the ISSC report, such characteristics are sufficient to delineate an MRS. This is all well and good and I doubt if anyone would dispute their placement of the MRS in this instance. The BSFR is the time surface that marks the beginning of base level fall. MacNeil and Jones have not demonstrated that the observable facies change they observed (at the base of the biostrome) formed at the instant that base level started to fall. As argued in “Shoehorning”, it is reasonably certain that the recognized facies change did NOT form at the moment of the start of base level fall but most likely occurred AFTER base level began to fall. Thus, in contrast to their MRS, their BSFR is highly disputable. The reason for this is there are no noticeable changes in sedimentation over most of the basin at the instant base level starts to fall and consequently there are no reasonable criteria for delineating such a time surface. The bottom line is that MacNeil and Jones have failed to provide “empirical data” that can be used to identify a “BSFR” (the paleo-seafloor at the instant when base level starts to fall). All they have done is to try to rationalize that the facies change they do see formed at the moment that base level started to fall. As noted earlier, such a rationalization “does not hold water”. “It is not very “real-world” to limit sequence stratigraphic interpretations and systems tract definition to whether or not a specific surface is identifiable across a study region – a criterion argued for throughout the ISSC paper.” If you want to delineate a given stratigraphic unit, say a TST in a given sequence, then one has to correlate the boundaries of that unit over the study area. If this cannot be done, such a unit has little to no utility for subsequent facies analysis and reconstructing depositional history. Of course it is recognized that at some localities the data may not be sufficient for the exact placement of a given boundary but this can only be determined if one knows what data are needed to recognize the boundary in the first place. In most cases the placement of the empirical surfaces of sequence stratigraphy can be reliably done at most localities at which strata are well exposed and the work of MacNeil and Jones clearly demonstrates this. Their MFSs and MRSs are reliably delineated in most sections. The same cannot be said for the BSFRs and CC. “Systems tracts are delineated by sequence stratigraphic surfaces, but they are defined by integrated data sets that include surfaces, stratal geometries/stacking patterns, and detailed analysis of facies associations.” This is a common error in understanding. The systems tracts are defined by their boundaries. Their boundaries are “defined by integrated data sets that include surfaces, stratal geometries/stacking patterns, and detailed analysis of facies associations.” It is important to understand the difference between how surfaces are defined and how systems tracts are defined. Data sets yield surfaces and surfaces yield systems tracts. Data sets do not directly yield systems tracts. One needs to go though the surface identification step first. “So yes, if one chooses to limit themselves to sequence stratigraphic surfaces, they may find themselves largely limited to defining T-R sequences. If one chooses, however, to actually include study of the sedimentology and stratigraphy of the deposits between the surfaces, they can conclude with much more advanced interpretations – and therein lies the powerful nature of sequence stratigraphic concepts and methods with significant implications for how we understand strata and advance the science”. We are getting close here. If we stick with surfaces, we can reliably identify an R-T sequence and a depositional sequence as defined in the ISSC report. So Alex is almost right with his first sentence. In his second sentence Alex suggests we can make some more advanced interpretations by examining the strata within the empirical systems tracts, the TST and the RST. There is no argument with this. Thus in the RST we can sometimes say these specific strata were deposited under conditions of rising base level and these specific strata were deposited under conditions of falling base level (e.g. they have a RSME at the base and a SU at the top). The critical point that must be stressed is that we CANNOT draw the boundary between rising base level deposits and falling base level deposits with any semblance of objectivity and thus cannot have separate stratigraphic units based on this concept. There will always be a large interval of uncertainty, and any attempt to draw a line between rising and falling base level deposits superimposes a very speculative and unsupportable interpretation on the data. This is the last thing we want to do. “I can not imagine that its guidelines, in the current form, will be accepted by the greater research community. In the very least, they need to correct their assertions that the BSFR is purely theoretical and has never been demonstrated in core or outcrop, as this is incorrect.” I like to think most sedimentary geologists are rational, critical thinkers. To me, when one applies critical thinking to Catuneanu (2006) and the IWGSS draft report, the fruitlessness of trying to delineate abstract time surfaces and to use them for unit boundaries becomes readily apparent. It just seems few have ever thought about the problems of such an approach. I hope Alex will do such critical thinking and always ask himself “Can this sedimentological change I want to label as a BSFR be instantaneous over the basin and coincide with the moment of base level fall?”. Based on current data, including the MacNeil and Jones paper, we stand by our “assertions that the BSFR is purely theoretical and has never been realistically demonstrated in core or outcrop”. Summary – I again thank Alex for his very useful and “on topic” contribution to the debate. His work illustrates a critical problem with sequence stratigraphy –forced and inappropriate interpretations of a BSFR and a CC. The interpretations that facies contacts are time surfaces demand the past occurrence of non-actualistic sedimentological processes and events. Ashston Embry <AEmbry@nrcan.gc.ca> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On June 3rd 2007 Ashton Embry wrote: Reply to Bob Dalrymple These are by far the most comprehensive and insightful comments received so far and they will be of great value for revising our report. Thank you very much Bob for taking the time to be so helpful. Many of Bob’s comments will require only wording or “tone” changes in the report and I will not bother to discuss these. I would add here that such clarifications are very important and I am not trivializing such comments and the changes necessitated by them. It is just that they do not have anything to do with the main aspects of the debate which centre on classifying sequence stratigraphic surfaces and units. Below are comments on some of Bob’s “bigger” points that I think are worth considering in this forum. Bob’s “process sedimentologist” perspective of sequence stratigraphy has been especially enlightening and has helped me understand how the current problems have arisen. I can assure Bob and everyone else on this forum that any well-supported, substantive comments will bring about substantive changes in the Report. ISSC is looking to provide the best recommendations possible. That is why we have circulated the Report as widely as possible. Specific Comments ‘as the word “science” appears to be used in an extremely loose manner in the ISSC report. As Ashton himself has said, the deductive approach is an acceptable scientific methodology. Furthermore, the proposals that have been made by the deductive approach are definitely testable, one of the hallmarks of science.” I expect Bob is right that we went overboard with the adjective “unscientific’ and this will be easily remedied in the revised Report. I think we all agree that deduction is an important part of science. However we enter the area of non-science (junk-science) when one uses a deduced entity, for which no empirical support has been yet demonstrated, as a real entity upon which other entities are then based (classic house of cards). Thus one can deduce almost anything imaginable and examples include a neutralino, a powerful deity, a Martian, and a stratigraphic surface generated at a specific instant in time (as defined by a specific event at a specific geographic location). Science thrives on imaginative deduction. For a deduced entity such as a Martian to enter the world of science, it is crucial to accumulate empirical data which support the existence of such a deduced entity. Thus neutralinos, powerful deities and Martians are NOT scientific entities because we have no empirical evidence to support their existence. A recent headline in our local paper announced that “The Museum of Creation Science” has just opened (only in Alberta you say). Lots of deduced concepts but not much science. I would claim that stratigraphic surfaces formed at the start and end of base level fall are also “Not Yet Scientific Entities” simply because we have no empirical evidence for their existence. If we did, we would not be having this debate and it would be clear how one might recognize a BSFR as we now recognize a MRS or an MFS or any other widely accepted, empirical surface of sequence stratigraphy. Notably, every attempt to delineate and correlate such time surfaces has failed (i.e. the deduced entities are falsified so far). Given this, to use such entities as a basis for defining specific types of stratigraphic units (e.g. FSST), has no scientific support. I will deal with a few attempts to delineate such baseless entities in a separate post on “Shoehorning and its Consequences”. All I will say now is, calling a dog a Martian does not make Martians (as originally deduced) real entities. Thus I would disagree with Bob’s assertion that “Therefore, none of the work being criticized is unscientific”. Searching for empirical data to support the existence of a BSFR is scientific activity. Claiming a BSFR exists in spite of no supporting empirical data and then using it to define a stratigraphic unit is not scientific. “ I’m sure that the authors of the ISSC report, despite their claims of being “data driven” empiricists, must make “informed decisions” (i.e., model-driven interpretations) every time they construct stratigraphic sections. What they are attempting to do in this report is to create a highly simplified scheme for erecting sequences within sedimentary basins, so that the amount of interpretation is minimized.” When it comes to correlation and defining stratigraphic units, it is most important to do these activities with “real” surfaces. Once we allow entities that have no empirical support into the mix, we have crossed a major line between science and non-science. We certainly want to create as simple a classification scheme as possible while at the same time honoring the existence of all real sequence stratigraphic surfaces. This debate concerns the realness of the BSFR and CC. If they can be demonstrated to be real, then a seven-surface/four systems tract classification scheme is simplest. If not, a five-surface/ two-systems tract scheme is simplest and best. “If I understand the ISSC Report, its authors have a very restricted purpose behind the proposals that they make, namely to provide a simple and straight-forward, pragmatic means of mapping sequences throughout a sedimentary basin, and, to a lesser degree, to assign an order to each of them.“ I see this as the “process sedimentologist” perspective on sequence stratigraphy. To stratigraphers, sequence stratigraphy is fundamentally all about correlation. And correlation entails the recognition of specific types of stratigraphic surfaces and matching them to specific, equivalent surfaces at other localities. Specific types of units “fall out” of such a correlation framework, being bounded by specific types of surfaces. A very important part of the guidance in the International Stratigraphic Guide is the definitions of various surfaces and units which are part of a given discipline. The ISSC Report on Sequence Stratigraphy provides recommendations on what surfaces and units are best to use when doing a sequence stratigraphic analysis. This is the heart of sequence stratigraphy and once such a correlation framework is established then the interpretations of facies relationships and depositional history naturally follow. Such interpretations can be considered a part of sequence stratigraphy but they are completely dependent on the established sequence stratigraphic framework. Bad framework, bad interpretations. We are trying hard to get some logical, well-supported feedback on the recommendations or surfaces and units but as can be seen from many of the replies on this webpage, such science-based feedback is tough to extract. If people do not like the recommendation that a BSFR is not an acceptable sequence stratigraphic surface (it is actually a chronostratigraphic surface) or that a FSST is not an acceptable sequence stratigraphic unit (it is, in part, a chronostratigraphic unit [chronozone]), we would really love to receive a good scientific argument why they are. “The more provocative proposal to use the MRS as the basinward extension of the subaerial unconformity is, from a strictly mapping point of view, perhaps defendable. However, I believe that the authors downplay the diachroneity of this surface” There is little doubt that in many cases the MRS joins the flanking unconformity simply because the basinward portion of most flanking unconformities is an SR-U. In fact this is the situation in almost all carbonate depositional systems (i.e. marine strata overlie the unconformity). Thus, on both empirical and theoretical grounds, the MRS is most often (not always!) a correlative surface of a basin flank unconformity. The diachroneity of the MRS is not a big issue because it is less than that of a within trend facies contact and it varies greatly in absolute terms depending on the order of the given MRS. Miscorrelation of MRSs is also another big problem which can lead to a false impression of excessive diachroniety. There is no doubt that the interpreter has to appraise the diachroneity when interpreting depositional history. “If the RSL lowstand is of longer duration than is argued (on flimsy grounds) in the ISSC report, then the amount of diachroneity of the MRS may be large.” This question of the lag between the times of start base level rise and start transgression is a very important one and requires much more empirical data to resolve it. I have not found any empirical evidence to support an interpretation that such a lag time is long and have found considerable evidence that it is short to basically non-existent. Again for those who disagree with this assertion, we would appreciate receiving empirical data (or references to such) that support their position of a long lag time. As noted in my reply to the IWGSS report, all the references that were used to support the existence of a unit deposited after the start of base level rise but before start transgression, turned out not to include such evidence. “I would suggest that the correlative conformity (i.e., the surface/timeline that truly does extend from the tip of the subaerial unconformity; the MRS is erroneously shown as tying to this position in some of the ISSC figures!) is a better surface on which to base sequences. Embry et al. argue strenuously (both in the ISSC report and in subsequent rebuttals to comments) that this surface is “invisible” (i.e., it is not easily seen in outcrops and cores). While I have not worked a great deal in the relevant deposits, I do have some minor experience (see MacNaughton et al., 2000, Canadian Jour. Earth Sci., 37: 997-1020). In that paper, we were able to identify on objective grounds (grain-size changes as controlled by the trapping, or not, of coarse material further updip in the source-to-sink system; the degree of development of slump and other mass-movement deposits, as a proxy for sedimentation rate; etc.), the boundaries of the four systems tracts that have been proposed by recent sequence-stratigraphic schemes.” It is most encouraging to have a member of IWGSS finally address the main point that distinguishes the different approaches taken by ISSC and IWGSS. Bob has offered a reference which demonstrates how he and co-workers recognized BSFRs and CCs in a well-exposed rock succession and thus delineated the 4 systems tracts in the succession. I had read this paper when it came out and have just finished re-reading it. I will discuss in some detail in a separate post on “Shoehorning in Sequence Stratigraphy and its Consequences” how Bob and his colleagues delineated the BSFRs and CCs and the consequent enclosed systems tracts. Some very useful insights into the problems with delineating the BSFR and CC time surfaces and the consequences that result can be extracted from a look at how MacNaughton et al “shoehorned” their excellent data into the four systems tracts approach now advocated by IWGSS. “As to the other “invisible” surface… the base of forced regression… my tendency is to agree with Embry et al. that this is not a realistic surface for correlation, or on which to base the definition of a sequence. . Embry et al. are wrong, however (in my opinion), to assert that there is no change in stacking pattern at the time when this surface forms.” It is nice to hear Bob agreeing that the time surface at the start of base level fall “is not a realistic surface for correlation” or to use to define a sequence stratigraphic unit. I would still claim there is no recognizable change in stacking pattern (in conformable successions) at such a time surface because regression simply continues in the offshore area and there is no discernable difference between a small scale sequence (depositional or R-T) directly below such a time surface and that directly above. Only in an unrealistic, perfect setting where each stacked unit represented the same time interval, each filled exactly to sea level and there was no erosion during transgression could such a difference possibly be determined. If anyone has empirical data which shows a change in stacking pattern at such a theoretical time surface, we would be pleased to receive them. We have not been able to find such data despite a focused search for them. “Sequence stratigraphy, as it is now practiced by most people, is much more than a tool for mapping. Instead, it encompasses a very sophisticated and holistic conceptual framework for interpreting many things about a sedimentary succession, including but not limited to: the presence/absence and location of specific facies, the vertical and lateral distribution of grain sizes, etc., etc. For this purpose, people require the most detailed model possible, because the system being interpreted is enormously complex. On this basis, the scheme proposed in the ISSC report is simpler than most other schemes and hence is a “step backward” as has been noted by some commentators”. This again is the “process sedimentologist” perspective which fails to appreciate the overwhelming importance of correlation in sequence stratigraphy. There is no doubt that once sequence stratigraphic surfaces have been delineated and correlated that many useful interpretations as to facies relationships, depositional history and paleogeographic evolution can be made. This point was clearly made in the Report. However unless such surfaces are properly defined, delineated and correlated then there is no “sophisticated and holistic conceptual framework for interpreting many things”. And that is the problem with current sequence stratigraphic practice. The deductive approach demands the recognition of unrecognizable time surfaces and many practitioners such as MacNaughton et al are using inappropriate stratigraphic surfaces as “proxies” for such unrecognizable time surfaces. This results in unrealistic interpretations of sedimentation processes, facies relations and depositional history as is discussed in the “Shoehorning” post. It seems to me all process sedimentologists should be concerned about this. I agree we need the most “detailed model” possible but it must be scientifically defendable and applicable to real world situations. We have argued that the seven-surface/four systems tract model is not scientifically defendable and is not applicable to real world situations. The five surface/two systems tract approach appears to be as detailed as we can presently get while at the same time having a scientifically defendable approach which is applicable to real world situations. “the formal recognition and endorsement of the Embry et al. scheme by the ISSC may, for some workers, give it a greater degree of sway than it should receive. If people attempt to use the Embry et al. scheme for detailed interpretations (as opposed to mapping), then their work will be limited unnecessarily.” This is very doubtful. People tend to apply what works. A classic example is the ISSC recommendation for the use of a “synthem” in the 1994 edition of the ISG. I did not detect “a greater degree of sway than it should receive” for the synthem. I ask everyone reading this post to reflect on how much he/she was influenced by the 1994 International Stratigraphic Guide and the specific recommendation for applying synthems. “.I suspect that many of the more vitriolic attacks on the ISSC proposal stem from the belief that this document proposes to discourage the application of other schemes (e.g., a four systems-tract model). This is (hopefully) not intended and should be explicitly stated, with guidelines as to situations where the use of each may be more appropriate.” I agree with Bob that the somewhat mean-spirited attacks we have received stem from the recommendations that discourage the application of a mixed, material-based/time-based approach to sequence stratigraphy which they strongly advocate. A number of the members of IWGSS including the chair have their careers closely associated with such an approach through the books they have written and the courses they teach. It is not surprising that these people have reacted in a very emotional manner and I understand why they are so upset. I would emphasize that the draft ISSC report DOES recommend that the four systems tract approach NOT be used until such time that empirical data are available to demonstrate the existence of the deduced time surfaces labeled as BSFR and CC. To us, this is a scientifically responsible approach and, if it upsets some people, so be it. We did not find any situations where the use of such an approach would be appropriate because we could not find any evidence to support it. We found lots of attempts to use it (e.g. MacNaughton et al, 2000; MacNeil and Jones, 2006) but in every case inappropriate stratigraphic surfaces were used as proxies for the time surfaces with less than satisfying results. We will discuss the 4 systems tract approach more explicitly in the revised text and outline our serious concerns (with real world examples such as MacNaughton et al) with it. “I urge the ISSC to explore this “pragmatic” alternative to the contentious ideas put forward in the Embry et al. report” Actually Bob’s proposal for a pragmatic approach is quite similar to that proposed in the ISSC report. We both agree that only empirical surfaces be used. We define our two types of sequences on the two primary types of regional unconformities which have been described - the SU/SR-U combo and the unconformable portion of the MFS. The only difference between our approach and that advocated by Bob is that we follow Vail et al’s innovation that the sequence boundary is extended along correlative surfaces (i.e. surfaces which join the unconformity so as to form a single through going boundary). The advantages of this approach over Bob’s are described in the report and are well accepted by most. Extension of the unconformable MFS boundary along the conformable portion of the MFS is straightforward and is not contentious. I don’t think anyone has a problem with the definition and delineation of an R-T (genetic stratigraphic) sequence. I think we all agree that a depositional sequence has an SU as its defining unconformity following Van Wagoner et al (1988). The disagreements come with what are the correlative surfaces for the SU. I would hope that in many cases there is no doubt that the SU correlates with (i.e. joins with) an SR-U and an MRS. Thus this is one type of viable depositional sequence and there are innumerable published examples of such a depositional sequence. The ISSC Report takes a very flexible approach to sequence definition and other types of depositional sequences can be proposed. All we ask is that the correlative surfaces used to extend the depositional boundary be empirical surfaces which can be recognized on the basis of observable characteristics. This does not seem to be an onerous and unreasonable requirement given that every boundary, for every defined stratigraphic unit, in every material- based stratigraphic discipline has the same requirement. In summary Bob’s proposal is perhaps too simplistic and, as he himself states, we need as detailed approach as possible. I see the empirical, five surface/two systems tracts approach as representing the most detailed approach that our current technology and understanding will allow. Attempts at more detail (e.g. the seven surface/four systems tract approach) unfortunately are not practical or scientifically defendable at this time. “The Embry et al. scheme is not very flexible, given than there are no alternatives that are to be allowed. This will be a disappointing outcome if it is accepted. The more flexible approach that has been suggested (in preliminary draft form only) by Catuneanu allows more freedom of choice and, thus, may be more readily embraced by the scientific community” I would counter that the recommended approach is extremely flexible with the caveat that only surfaces with empirical support can be used. Including deductive, time-based surfaces, which have no empirical support as Catuneanu (2006) and now the IWGSS advocate, does not add flexibility. It is a recipe for stratigraphic anarchy because material-based observation does not guide the recognition of abstract entities such as the BSFR or CC. “I would note that the guidelines for the creation of “formations” (for example) to not specify the precise rules to be used in the establishment of formation boundaries.” Actually the rules for the formalization of a lithostratigraphic unit such as a formation are rather rigorous. I can assure Bob that formation boundaries have to be observation-based, empirical boundaries that represent a major change in lithology. If a “flexible” approach of using deductive surfaces which had no material-based characteristics was adopted for formation boundaries, I would hate to see the resulting geological maps, not to mention the tops picked for a given well. “Sequence stratigraphy is such an important branch of stratigraphy that the entrenchment of ideas that do not have wide acceptance will do more harm than good.” The main problem currently dogging sequence stratigraphy is not the entrenchment of ideas which do not have wide acceptance (this seems to be a contradiction) but is the unfortunate entrenchment of ideas that are not supported by solid, empirical science. “I’m unclear what is meant by “concrete”. I very much doubt that most previous work on sequence stratigraphy is “non-concrete” because it does not agree with the authors’ ideas. I would also note that there are no rigid statements of where a formation boundary should lie within the Guide or Code, so I see no reason why rigorous statements should be made in the case of sequence stratigraphy.” As noted in past posts, I think we will use the term material-based (as NACSN does) for what was referred to as concrete in the Report. Past work, which advocated the use of non-material based surfaces (i.e. time surfaces) for sequence stratigraphic units and correlation was non-material-based (non-concrete). Bob might want to dig into the struggles that lithostratigraphy and biostratigraphy had in shedding their methods and units of non-material based entities. Hedberg wrote a number of excellent papers on this theme. I can assure Bob that all formation boundaries are material-based (time surfaces need not apply) and the observable characteristics which define the boundaries are described in the formal definition of a given formation. “All stratigraphic units are fundamentally defined by their boundaries” I agree completely and that is why we need material-based boundaries with agreed upon observable criteria for their recognition. “We will be concentrating on the second use of sequence stratigraphy, that is, the definition of stratigraphic surfaces and units within the discipline”. This is a narrow and incomplete view of sequence stratigraphy. Therefore, any formulation that is based on it will be limiting as opposed to enabling, which is what any formulation should be.” This is a fundamental part of any stratigraphic discipline and is ISSC’s main concern. If “the definition of stratigraphic surfaces and units within the discipline” is not done well, the entire discipline will be in complete disarray. Because this has not been done well in the past for sequence stratigraphy, the discipline is experiencing some substantial problems and we are having this debate. “Emphasis is placed on the relative objectivity and reproducibility of data in defining units in each category” (NACSN, 2005)”. This statement does not require absolute objectivity, nor does it specify rigorously which surfaces are used, which it what these authors are doing.” There is a fundamental difference between recognizing an empirically based entity which is defined by observable characteristics (e.g. an MFS) and recognizing a deductive based entity which has no observable characteristics (e.g a BSFR). I suspect the failure of some people to understand this fundamental difference lies at the heart of the problems which beset sequence stratigraphy. “Farther basinward, a surface, which marked the depositional change from coarsening upward to fining upward and which was interpreted to represent the change from regression to transgression, was employed as the correlative conformity portion of the boundary (Fig. 8b)”. Simple logic (based on an understanding of the factors controlling sediment transport and deposition) shows that the surface defined in this quote forms later than the seaward-most extension of the subaerial unconformity where there is a period of relatively slow base-level rise and onlap of the landward-most point of deposition on top of the subaerial unconformity (see Fig. 11). Therefore, correlating it with the subaerial unconformity is inappropriate in the general case.” As has been described in the report, the MRS connects with (correlates with) the SU by means of the SR-U. This occurs in the vast majority of situations but certainly not all. This will not occur if a theoretical LST develops and separates the MRS from the SU. Real world examples of such a theoretical LST are exceedingly rare. Our request for references which illustrate such a stratigraphic relationship has been met with a telling silence. Thus correlating the MRS to the SU (through the SR-U) is the general case and I am sure rare exceptions to this do occur even if people have not been able to find any appropriate references. When such exceptions occur, extending the unconformable boundary of the depositional sequence basinward becomes problematic and perhaps impossible. “The surface must have various defining properties (i.e. physical features) which allow it to be recognized with reasonable objectivity in core or well-exposed strata and over a reasonable geographic extent so as to allow correlation to other sections and the establishment of mappable units”. The lack of reference to well logs and seismic data sets is an inappropriate limitation of sequence stratigraphy.” The point being made is that a given surface, above all, must be recognizable in rock because rock provides the most detailed and high resolution data and allows a given type of surface to be defined in the first place. Once it is recognizable in rock and everyone understands what it is (e.g. a subaerial unconformity), it then can be interpreted on lower resolution well log and seismic data. Such interpretations are somewhat speculative if no rock data are available to calibrate the seismic or logs. All the five accepted empirical surfaces of sequence stratigraphy were first recognized in rocks and are now interpreted on log and seismic sections. It is not reasonable to claim a bona fide sequence stratigraphic surface is recognizable on only seismic data because seismic data do not provide suitable information to characterize a surface. Seismic can be used to help recognize a new surface type in rock (e.g. it can help to pinpoint the stratigraphic position to determine if a new type of stratigraphic surface really exists or not). “For old deposits, there is no independent way of knowing what the actual sea-level curve was. To use the shape of the sea-level curves from the Quaternary as a model for ancient sea-level changes is highly problematic.” I completely agree with this and such a statement is important. The seven-surface/four systems tract approach ignores this statement and a priori uses a sinusoidal shape for the base level curve. Such a shape dictates the theoretical existence of four systems tracts. To paraphrase Bob “To use a sinusoidal shape for the base level curve as a model for ancient base level changes is highly problematic.” The interpretation of the base level curve shape should come from the observed stratigraphic architecture and the stratigraphic architecture should not be shoehorned into the a priori sinusoidal shape for the base level curve as Catuneanu (2006) and the draft IWGSS report advocate. “To us, it is essential that a depositional sequence be defined such that its boundaries are valid, recognizable sequence stratigraphic surfaces which all join together to form a continuous, unbroken boundary over much or all of a basin”. This requirement was not demanded of an R-T sequence, so why is it demanded here? This is inconsistent and suggests (to a cynical observer) that the stage is being set to promote the authors’ choice of the MRS as the surface that should be the basinward equivalent of the SU.” This requirement holds for the R-T sequence as well as any new type anyone wants to propose. For the R-T sequence it just happens the MFS extends over the entire basin and thus no other type of surface is need to extend its boundary. There is no stage setting in such a requirement. It is difficult to imagine such a requirement not being part of any means of defining the boundary of any specific type of sequence. If you have a boundary of a unit you want it to be continuous. Thus if you have more than one surface forming that boundary they have to join together. “p. 55, Figure 13—This figure conveniently omits all reference to the deposits that form during the turn-around from regression to transgression, and thus inappropriately ties the MRS with the very tip of the SU. This figure also inappropriately shows the CC as not tying to the very tip of SU.” First of all the figure does NOT tie the MRS to the “very tip of the SU”. It ties it to the very tip of the SR-U which is both theoretically reasonable (both start to form at the initiation of transgression) and empirically demonstrable. As noted previously, if any deposits are formed after base level rise but before transgression , they are rarely preserved on the SU. Thus this diagram represents the general (but not every) case. The CC does not tie to the tip of the SR-U (not SU) as shown on the diagram because the SR has eroded the former tip of the SU which indeed did join the CC. “The “magnitude” of a sequence boundary is stated in two very different and inconsistent ways here. Explicitly, it is stated to be the “magnitude of the base-level change” that defines the magnitude of the sequence boundary, with the given example being that a 500 m change being a large-magnitude sequence boundary. However, most of the criteria given as “objective” criteria for determining the magnitude of a sequence boundary relate to the duration of the time gap! “ Actually none of the criteria relate to the duration of the gap. “p. 80—“The first is the reality that sequence methodology and terminology is still a hotly debated topic and only time will determine if the methods and units advocated herein are widely accepted”, and “Secondly, because sequence boundaries are somewhat interpretive and sequence designation depends to an extent on the establishment of a hierarchy which also has elements of subjectivity…” Statements such as these fly in the face of the call for strict “objectivity” earlier in this paper. Taken at face value, such statements mean that the formalization of sequence stratigraphy should stop now. This is, in fact, advocated by the subsequent statement: “Thus it may be best if formalization is never instituted so as to allow the easy and possibly frequent reinterpretation of sequence boundaries and their enclosed sequences” What is meant by objectivity in the Report is that interpretations are based on material-based observations. All such interpretations have an element of subjectivity but, because the interpretations are material based, scientific discussion is possible. There is no doubt that sequence stratigraphic interpretations are more subjective than lithostratigraphic ones but still include lots of reasonably objective observations. Interpretations of deductive surfaces that have no material based characteristics (BSFR) have no objectivity whatsoever. We are not recommending the formalization of INDIVIDUAL sequence stratigraphic units for the reasons stated but we are recommending some standardization of types of sequence stratigraphic surfaces and units. There is an important difference between these two approaches to classification. I trust all IWGSS understand this difference Summary – Bob has made some very good points and many of his comments will help us improve the Report. I am pleased he addressed the fundamental topic of the debate – the recognition of and use of time surfaces in sequence stratigraphy. I am pleased he agrees that the BSFR is not a recognizable surface. It has also been helpful to draw my attention to the MacNaughton et al paper which allows me to discuss the problems and consequences of attempts to apply the theoretical four systems tract approach in the real world. We would appreciate receiving other examples of attempts to apply the four systems tracts to real world stratigraphic successions. These provide the acid test of whether or not such an approach can be used in a “value added way” or if successions are basically being forced into an entrenched but unworkable approach. Ashton Embry: AEmbry@nrcan.gc.ca ++++++++++++++++++++++++++++++++++++++++++++++ On 15th May 2007 John Anderson wrote: Dear All I may well be the last one to register my comments on the Sequence Stratigraphy Report, but I just this week returned from a six week cruise in Antarctica where I did not have access to the web, and hence the report. I must confess that I still have had little time to do much more the skim the report and read letters from colleagues, some of which were forward to me on the ship. I agree, with most of the comments that have already been sent. I do not understand the need for a strict deadline on an issue of this type and, based on what I have read, I do not particularly appreciate the tone of the report or the approach that is taken. Many papers on the subject have passed the peer review test and it would appear that proper treatment of the literature is lacking. As one who has spent his time testing sequence stratigraphic concepts using the Quaternary stratigraphic record, I was annoyed to see that so little of this type of information is considered. It is my opinion that some of the questions raised by the report concerning various surfaces have been addressed by examination of strata deposited at a time when we know well the history of sea level rise and fall. I am not sure how we should proceed, but it is clear that the process of scientific debate has been short circuited and a different approach is needed. I for one would welcome a research symposium where those of us who are interested in this debate could present our most compelling results and try to resolve some of the issues raised in the report. Kind Regards John Anderson <johna@rice.edu> Maurice Ewing Professor of Oceanography Department of Earth Sciences Rice University Houston, Texas 77251-1892 +++++++++++++++++++++++++++++++++++++++++++++++++++++ On 9th May 2007 Rune Mjøs wrote: COMMENTS TO SEQUENCE STRATIGRAPHY AS A “CONCRETE” STRATIGRAPHIC DICIPLINE I agree with much of the discussions related to the difficulty of finding good criteria for determining sequence stratigraphic surfaces defined and picked by the “Exxon school”. Also, in some cases, poor stratigraphic conceptual thinking is responsible for the “Hunt for Surfaces” practise. I think the regressive – transgressive cyclic approach is simple and very useful in coastal areas. However, the focus on surface definitions and surface extensiveness may be dangerous and too simplistic. This is a report which focuses a lot on surfaces; the sedimentary facies in between the surfaces are not much incorporated in the discussions. This is strange since the surfaces itself are picked and defined based on sedimentary facies contrasts from below to above the surfaces. Also, a change in depositional trend is mentioned to be important in order to pick and define a seq strat surface. The meaning of ”change in depositional trend” is not really pointed out clearly. There are spatial changes in depositional trends, pointed out earliest by A. Gressly (Cross and Homewood, 1997,) and followed by J. Walther (G.V. Middleton, 1973). The authors of this report are probably thinking along other lines, but please, do not ignore basic conceptual thinking given by Gressly and Walthers. - In the report base level is synonymous with sea level, hence nearly all discussions are related to absolute sea level changes as the only reason for changes in stratigraphic pattern. The approach in this report is therefore similar to the Exxon school. The terminology “stratigraphic base level” (Wheeler, 1964, Cross and Homewood, 1997, Cross and Lessenger, 1998) is not discussed. This terminology honours that the stratigraphic pattern is a response to interaction of several process variables; hence, base level is put into an energy domain by T. Cross and others. - Relationships to mass balance is not emphasized: - An important aspect of a significant erosional surface is that it correlates down dip to a pile of sediments. - An important aspect of a significant marine condensed horizon is that it correlate up dip to a pile of sediments - Stacking pattern of facies tracts is not mentioned at all, although in the Spitsbergen example at the report end (Fig 25) the importance of ”shoreline trajectory” stacking pattern is shown. - Sediment volume partitioning is not mentioned at all. Sediments accumulate at different places along a depositional profile during a base level cycle. - Correct sediment volume partitioning is not shown in seq strat sketch figures; only surface drawings without much thickness considerations. - Geometry and stratal patterns are not discussed as being an important part of the sequence stratigraphy. Toplap, onlap and downlap terminations are not incorporated as being an important part of the sequence strat. In fact, sketches like Fig 11 shows no indication of that down lapping occur, although it’s quite obvious that MFS is a downlap surface. The petroleum industry would expect that stratal pattern from seismic would be important to incorporate in a sequence stratigraphic analysis. - Discrimination between an SU and SR is not a trivial task. Not as straightforward as it sounds after many repetitive arguments in the report. Also, in a vertical section the MRS or RS would probably be picked at the top of a coarse grained sandstone where a distinct deepening occurs above (examples figs 23 and 24). However, in fig 25, excellent outcrop exposures show that the SU-SR is located at the base of a thick medium-coarse grained sandstone package. Traditionally, the MRS/SR pick is at the top of a coarsening (cleaning) upward trend. Fig 25 shows that this is not exactly the case; the SR is located beneath a medium-coarse grained tidal sandstone some distance beneath the sandstone top. - Most figures show a seaward inclined coastal plain/shoreface facies tract implying that distinct absolute sea level falls are the most common feature. In the examples, figs 24 and 25, an absolute sea level fall is not demonstrated. And for fig 23, basin margin uplift seems to be the most plausible explanation for erosion in the southwestern area. Absolute sea level falls are not easy to demonstrate in aggradational basins. But stratigraphic base level fall and rises occur, but an absolute sea level drop is not required. Again, the word base level synonymous with sea level is not appropriate to use in seq strat analysis. - It is assumed that an MFS is easy to pick because it always occurs as a condensed bed on the shelf or?. The thinking seems to be that there is a global shut down of sediment supply at that level or only one surface exists (MFS) related to the shoreface/estuary/coastal plain/continental complex at the turnaround from transgression to regression. This is in contrast to the variety of surfaces incorporated (SU, SR-U, SR-N, MRS, RMSE) related to the regressive-transgressive turnaround. - Basic conceptual understanding cannot be discarded. It is a poor argument that there is a poor stratigraphic conceptual understanding among scientists, both now and in the past. It’s not an argument against the importance of conceptual understanding. ”It’s extremely difficult to correctly describe what is not understood”, this phrase is also valid for a sequence stratigraphic description. - Accomodation and accomodation/sediment supply ratio changes as controls to understand depositional pattern through a stratigraphic base level cycle are ignored. Similarly, for stratigraphic base level and energy considerations. The accommodation concept is used by several in sequence stratigraphy, and Muto and Steel (2000) concluded that the definition published by T. Cross (1988) is to recommend for future use. - The continental environments in a seq strat framework are not well understood since this sequence strat framework proposal is completely based on identification of regressive-transgressive trends at the coast. Many arguments in the report (i.e. fig 15) indicate that ”base level” curves are similar to rock trends at the coast (for instance asymmetric coarsening-fining up trend indicate similar base level curve). This is not correct, modelling by T. Cross (1988) shows that sinusoidal sea level changes give different facies expressions along a depositional profile. Facies signature at one point on this profile does not give the true picture of the real base level (sea level) changes. - In order to understand the continental to coastal plain environments in a seq strat context, sediment volume partitioning during a base level cycle along a depositional profile must be understood. Without this understanding many wrong correlations will be undertaken from the sea to the continental facies tracts. Text comments: Subaerial Unconformity (p 31). Strange criteria that there must be non-marine strata on top of an SU. Certainly, there are cases where tidal deposits cap SU surfaces (i.e in.valleys). Marine floodings do not necessarily imply ravinement erosion at all places. Only at locations with high energy take ravinement erosion place. In carbonate strata, I would think that karst can form an SU surface which partly can resist ravinement erosion. Also, a direct connection between SU formation and base level (sea level) is assumed. Again, a too simplistic approach which is easier to discuss if stratigraphic base level and its control on deposition and unconformity formation is understood. Fig 10. In practise, it’s not easy to discriminate between SU and SR-N. Often the cross-bedded sandstones (within channel/valley) are interpreted as fluvial in line with Exxon (Van Wagoner et al) practise and both SU and SR-N are interpreted in this context. However, if the cross-bedded sandstones are more likely to be of tidal origin, only an SR can be interpreted. Again, this play with surface terminology is very dependent on facies tract interpretations. In order to get better control on stratigraphy and sedimentology, facies tract stacking patterns combined with sediment volume partitioning (mass balance) considerations should be included. BSFR (p 41-42). It’s unfair to include a new type of argument, geographic variations in subsidence rate, related only to this surface. In general, I think the discussion reflects the view that ”start base level fall” is only thought of as an absolute sea level change issue. In reality, the regression is a response to stratigraphic base level changes due to interactions between gradients, sediment supply, subsidence and eustasy. Marine flooding surface (p 47) is not recommended used in seq strat in this report. If not, we have to use it in sedimentology and high resolution stratigraphic analysis. A marine flooding surface is the physically seen surface across which rapid deepening has occurred and no ravinement erosion is necessarily involved. It is an objective surface to interpret since its position can be many places within a depositional sequence. An MRS surface has similar attributes as a ”marine flooding surface” but an MRS interpretation require that you know the large scale stacking pattern (although, strangely, in this report the word ”stacking pattern” is not mentioned). For instance, in the Mesa Verde Gp in the San Juan Basin, the MRS is not positioned at the level where the large transgression (back step) occur (Mjøs et al, 1998). Without a detailed knowledge about the stacking pattern, the correct position of the MRS is difficult to pick in this case. R-T sequence (Galloway) model. Fig 8a does not show that MFS is present in marine and non-marine as stated in the text (p 52). There seems to be a fundamental problem in the understanding of MFS and time-space-sediment supply relationships. MFS is certainly not a condensed surface continuing from the offshore marine environment into the non-marine environments. The reason for condensation offshore is that sediments supplied are trapped in the non-marine environment due to high accomodation space creation. For the deep marine environments, are we sure the submarine fans have an offset stacking in all cases resembling the shallow marine R-T development? There is a good example in Fig 25, but is this always the case? For the discussions on page 56. Start of base level (sea level) rise is defined by being an onlap surface (and may be seen on seismic). This is not mentioned in the report. However, I agree, in the rock record such a surface may be difficult to pick because no distinct change in depositional environment is necessary to occur across the boundary. Fig 16. Is Fig 16B a time-space diagram (Wheeler diagram)? If so, why is no time gap related to the most offshore parts of MFS? There is a big time gap between two SR surfaces; the reality is that the SU is responsible for this time gap. SR surfaces are not responsible for much of a time gap. The RSME surface is responsible for a distinct time gap in the figure, but no sandstone deposition occur seaward of the surface. This is an unrealistic case and shows a lack of mass balance thinking. Fig 17. I presume the lower figure is a time-space diagram of the upper one. But why two different legends? Page 77. “unconformity and shoreline facies penetrate far into the basin”. The terminology “penetrate” is not appropriate in this context. Fig 23. The unconformity at the basin margin is interpreted as formed by ravinement erosion, why not a subaerial erosion and bypass surface? An RSME surface is interpreted at NW Hecla M-25, but the offshore-lower shoreface succession at Roche Point J-43 has the same thickness as at NW Hecla M-25, and hence, a seaward sandstone wedge related to the RMSE is difficult to pick up (at Roche Point J-43). The “upper shoreface-foreshore-backshore” facies tract is only present at NW Hecla M-25. I assume that the reason for the non-existence of this facies tract at SW Hecla C-58 is due to basin margin low accommodation space creation and the non-existence of this facies tract at Roche Point J-43 is due to offshore condensation. The NW Hecla is then an area of aggradational deposition of an “upper shoreface-foreshore-backshore” facies tract. Instead of an RSME surface interpretation at NW Hecla, this surface could mark a change from progradational to aggradational deposition (and is therefore an RS surface) Comparing Fig 23 and Fig 25, a more consistent interpretation would be to pick the SR in Fig 23 at the base of the “upper shoreface-foreshore-backshore” facies tract (similar to base “tidal facies tract” in Fig 25). This discussion related to the figure examples shows that sequence stratigraphy and surface definitions are not as straightforward or concrete as we should wish, correct conceptual understanding must be included as part of the stratigraphic analysis. REFERENCES Cross, T.A. 1988. controls on coal distribution in transgressive-regressive cycles, Upper Cretaceous, Western Interior, U.S.A. In: Wilgus, C.K. et al (eds), Sea-Level changes: An Integrated Approach. Soc.Econ.Paleontol.Mineral., Spec. Publ. 42, 371-380. Cross, T.A., and Homewood, P.W., 1997. Amanz Gressly's Role in Founding Modern Stratigraphy: Geological Society of America Bulletin, v. 109, p. 1617-1630 Cross,T.A. and Lessenger, M. 1998. Sediment volume partitioning: rationale for stratigraphic model evaluation and high-resolution stratigraphic correlation. In: Gradstein, F.M. and Sandvik, K.O. (eds), Sequence Stratigraphy – Concepts and Applications. Norwegian Petroleum Society (NPF), Special Publication No 8, p 171-196. Elsevier, Amsterdam. Middleton, G.V. 1973. “Johannes Walther’s Law of the Correlation of Facies”. Geol. Soc. Amer. Bull., 84, p 7979-988. Mjøs, R., Hadler-Jacobsen, F., and Johannessen, E.P., 1998. The distal sandstone pinchout of the Mesa Verde Group, San Juan Basin and its relevance for sandstone prediction of the Brent Group, northern North Sea. In: Gradstein, F.M. and Sandvik, K.O. (eds), Sequence Stratigraphy – Concepts and Applications. Norwegian Petroleum Society (NPF), Special Publication No 8, p 263-298. Elsevier, Amsterdam. Muto, T. and Steel, R., 1998. The accommodation concept in sequence stratigraphy: some dimensional problems and possible redefinition. Sedimentary Geology, 130, p. 1-10. Wheeler, H.E., 1964. Baselevel, lithosphere surface, and time-stratigraphy. Geol. Soc. Am. Bull., 75, p. 599-610. Rune Mjøs Email: rumj@statoil.com Lead Geologist and Specialist, Statoil. D-1184, ST-FV Statoil ASA Svanholmen 6 N-4035 Stavanger Norway Phone: +47 95230690 ++++++++++++++++++++++++++++++++++++++++ On 3rd May 2007 10:17 am Robert Dalrymple wrote: My input to the debate is: Comments on Embry et al. (“Sequence Stratigraphy as a “Concrete” Stratigraphic Discipline”) INTRODUCTION I approach these comments on the ISSC report with trepidation for three reasons: 1) I have not been at the forefront of developing sequence-stratigraphic thought, as many of the other respondents have and so have less experience on which to base my comments; 2) the tone of the "debate" has not been particularly polite and I fear that any comments that I might make will bring down the wrath of one or more of the principals in the discussion, and 3) I have little expectation that these comments will lead to substantive changes in the recommendations contained in the report. However, as a process sedimentologist, I am very interested in the role that accommodation plays in the sequestration of sediment and the resulting expression of these changes in the sedimentary record. Thus, I have a stake in the outcome of the debate. After having read the ISSC report in considerable detail, before reading the subsequent series of e-mail messages so I was not biased unduly by them, I suspect that I have a view that is somewhat different from those expressed by others, although I am in fundamental agreement with most of the other people who have provided comments. GENERAL AND PHILOSOPHICAL COMMENTS To begin, I feel compelled to address the tone of the original report and of the responses that it generated. (Let's get this out of the way so we can turn to the science….) It seems to me that the old scientific law "To every action, there is an equal and opposite reaction", and the saying "As you sow, so shall you reap", are definitely appropriate here. While the authors of the ISSC report may not have intended it, and seem surprised by the tone of the responses they have received, those responses are simply responding in kind (although this does not justify the offensive tone of some of the comments). In my opinion, the language used in the ISSC report was beyond the normal bounds of polite scientific discourse. There are numerous examples of provocative language in the original report that I'm sure has caused offense to those against whom it was directly, or indirectly, addressed. This is compounded by the use of first names in the early stages of the report, thereby personalizing the presentation. As one notable example, to have one's work said to be unscientific is an insult, especially as the word "science" appears to be used in an extremely loose manner in the ISSC report. As Ashton himself has said, the deductive approach is an acceptable scientific methodology. Furthermore, the proposals that have been made by the deductive approach are definitely testable, one of the hallmarks of science. Even "shoehorning" results into the deductive model in a publicly published paper is acceptable science because others can assess the appropriateness of the approach, as the ISSC authors have done on reading those papers. Therefore, none of the work being criticized is unscientific and the ISSC report authors are strongly encouraged to delete all such references in their revision. Two other examples of inappropriate language are cited in the detailed comments below re. page 24. On a second general point, the ISSC report falls into a common error of blaming a model for the way the model is used. (Here, I'm using "model" in a general way to include any one of the extant formulations of sequence stratigraphy, although I also agree with the idea that "sequence stratigraphy" is a methodology). Just because some workers use a model inappropriately does not mean that the model itself is necessarily wrong. Similarly, most people realize that a model cannot possibly hope to encompass all possible variations that may occur in nature. (Readers are referred to Roger Walker's excellent summary of the formulation and use of models in the 2nd edition of the book "Facies Models". That presentation, although it is about facies models, is none-the-less relevant to sequence-stratigraphic models). Therefore, deviations from the Exxon (or any other) model are to be expected. People who use models must, however, have the courage to report deviations from the model if such deviations are dictated by their data. This leads to one of the central themes of the ISSC report, which is framed in slightly different words here than they are in the report: data-driven vs. model-driven interpretations. (The first of these would be termed "empirical" in the phraseology of the report, whereas the second is more related to what they call the "deductive" approach). Both have validity. While data-driven interpretations (of depositional environments, the positioning of significant surfaces, etc.) are probably the ideal, most, if not all, actual studies lack sufficient data to make unequivocal interpretations from the data alone. In such cases, a worker must make use of the best possible conceptual framework (or model) to fill in the blanks where data are not available. This is done every time one correlates between isolated sections. I'm sure that the authors of the ISSC report, despite their claims of being "data driven" empiricists, must make "informed decisions" (i.e., model-driven interpretations) every time they construct stratigraphic sections. What they are attempting to do in this report is to create a highly simplified scheme for erecting sequences within sedimentary basins, so that the amount of interpretation is minimized. THE PURPOSE OF THE ISSC REPORT AND AN ALTERNATIVE PROPOSAL "QUALIFIED SUPPORT FOR PROPOSAL" If I understand the ISSC Report, its authors have a very restricted purpose behind the proposals that they make, namely to provide a simple and straight-forward, pragmatic means of mapping sequences throughout a sedimentary basin, and, to a lesser degree, to assign an order to each of them. Once the sequences are so mapped, their spatial distribution and thickness can then be used to deduce something about the evolution of the basin, but such subsequent uses are not explicitly discussed in the report. Within that extremely limited framework, the report may have some utility. The criteria by which unconformities (of various origins) are recognized are relatively well known, so the delineation of such surfaces is not unduly problematic. The more provocative proposal to use the MRS as the basinward extension of the subaerial unconformity is, from a strictly mapping point of view, perhaps defendable. However, I believe that the authors downplay the diachroneity of this surface. In the case of a ramp margin with no pronounced slope leading to a deep-water basin, the MRS may have the characteristics stated in the ISSC report. However, in cases where there is bypass of sediment down a slope into deep water, the coarsest sediment is likely to reach the basin floor at the lowest point on the relatively sea-level curve (as determined for the shoreline). After this time (i.e., at the onset of lowstand progradation with coastal-plain aggradation and onlap… i.e., when the shoreline begins to have an upward trajectory), there will be sequestration of coarse material in the aggrading fluvial system, such that only relatively finer material will reach the river mouth and, hence, the deep basin. Therefore, the coarsest deposits in the deep basin are likely to predate the MRS as defined for shelf depths. I believe that this is the point being made by Catuneanu and Plint in some of their remarks. If the RSL lowstand is of longer duration than is argued (on flimsy grounds) in the ISSC report, then the amount of diachroneity of the MRS may be large. If this is not a problem, then workers are free (in the democratic field of science) to use the MRS surface for mapping purposes. However, if the objective is to determine a unit that has the maximum value as a chronostratigraphic unit, then I would suggest that the correlative conformity (i.e., the surface/timeline that truly does extend from the tip of the subaerial unconformity; the MRS is erroneously shown as tying to this position in some of the ISSC figures!) is a better surface on which to base sequences. Embry et al. argue strenuously (both in the ISSC report and in subsequent rebuttals to comments) that this surface is "invisible" (i.e., it is not easily seen in outcrops and cores). While I have not worked a great deal in the relevant deposits, I do have some minor experience (see MacNaughton et al., 2000, Canadian Jour. Earth Sci., 37: 997-1020). In that paper, we were able to identify on objective grounds (grain-size changes as controlled by the trapping, or not, of coarse material further updip in the source-to-sink system; the degree of development of slump and other mass-movement deposits, as a proxy for sedimentation rate; etc.), the boundaries of the four systems tracts that have been proposed by recent sequence-stratigraphic schemes. I must acknowledge, however, that there are limitations to this study because it was essentially one dimensional (the full proximal-distal transect could not be seen). Embry et al. might also argue that there is an element of model-driven interpretation in that study, because we fit our observations to the four-systems-tract model. However, a reader of that paper will see that we had extensive outcrop observations, so our interpretation is a typical "hybrid" that uses a model to interpret the empirical observations. [As to the other "invisible" surface… the base of forced regression… my tendency is to agree with Embry et al. that this is not a realistic surface for correlation, or on which to base the definition of a sequence. I think I understand why Posementier and Allen suggest this surface… on a flat shelf with low-resolution data, the regression of the shoreline to the shelf edge may appear to be almost instantaneous as relative sea level falls. However, in ramp settings with a slow RSL fall, it is unlikely that there will be a single, through-going, easily identifiable surface. Locally, this surface may be clear, but I would not expect this to be true everywhere. Enbry et al. are wrong, however (in my opinion), to assert that there is no change in stacking pattern at the time when this surface forms. This timeline marks the change from "topset" aggradation (in the preceding highstand) to bypass and/or incision with no development of topssets (in the falling-stage systems tract, or late highstand systems tract). (Embry et al. will probably argue that this type of reasoning is "deductive" rather than empirical. I don't have the time to find the relevant examples, but they do exist. Also this reasoning is based on a good understanding of the physics of sediment transport, which should not be discounted)]. "CONCERN WITH THE PROPOSAL" Although I have expressed (heavily) qualified support for the mappability of sequences using the criteria proposed in the ISSC report, I am deeply concerned that any recommendations being made by the ISSC will be extended (by others) beyond the very limited purpose for which they are being created (mapping sequences). Sequence stratigraphy, as it is now practiced by most people, is 'much more' than a tool for mapping. Instead, it encompasses a very sophisticated and holistic conceptual framework for interpreting many things about a sedimentary succession, including but not limited to: the presence/absence and location of specific facies, the vertical and lateral distribution of grain sizes, etc., etc. For this purpose, people require the most detailed model possible, because the system being interpreted is enormously complex. On this basis, the scheme proposed in the ISSC report is simpler than most other schemes and hence is a "step backward" as has been noted by some commentators. It is certainly possible for several distinct worldviews to co-exist, each having its own relevant application. However, the formal recognition and endorsement of the Embry et al. scheme by the ISSC may, for some workers, give it a greater degree of sway than it should receive. If people attempt to use the Embry et al. scheme for detailed interpretations (as opposed to mapping), then their work will be limited unnecessarily. I'm not sure how to get around this potential problem. I would suggest that the minimum course of action must be for the authors of the ISSC report to more clearly articulate the situations where the proposed scheme is intended for use, and those where another scheme may be more appropriate. I suspect that many of the more vitriolic attacks on the ISSC proposal stem from the belief that this document proposes to discourage the application of other schemes (e.g., a four systems-tract model). This is (hopefully) not intended and should be explicitly stated, with guidelines as to situations where the use of each may be more appropriate. "AN ALTERNATIVE PROPOSAL" In an attempt to be constructive and to diffuse the war of the worldviews that is being waged, I would suggest that the ISSC take a fresh look at the idea of defining sequences (for mapping purposes) as being bounded solely by unconformities. All of the controversy lies in which correlative surface(s) are used to extend the sequences beyond the end of the unconformities, so why not avoid the problem, at least for the time being, by stopping sequences where the bounding unconformities end. Embry et al. have argued in their historical section that this early approach led to the failure of sequence stratigraphy to catch on. I disagree. The early work on sequences by Sloss, Wheeler, etc., was supplanted by the facies bandwagon and the move away from stratigraphy in all forms by most workers, and was not consciously rejected because of perceived weaknesses. The proposal that I would put forward would be to erect two different types of sequences (that are very different from the two types advocated by Embry et al.): one type would be bounded by subaerial unconformities and would be restricted to the shallow-water parts of the basin, whereas the second type would be bounded by the unconformities that occur at maximum flooding surfaces (I do believe that there are unconformities at these surfaces, as I have been saying in talks recently) and would be restricted to the deeper-water parts of the basin. The two types of sequences would interfinger in a transition zone, where a more detailed mapping scheme would be possible. This scheme has two significant advantages: 1) it is based on surfaces, the recognition of which, is not particularly contentious; and 2) it avoids the necessity, at least right now, of defining which surfaces will be used to extend the sequence beyond the tip of the defining unconformities. It is also consistent with the prevailing practice in lithostratigraphy (or biostratigraphy) of limiting the concrete unit to the area where the defining parameter exists: we have no significant problem with the fact that a shallow-water sandstone formation is replaced seaward by a shale-dominated formation, the vertical extent of which may be quite different from that of the laterally correlative sandstone unit(s). There are also potential difficulties with this scheme, but these don't seem insurmountable: 1) the sequences so defined do not extend across the entire basin, although cross-correlation will be easy because they overlap in their areal extent; 2) the specification of the age of a sequence becomes somewhat more complicated because there are more of them (however, the interfingering of sequences in the transition area will allow general ages to be extrapolated with some degree of confidence); and 3) any informal naming of sequences will be complex, especially in the transition area where the two schemes overlap. I urge the ISSC to explore this "pragmatic" alternative to the contentious ideas put forward in the Embry et al. report. SUMMARY The stratigraphic record is complex and flexibility is required in any scheme that is to be used to delineate units. The Embry et al. scheme is not very flexible, given than there are no alternatives that are to be allowed. This will be a disappointing outcome if it is accepted. The more flexible approach that has been suggested (in preliminary draft form only) by Catuneanu allows more freedom of choice and, thus, may be more readily embraced by the scientific community. I would note that the guidelines for the creation of "formations" (for example) to not specify the precise rules to be used in the establishment of formation boundaries. Thus, the proposals in the report differ conceptually from the loose guidelines provided in the other branches of stratigraphy. I would make the following recommendations: " Tone down the language within the report, to conform with the excepted standards of polite scientific discourse, avoiding personal references (direct or implied). " Avoid the many unsupported assertions that pepper the report (some are discussed in the detailed comments below). " Clearly state the intended use and limitations of the proposed scheme. " Consider seriously the alternative scheme for the creation of mappable sequences proposed above. " Do no rush the process of discussion and evaluation of proposals. Sequence stratigraphy is such an important branch of stratigraphy that the entrenchment of ideas that do not have wide acceptance will do more harm than good. DETAILED COMMENTS The following comments are tied to the text of the ISSC report by page numbers. In cases where a specific piece of the text is under discussion, I have reproduced that text in italics. p. 2-"Proposed systems tracts, which require unrecognizable time surfaces or highly diachronous surfaces as part of their boundaries, are not scientifically valid and should be avoided".-The phrase "not scientifically valid" is inappropriate, in that the logical deduction that went into their formulation and the ability to test whether they exist conform to the usual definitions of "science". It is an entirely different question whether they are easily found, or not, but practicality is not a part of the usual definition of "science". p. 3-"The application of empirically-based methods and terms and the avoidance of theoretically-based concepts with no empirical support allow sequence stratigraphy to become a concrete stratigraphic discipline"-I'm unclear what is meant by "concrete". I very much doubt that most previous work on sequence stratigraphy is "non-concrete" because it does not agree with the authors' ideas. I would also note that there are no rigid statements of where a formation boundary should lie within the Guide or Code, so I see no reason why rigorous statements should be made in the case of sequence stratigraphy. p. 3-"… this has contributed to the lack of any standardization in methodology and terminology". There is no rigorous definition of a formation boundary, or of a biozone boundary, so the lack of standardization does not need to be an impediment to some degree of formalization of sequence stratigraphy. p. 5-"Sequence stratigraphy differs somewhat from the other widely accepted stratigraphic disciplines in that the units are defined mainly by specific sequence stratigraphic surfaces and individual sequence stratigraphic units often do not have any specific and characteristic properties per se. The recognizable property change of strata that allows sequence stratigraphic surfaces to be defined and delineated and provides the rationale for sequence stratigraphy being a distinct stratigraphic discipline is a change in depositional trend (Embry, 2002)". All stratigraphic units are fundamentally defined by their boundaries (i.e., the "Golden Spike" concept applied more generally). p. 6-"We will be concentrating on the second use of sequence stratigraphy, that is, the definition of stratigraphic surfaces and units within the discipline". This is a narrow and incomplete view of sequence stratigraphy. Therefore, any formulation that is based on it will be limiting as opposed to enabling, which is what any formulation should be. p. 6-"The article concludes with a list of 20 specific recommendations that will allow sequence stratigraphy to become a concrete stratigraphic discipline, free from a dominance of abstract concepts and invisible surfaces". Sequence stratigraphy is, as the authors themselves say, based on the abstract concept of "base level". Therefore, it is not possible to free it from abstract concepts. p. 7-" "Emphasis is placed on the relative objectivity and reproducibility of data in defining units in each category" (NACSN, 2005)". This statement does not require absolute objectivity, nor does it specify rigorously which surfaces are used, which it what these authors are doing. p. 8-"… those who follow such a model-driven approach tend to ignore data which doesn't fit the model or, at best, to "shoehorn" collected data into the model. The model remains inviolate regardless of subsequent empirical observations". This does not invalidate the use of models. Rather, it is an indictment of the people who do the work, not the model. p. 9-"Thus the significant gap in the stratigraphic record represented by each of Sloss' unconformities is due to a combination of removal of previously deposited strata and the lack of any deposition during the time of erosion". The words after the "and" are strictly never true: there is always deposition somewhere while there is erosion in another area. Therefore, such a statement is only true within a restricted area. p. 10-"Wheeler (1958, 1959) provided real-world examples of unconformity-bounded sequences to support his model. In most cases, the recognized unconformities were of smaller magnitude than the continent-wide unconformities of Sloss (1963) and many of the unconformities of Wheeler (1958, 1959) disappeared in a basinward direction". This is written in such as way as to imply that the unconformities recognized by Sloss do not disappear into basins. All unconformities must end somewhere! Simultaneous world-wide erosion is impossible! p. 10-11 (Fig. 1)-This lateral limitation of sequences is no different than the fate of formations as one moves from the basin margin into the basin centre. p. 18, Fig. 4- The sentence that talks about the time-of-origin of the basinward extension of the subaerial unconformity cannot be a universal statement, as the onset of turbidite deposition will differ from case to case, based on the position of the shoreline relative to the basin margin at the onset of base-level fall. p. 19-"This contrasted sharply with the time surface just after the start of fall which was used for the correlative conformity portion of a Type 1 boundary". I do not see the difference that is stated here re. the position of the correlative conformity. This difference exists only because of the supposed timing of the onset of turbidite deposition in the case of a Type 1 sequence boundary. However, this timing cannot be universal. p. 24-" Making a bad situation worse…" and "The Naish and Kamp (1997) proposal is a fine example of thoughtless nomenclature…" Such value-laden statements push (or exceed) the bounds of polite scientific discourse! Such statements are made worse in many cases by the use of first names when discussing the published literature. p. 26-"Farther basinward, a surface, which marked the depositional change from coarsening upward to fining upward and which was interpreted to represent the change from regression to transgression, was employed as the correlative conformity portion of the boundary (Fig. 8b)". Simple logic (based on an understanding of the factors controlling sediment transport and deposition) shows that the surface defined in this quote forms later than the seaward-most extension of the subaerial unconformity where there is a period of relatively slow base-level rise and onlap of the landward-most point of deposition on top of the subaerial unconformity (see Fig. 11). Therefore, correlating it with the subaerial unconformity is inappropriate in the general case. (The lack of onlap over a long distance from the most seaward shoreline position shown in Figure 11 is only one possible situation and is not universal. It requires rapid (essentially instantaneous) transgression and/or a significant depth of ravinement. Such circumstances are not present in all settings.) p.30-"The surface must have various defining properties (i.e. physical features) which allow it to be recognized with reasonable objectivity in core or well-exposed strata and over a reasonable geographic extent so as to allow correlation to other sections and the establishment of mappable units". The lack of reference to well logs and seismic data sets is an inappropriate limitation of sequence stratigraphy. p.31-"An important characteristic of a subaerial unconformity is that non-marine strata overlie the surface. When marine strata overlie strata that had been formerly exposed and eroded, the surface marking the contact is not a subaerial unconformity because that surface was previously eroded with the passage of marine waters over it". Passive onlap is readily possible, such as where back-barrier deposits onlap a paleosol as base-level rises. Indeed, the preservation of a paleosol in (more or less) its entirety, requires passive onlap. The statements copied here represent a common, but not universal, situation. All models (whether deductive or inductive) are limited to some degree by the range of situations considered. p. 35-"Following Embry (2002, in press), we refer to a shoreline ravinement which has not eroded through a subaerial unconformity as a normal shoreline ravinement. Such a surface is a sharp, scoured contact, is underlain by non-marine strata and is overlain by estuarine or marine strata that deepen upward (Fig. 9, 10)".. A normal shoreline ravinement surface as defined here need not have non-marine strata beneath it! There are countless examples in the stratigraphic record where marine strata of the preceding parasequence (i.e., a regressive episode that is not capped by a subaerial unconformity; as example would be the flooding of an abandoned delta lobe) occur directly beneath the ravinement surface. p. 38-"It is one of the more easily recognized sequence surfaces in clastic marine strata where it separates coarsening upward strata from fining upward strata (Fig. 9) (Embry, 2001) and represents a change from a regressive trend to a transgressive one". This signature of the MRS is not universal… it may be specific to the area immediately seaward of the most seaward shoreline position and may not extend more than a few 10s of kilometers seaward of this. p. 39-"However, when actualistic base level curves are employed…" For old deposits, there is no independent way of knowing what the actual sea-level curve was. To use the shape of the sea-level curves from the Quaternary as a model for ancient sea-level changes is highly problematic. p. 40-With respect to the MFS, I find it strange that no mention is made of the fact that it separates transgressive deposits from regressive deposits. In the vicinity of the shoreline, this formulation is perhaps more useful than deepening/shallowing, because the changes in water depth in the transition from estuarine to deltaic are small and/or non-existent. p. 42-"scientific objectivity" Are there different kinds of objectivity, which would require the use of an adjective? p. 49-"objective science" Such a characterization of science is inappropriate. p. 53-"To us, it is essential that a depositional sequence be defined such that its boundaries are valid, recognizable sequence stratigraphic surfaces which all join together to form a continuous, unbroken boundary over much or all of a basin". This requirement was not demanded of an R-T sequence, so why is it demanded here? This is inconsistent and suggests (to a cynical observer) that the stage is being set to promote the authors' choice of the MRS as the surface that should be the basinward equivalent of the SU. p. 55, Figure 13-This figure conveniently omits all reference to the deposits that form during the turn-around from regression to transgression, and thus inappropriately ties the MRS with the very tip of the SU. This figure also inappropriately shows the CC as not tying to the very tip of SU. p. 56-"At its seaward edge, the SR-U joins the MRS…" I can think of cases where this will not be true. This would be so in the case of a transgression that was punctuated by a period of regression (that was then not removed by ravinement), such that the age of the SR-U in landward areas was younger than the MRS. p. 57-"many empirical observations demonstrate that the three types of surfaces do indeed join and form one continuous boundary." The objectivity of the correlations on which this statement is based is suspect in most instances, because it if almost always impossible to physically trace surfaces over the many kilometers that would be needed to demonstrate their physical continuity. Therefore, some amount of "interpretation" is almost always involved. p. 59-"Given that either eustasy or tectonics is the primary driver of base level change, Embry (2006, in press) suggested that an input base level curve based on eustasy (Shackleton, 1987) or tectonism (Gawthorpe et al, 1994; Embry, 1997) is more appropriate for a sequence stratigraphic model. Each of these base level curves is punctuated by pulses of fast rises followed by either slow rise or standstill". This sounds like just as "theoretical" an argument as the assumption of a sinusoidal curve! The only one of the three references cites (other than Embry, 2006, in press) that is not based on interpretation of an ancient succession is Shackleton which is potentially unique to the Quaternary (or at least can't be shown to be universal). p. 76-The "magnitude" of a sequence boundary is stated in two very different and inconsistent ways here. Explicitly, it is stated to be the "magnitude of the base-level change" that defines the magnitude of the sequence boundary, with the given example being that a 500 m change being a large-magnitude sequence boundary. However, most of the criteria given as "objective" criteria for determining the magnitude of a sequence boundary relate to the duration of the time gap! These two attributes (base-level excursion and duration of the gap) are not the same thing! Only point 5 (and point 4 to a much lesser extent) is relatively directly related to the size of the base-level fall. (The amount of deepening above a sequence boundary is more likely to be related to the rate of sediment supply at the onset of deepening, rather than to the actual amount of deepening. Slow rates of sediment supply (as commonly occur at the start of a basin-fill cycle) are commonly low which leads to an apparent large-magnitude deepening, while the actual movement of base level (i.e., sea level in most cases) may not be great.) p. 80-"The first is the reality that sequence methodology and terminology is still a hotly debated topic and only time will determine if the methods and units advocated herein are widely accepted", and "Secondly, because sequence boundaries are somewhat interpretive and sequence designation depends to an extent on the establishment of a hierarchy which also has elements of subjectivity…" Statements such as these fly in the face of the call for strict "objectivity" earlier in this paper. Taken at face value, such statements mean that the formalization of sequence stratigraphy should stop now. This is, in fact, advocated by the subsequent statement: "Thus it may be best if formalization is never instituted so as to allow the easy and possibly frequent reinterpretation of sequence boundaries and their enclosed sequences" (p. 80). R.W. Dalrymple Professor and Head Dept. of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6 Canada phone- 613-533-6186 fax- 613-533-6592 <dalrymple@geol.queensu.ca> +++++++++++++++++++++++++++++++++++++++++++++++++++++ On 3rd May 2007 9:19 am Alex MacNeil wrote: Dear ISSC Chair, I have had the chance to review the ISSC draft paper by Embry et al. (2007), and would like to make a few comments. 1) In several parts of the ISSC draft paper (e.g., page 44), the authors assert (rather strongly) that no one has ever presented any defining characteristics of a BSFR or demonstrated such a surface in core or outcrop. Indeed, Dr. Embry reiterates this in his reply to several comments challenging various aspects of the draft paper that have been posted on the USC website. Before making such strong assertions, the authors should have taken the time to familiarize themselves with the recent literature on sequence stratigraphy. The MacNeil and Jones (2006) paper in Sedimentology documented the BSFR in outcrop and core, and undermines some of the key arguments made in the ISSC paper. In our study on the Late Devonian Alexandra Reef System, located in the Northwest Territories of Canada, off-reef ramp deposits of the highstand systems tract are sharply overlain by biostromal deposits that accumulated during base level fall. These, in turn, are overlain by shallower water carbonate sediments that developed a new reef complex in the lowstand systems tract (note that this is a ramp-type platform, not shelf-type platform). The Basal Surface of Forced Regression in this succession lies a few meters above the MFS which is marked by a firmground and incipient hardground (e.g., we have not misinterpreted the MFS as a BSFR). The BSFR is traceable over 10 – 15 km (the study deals with fourth-order sequences), and is marked by (1) local erosion that can be attributed to a lag between deeper water deposition and shallowing-water deposition as the carbonate regime changed in response to changes in water depth (keeping in mind that carbonate-producing organisms are highly sensitive to such changes), and (2) a sharp biofacies change (entirely metazoan-dominated to metazoan-microbial domination) that can be attributed to changing nutrient regimes that were attendant with relative sea level fall. The latter is the focus of a paper in the SEPM Special Publication 89 that is currently in press, edited by J. Lukasik and T. Simo. Deposits above the BSFR include blackened lithoclasts that are also found at the subaerial unconformity that was developing landwards (MacNeil and Jones 2006 in JSR). Also note that as the deposits above the BSFR are biostromal, any argument that they were shed from the platform in the highstand systems tract and the BSFR is actually a MFS is precluded. Likewise, the biotic composition of the biostrome is consistent with an intermediate water depth (ironically, I can reference some of Embry’s original work from the Arctic on suggested absolute water depths for Devonian carbonate facies to support the intermediate water depth) between deeper water, nearly-drowned carbonates of the TST and HST and shallow water, high-energy carbonates of the ensuing LST. Basinwards, the BSFR changes from being a sharp surface to a gradational change that takes place over a stratigraphic thickness of ~ 1 m. Landwards, mixing of sediment and probably regressive marine erosion blurred expression of the BSFR (lateral variations in surface expression are summarized in Figure 20 in MacNeil and Jones). In the case of the Alexandra Reef System, recognition of the BSFR is critical to correctly understanding the sequence stratigraphic evolution of these deposits, and serves to accurately differentiate the falling stage systems tract from the preceding highstand systems tract. 2) In carbonate systems with ramp-type geometries, the Basal Surface of Forced Regression can be identified as the surface that separates depth-controlled biofacies, and biofacies controlled or influenced by trophic resources, if the change in base-level shifted nutrient regimes. A firmground or possibly even hardground may be present, depending on the rate of sediment production/accumulation as the communities of sediment-producing organisms changed, seafloor cementation, and the degree of sediment shedding from shallower parts of the ramp in the preceding HST. Note that sediment shed in the HST has a specific grain size range (for review see MacNeil and Jones 2006) and changes in grain size can be used to constrain the shift from normal to forced regression. Delineation of the BSFR with these empirical criteria is consistent with the type of data that Embry et al. argue (e.g., last paragraph on page 40) can be used to define the MRS and MFS. As such, to argue that there is no empirical data that can be used to identify a BSFR invokes a double standard and is incorrect – at least in the case of some types of carbonate systems. This fact underscores the importance of acknowledging variability between different types of sedimentary systems and being able to apply different models of sequence stratigraphy (e.g., T-R model, four-system tract models) as the situation permits – fundamental aspects that the draft ISSC paper has entirely failed to embrace. 3) It is not very “real-world” to limit sequence stratigraphic interpretations and systems tract definition to whether or not a specific surface is identifiable across a study region – a criterion argued for throughout the ISSC paper. Systems tracts are delineated by sequence stratigraphic surfaces, but they are defined by integrated data sets that include surfaces, stratal geometries/stacking patterns, and detailed analysis of facies associations. Indeed, sequence stratigraphic surfaces may vary in physical expression between areas, and some surfaces may share the same physical features, as determined by ambient depositional conditions, even though they mark opposite changes in depositional trends. For example, Embry et al. correctly noted (page 38) that the MRS in deep-water parts of carbonate systems may be marked by a submarine unconformity – but hardgrounds and submarine unconformities also mark late stages of transgression and MFS’s, as documented in numerous studies (e.g., Kendall and Schlager 1981; Hillgartner 1998; Kenter et al. 2001; Lukasik and James 2003 to name only a few). So yes, if one chooses to limit themselves to sequence stratigraphic surfaces, they may find themselves largely limited to defining T-R sequences. If one chooses, however, to actually include study of the sedimentology and stratigraphy of the deposits between the surfaces, they can conclude with much more advanced interpretations – and therein lies the powerful nature of sequence stratigraphic concepts and methods with significant implications for how we understand strata and advance the science. While I appreciate the time and effort that Embry et al. put into the draft ISSC paper, I can not imagine that its guidelines, in the current form, will be accepted by the greater research community. In the very least, they need to correct their assertions that the BSFR is purely theoretical and has never been demonstrated in core or outcrop, as this is incorrect. Best regards, Alex MacNeil alex.j.macneil@esso.ca ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On 3rd May 2007 9:18 am Bruce Sellwood wrote: Dear All, I wholeheartedly agree with Tucker, Posamentier, Tesson, Martinsen and Flint. It doesn't need fixing. Just keep it practical! Bruce Sellwood Professor Bruce Sellwood Emeritus Professor of Applied Sedimentology Editor-in-Chief, Sedimentary Geology <b.w.sellwood@reading.ac.uk> +++++++++++++++++++++++++++++++++++++++++++++++++++++ On May 3rd 2007 at 7:20 am Vitor Abreu wrote Dear Dr. Cita, I am writing this email to add my comments to the ongoing debate about the ISSC proposal to provide guidelines to sequence stratigraphy. Considering that we were never introduced, please allow me to give you some of my background pertinent to this discussion: I am currently Stratigraphy Advisor of ExxonMobil Exploration Company, Adjunct professor at Rice University teaching Sequence Stratigraphy since Peter Vail's retirement 6 years ago, current AAPG Distinguished Instructor ("Sequence Stratigraphy Applied to Exploration"), teaching short courses in several universities and societies around the globe. First, I would like to give my support to IUGS in this new attempt to provide guidance to Sequence Stratigraphy through the International Sub commission on Stratigraphic Classification (ISSC). However, I have strong concerns about it in 3 different areas: the process, the ISSC leadership and the science. The Process The scientific community was rushed to review this proposal. The proposed May 1st deadline was never justified and clearly was not time enough for a thorough review of the lengthy manuscript. This observation is supported by several of the comments posted on Chris Kendall's website http://strata.geol.sc.edu/SeqStratForm.html. The ISSC committee is also considered very small, with no representation from the various different schools of thought on sequence stratigraphy. Noticeably, none of the original proponents of the method were invited to participate (Peter Vail, Bob Mitchum, John Van Wagoner, Henry Posamentier, etc...). Also, it would have given more credibility to ISSC to have chosen a less biased scientist to chair the committee, preferentially one that would not be proposing his own ideas as the basis for the guidelines. The ISSC Leadership Ashton has chosen a very confrontational way to lead. The tone of the proposal is very aggressive and sometimes derogatory to all of the early proponents of the methodology. The result of this strategy is that several scientists that should be involved in this discussion prefer not to, because of the belligerent response that it may trigger. This is leadership by intimidation, in an attempt to "bully" the community to his own ideas. The Science Sequence Stratigraphy is not a concept or a model - it is a methodology. The proposal is not addressing this key point and does not offer a method for interpretation of different data sets. In my opinion, the major flaw of the proposal is not including seismic stratigraphy. As it was pointed out several times before, Sequence Stratigraphy in its current form, which is the subject of the debate, started 30 years ago with AAPG Memoir 26 and later revised in the SEPM Special Publication 42 (1988) through the interpretation of seismic reflection profiles and well-logs, and later was applied to outcrops and cores. The key definitions like depositional sequence and methodology utilizing stratal terminations are defined in Memoir 26 and SEPM 42. The ISSC proposal does not build on that foundation but rather dives into attack to discredit this work as a justification for supplanting accepted definitions with its own version - the very reason we are in the current state of confusion. The "science" of this proposal is mostly opinion (e.g. the whole "concrete" discussion) and the lack of data integration from multiple disciplines and multiple depositional settings illustrates the limited applicability of this proposal and calls into question the ability of the committee to grasp the nature of the problem. The ISSC report's dominant focus on outcrop expression, from a single tectonic setting (distally-steepened ramp) illustrates the inadequacy of the report to be widely acceptable. Outcrop is just one data type; an effective sequence stratigraphic methodology should be inclusive of seismic, well-logs, outcrops, cores, biostratigraphy, paleoecology, geochemistry (organic and inorganic), sedimentology, among others. The current proposal is too limited in this aspect. Is a surface defined by downlaps on a seismic profile "unreal"? To interpreted such a surface as a MFS is "not scientific" or "not concrete"? Misrepresentations of past work on the subject in the ISSC report create an "I'm right, they're wrong" climate. Our job is not to teach the committee to understand before criticizing, but rather to point out the errors and suggest a more inclusive approach, including the consultation of scientists who do understand the breadth of the issues and can sort out the confusion. Octavian Catuneanu has taken the time to point out misrepresentations and errors in the ISSC report and I agree with most of his points (although they are presented with a little unnecessary but completely understandable heat at times). Final Remarks The scientific community has spoken, despite the short time that was given to reply. The great majority of the comments are against the current proposal, coming from scientists that have been active contributors in the field and are well-known sequence stratigraphers. The current proposal represents views that are accepted just by a minority of the community and it basically proposes replacing the widely accepted methods and nomenclature of Sequence Stratigraphy with the R-T cycle nomenclature of Ashton. In short, Ashton is the Co-chair of ISSC and leader of a small committee trying to impose his own work to the community. Ashton is behaving as the judge, jury and executioner of what is "scientifically acceptable" in Sequence Stratigraphy. Ashton is claiming "Science" for himself - he is now defining what is scientific and what is not. In his replies to the overall negative reviews, he is always "challenging" the reviewers to demonstrate the validity of other methodologies or approaches. It is not the task of the reviewers to teach Ashton Sequence Stratigraphy. His committee is responsible for making a thorough review of the literature, making sure that the different aspects of the subject are covered. It helps to have a larger committee with a broader background to facilitate this task. His committee is clearly failing in demonstrating the utility of the proposed scheme in different settings and using different tools/data sets, besides outcrops. I understand that this note is coming after the May 1st deadline, but considering Ashton's responses to my fellow colleagues so far, there is a good chance that my comments would not be considered anyway (I would probably get the "None of his arguments have any notable effect on our recommendations." - it is my personal favorite). However, this exercise had one major positive outcome: it united the entire community for the first time - against the proposal. Now we have to build upon this momentum and find a more agreeable alternative to what Ashton is proposing. Respectfully, Vitor Abreu Advisor, Stratigraphy ExxonMobil Exploration Office: GP3-398 • Tel: (281) 654-2670 • Fax: (281) 654-7726 E-mail: vitor.abreu@exxonmobil.com Mailing Address: P.O. Box 4778 • Houston, TX 77210-4778 +++++++++++++++++++++++++++++++++++++++++++++++++++ On May 3rd 2007 at 7:01 am Stephen Flint wrote Dear Colleagues, I am using Maurice Tucker's email list for convenience as I am in the field, on a poor connection (hence mising the May 01 'deadline'). To keep this brief, I agree with the comments made by Posamentier, Tucker, Tesson and Martinsen....sequence stratigraphy is not broken, so don't try to fix it by some attempt to formalise a single scheme. In the field (where I am lucky to spend 2 months every year with PhD students and Post-Docs), the emphasis is on a 'bottom-up' approach starting with sedimentology and then (later!) using the concept of seq strat to explain vertical and lateral stratigraphic relationships. We seldom have precise age control and, after a career working on many of the world's larger outcrops, I have to say that we very seldomly have exposed time-equivalent nonmarine to distal shelf of slope/basin sections. So, we use seq strat as a practical field tool to understand basins. As such, I do not recognise the description of 'Exxon sequence stratigraphy' described by Embry et al. Moreover, I have spent several months over the past 3 years working in the field with XOM sedimentologists and they work in the way I have described above. In fact, over my 18 years doing this stuff I have never met anyone who uses the 'deductive' approach outlined by Ashton. The ISSC report seems to me to relate to conference discusions in the 1990-1992 period, not to the state of sequence stratigraphy in 2007. Yesterday I was excited to find tidal flat heteroliths sharply overlying offshore shales (late Mississippian, Pine Mt, Kentucky). This field observation tells me something important about that basin. I call it a sequence boundary. I don't care too much what others call it as long as the recognition is there. I don't care about how it exactly relates to when fluvial incision started up dip or what the correlative conformity looks like in the deep basin, because I'll never find them. So, to end this contribution, I would respectfully ask colleagues who are very concerned with how the rest of us define surfaces to get out in the field more (or down to the core store). On with practical sequence stratigraphy! Steve Flint Professor Stephen Flint Director of the Graduate School University of Liverpool 4 Brownlow Street Liverpool L69 3GP U.K. Tel +44)(0)151 794 5192 +++++++++++++++++++++++++++++++++++++++++++++ On May 1st 2007 Ashton Embry wrote: Reply To Ole Martinsen’s Comments of April 26 It is nice to hear from Ole with whom I have been discussing sequence stratigraphy for nearly 20 years. Ole has been a champion of the deductive approach and his credibility is beyond question. He has criticized various aspects of the Report and such comments are of significant value for us. Also such criticisms allow us to clarify some important points that we likely did not express as well as we could in the Report. My response to his criticisms is below: 1) “ I must say I do not see the need for a fomalization of the method. Such a fomalization, whatever principles it is based on, will barely be used, and if a fomalization is based on the current document, it will be used by very few” The ISSC TG agrees with Ole and, unlike ISSWG, recommends that sequence stratigraphic units NOT be formalized. We want to provide some guidance on sequence stratigraphic methods and units. 2) “pragamatism and not formalism is needed to capture the immense variability of sedimentary system, clastic or carbonate” This is exactly the philosophy of the ISSC TG. Currently we are debating whether or not the use of two abstract time surfaces, which have not been characterized, is a pragmatic way to go in regards to unit definition and delineation. ISSC has concluded the use of the time surfaces is NOT pragmatic and we are asking people like Ole who disagree with such a conclusion to provide well supported arguments for their use. This is pretty straight forward but it does require serious input from people like Ole. 3) “It is a paradox to me that the authors of the report claim to be empirically based and pragmatic and everyone else is model-driven, when what they do is attempt to squeeze the enormous variability of sedimentary systems into a couple of systems tracts and separated by four surfaces.” Being empirically based simply means using surfaces that are well characterized by empirical data. I trust Ole agrees that the SU, SR, MFS, MRS and RSME are solid, empirical surfaces which are well characterized. Those who follow a deductive approach (referred to as “model-based in the Report) use these empirical surfaces (thus include a lot of empiricism in their work) BUT also want to employ a BSFR and CC for unit definition and delineation. Many people follow an empirical approach (e.g. Guy Plint and his students) but do not know how to express it in sequence stratigraphic terms. It would be nice to be able to have more precise units for sequence stratigraphy but no one, including Ole, has demonstrated how this might be done in a scientifically acceptable manner. If Ole thinks he can, we would appreciate receiving data which backs up such a claim. However it is important to note that the definitions of a sequence and a systems tract provided by the ISSC Report leave the door wide open for additional units to be proposed and adopted. Right now we are simply only recommending those with well characterized boundaries. As long as new units have well characterized boundaries, I am sure they will be accepted and employed by the stratigraphic community. 4) “the template suggested in the report is based on only one basin or basin complex (Sverdrup Basin) and one geologic time period (Triassic).” First of all we do not suggest a “template”. We reach the interpretation that there currently are only four specific surfaces that are sufficiently well characterized and which have the required attributes to serve as boundaries of sequence stratigraphic units. These have been recognized in basins throughout the world.Various units are then defined in terms of these four surfaces. A specific geometry (template) has no part here. Ole is perhaps referring to our proposed boundaries of a depositional sequence (SU + SR-U + MRS) as a “template”. I would emphasize that such a composite boundary for a depositional sequence is well established in many basins and in both clastics and carbonates. There are innumerable examples in the literature of an SU, SR and MRS forming a single, through- going boundary. This is theoretically reasonable given the SR and MRS form at the same time (are joined without a doubt) and that the SR almost always cuts into the SU (i.e. joins it). As mentioned in the Report there may well be cases that the three surfaces do not join to form a continuous boundary. In that case the interpreter would either have to find other suitable surfaces that join the SU to form a through- going depositional sequence boundary or would simply have to restrict the boundary in that instance to the SU only. Perhaps of most importance is that the Report does not restrict the definition of a depositional sequence to only a unit bound by an SU, SR-U and MRS. This is just one well established (ie empirical) combination that we do know exists. Other, presently unrecognized, well characterized surfaces of sequence stratigraphy, which join with the SU, may be identified in the future. Such a composite boundary for a depositional sequence would fit the definition. The key point here is that the depositional sequence is NOT defined as having an SU, SR-U and MRS as it boundaries. This is just the only well established boundary we know of right now. More may be found (no template existrs). 5) “the authors are model-driven to the utmost extent themselves by forcing the stratigraphic variability of all other time periods, sedimentary systems and basins into a template erected from one basin.” We simply base our recommendations on well established, well characterized empirical surfaces and stratigraphic relationships recognized in basins throughout the world. We have only rejected the concept of using hypothetical time surfaces, which are not characterized whatsoever (ie not empirically based), for unit boundaries. A deductive approach is based on a combination of empirical data AND theoretical concepts (ie the use of the hypothetical, uncharacterized time surfaces). If Ole has some empirical data from even 2 or 3 basins in the world which support his deductive approach, we would be pleased to receive it. I stress that seismic data are almost always highly equivocal and are not of much use for deciding between the deductive approach and the empirical one. 6) “My point here is how one can make such a template universally valid when the formation of the four surfaces they have chosen vary immensely in time and space” Again Ole keeps referring to a non-existent “template”. We have four surfaces which potentially can be objectively recognized in strata of any basin type. These surfaces then define and allow the delineation of various units of sequence stratigraphy. That is all there is to it. If Ole wants to use other surfaces, we need data which demonstrates their “realness” (ie empirical data which characterizes them) 7) “diachronous in relation to one another (the three first ones are not genetically related by definition; i.e. the subaerial unconformity does not form at the time that the maximum regressive "surface" forms). The maximum regressive "surface" is not a physically recognizable surface anyways in most instances: it is a conceptual break between regressive and transgressive stacking patterns (see Helland-Hansen and Martinsen, 1996; Martinsen & Helland-Hansen 1995)” The fact that in many instances the SU/SR-U and MRS combine to form a single boundary is empirically established beyond a reasonable doubt. Will such a combination be present in every basin in every instance? Probably not, but that doesn’t matter. Notably for carbonates such a combination is present almost always because marine strata almost always overlie the basin flank unconformity (an SR-U). Once marine strata overlie the unconformity that leaves no doubt that the MRS joins the unconformity. In siliciclastics, nonmarine strata sometimes overly the unconformity making it an SU. In this case the MRS may or may not join the unconformity. Detailed observations have to be made at the terminal end of the unconformity to resolve this. The MRS is a surface in the same way most boundaries in all the other material- based stratigraphies are surfaces. It is a horizon of change of a regressive trend to a transgressive one as noted by Ole. It is often a “conformity” which means there is no actual physical surface present just like there is no physical surface at a biostratigraphic boundary. A FAD is still seen as a correlatable surface. 8) “A very simple point here, partly touched on above: by choosing these boundary surfaces to redefine a depositional sequence the whole concept that sequences should have chronostratigraphic significance breaks down because the subaerial unconformity, the shoreline ravinement and the max regressive "surface" do not form a surface with chronostratigraphic significance as they are likely to form at different times and they separate various units. If this concept is chosen and formalized the whole point about sequence stratigraphy disappears.” As stressed in point 4, the depositional sequence is not defined as having an SU/SR-U and MRS as its boundaries. It is clearly defined in the Report as a sequence with an SU as its defining surface and the correlative surfaces of an SU. As noted in the Report, this is essentially the same definition as used by van Wagoner et al (1988) There is no doubt that one combination of surfaces which fits this definition is the SU/SR-U and MRS because it is empirically established that in many cases the SR-U and MRS are correlative surfaces of the SU. Such a boundary has chronostratigraphic significance because the MRS is a low diachroniety surface and the SU and SR-U are approximate time boundaries. Such a boundary is not a time surface but no empirical surface or combination of surfaces will ever be a time surface. Ole claims that the use of such a pragmatic surface, which can actually be mapped, destroys the point of sequence stratigraphy. There is no support for such a claim. If Ole thinks the point of sequence stratigraphy is doing chronostratigraphy, he is looking at the wrong discipline. That is why chronostratigraphy, which is wholly different from sequence stratigraphy, was established. 9) “Moreover, I think it is common knowledge that erosion somewhere leads to deposition elsewhere: thus one should not search for correlative surfaces everywhere, rather deposits, condensed or expanded, that correspond to the surfaces.” This is somewhat odd. When it comes to defining and delineating sequence stratigraphic units, we need clearly defined boundaries (empirically established surfaces). It has been well understood for 100 years (Grabau , 1906) that unconformities develop over time and that there are sediments that are equivalent in time to such an interval of erosion. This has nothing to do with defining and delineating units of sequence stratigraphy. 10) “I would almost feel better if the authors actually stated that sequence stratigraphy does NOT work based on what they say instead of making a new definition.” The good news is that sequence stratigraphy works great for correlation and unit definition and delineation as longs as one uses well characterized, empirical surfaces of sequence stratigraphy for these procedures. Sequence stratigraphy fails to work when practitioners try (in vain) to delineate abstract time surfaces in their strata. Notably biostratigraphy works great as long as well established empirical procedures are used. Once one tires to use abstract “biochronological” surfaces (ie time surfaces corresponding to a paleontologically defined events at a specific localities) it all goes to hell. 11) “The entire document is full of argumentation such as "we are right and they are wrong". Very few use this type of argumentation” I regard this as an unfair characterization of our work. We criticize past work and provide our arguments to support our critique. The reader is free to accept or reject such argumentation. We make recommendations but no where do we claim these are the right and only possible recommendations. They are just our recommendations based on the data and logical reasoning we provide. Our recommendations are quite simple actually. We basically recommend the use of only well characterized, empirical entities in sequence stratigraphy. This is hardly out of line given it accords with the procedures of the other material-based stratigraphies. We then define a sequence and a systems tract in a very flexible manner which will allow for new types of these units to be introduced at any time. 12) “It is not universal and the only empirical approach-a statement like that is very provocative towards the rest of the sedimentary geology community who have spent hundreds of months in the field or working the subsurface in other basins looking at other types of expressions of sequence stratigraphic variability.” Again I think Ole has to reread our definition of a sequence and a depositional sequence to understand we are not proposing a rigid model or template. Just a very flexible definition that can meet the needs of a stratigrapher in any basin. There are very many examples in the literature of inappropriate boundaries for sequence stratigraphic units with the most common one being a highly diachronous, within trend facies boundary (e.g. the contact of sandstone with an underlying shale/siltstone). Now this type of boundary does indeed defeat the purpose of sequence stratigraphy. The lack of acceptable guidance in combination with less than great critical reading/editing for sequence stratigraphy has allowed many rather bizarre interpretations to be published. A fine example is the type 1 sequence boundary of Posamentier et al (1988) which, as discussed in the Report, is theoretically impossible. I plan to make a collection of published, less-than-adequate, sequence stratigraphic interpretations that have littered the literature over the past two decades. 13) “We have all learnt that sequence stratigraphy is NOT ONE universal model, but a method that directs our thinking, makes us make predictions and guides interpretations.” The ISSC Report argues for this point very strenuously. Sequence stratigraphy is best seen as just another material-based stratigraphic discipline that is useful for correlation and unit delineation. Once the correlation is done it can be of great help for guiding interpretations of the strata and for making predictions. I would hope no one disputes this and this was clearly discussed in the Report. Ironically, the deductive approach followed by Ole does subscribe to the one universal model (Jervey Model) that Ole decries. This is perfectly clear in the ISSWG Report and the reader is referred to my comments on this report. 14) “Sequence stratigraphy guided us towards understanding stratigraphic breaks, missing sections and made us understand how we could predict what we could expect updip, downdip or laterally to our window to the world.” Petroleum geologists were doing this long before Vail and colleagues published their work and I am sure Bill Fisher can confirm this. They found a lot of petroleum with such reasoning back in the 1950s and 60s. I realize many process sedimentologists that started appearing on the scene in the 1970s and 1980s were not taught such things and that the work of Vail el al introduced them to a whole new world. 15) “I am not sure I follow the argumentation for concrete stratigraphic disciplines,” We will likely use the equivalent term “material-based” in the revised report to ensure no confusion. The terms concrete and material-based simply mean the entities in such stratigraphic disciplines are based on observable (empirically demonstrable) features within the strata. Examples include lithology, fossils, facies trends etc. This contrasts with the non-material based chronostratigraphy which depends on highly interpretive, abstract concepts for unit definition. Mixing the two in a single discipline has always created great problems as discussed in my comments on the ISSWG report. The current mixing of the empirical with the conceptual in sequence stratigraphy is perhaps the root cause of the problems now confronting sequence stratigraphers 16) “Thus, most of the geological community is content with sequence stratigraphy as a "tool box" by which we can interpret our sedimentary section or basin fill, and we should not limit its application to strict rules and formalism but rather appreciate the variability. And we have enough "tools" by which to do that.“ It is fine to regard sequence stratigraphy as being a tool box but it is important to understand what tools you have in the box and what each is used for. No one is trying to apply strict rules and formalization (except perhaps ISSWG). ISSC is looking to provide some guidance mainly on unit classification. It would be helpful if we had some common understanding when it came to units if only for effective communication. An “anything goes” approach is not the best and that includes for effective exploration. I would add that the tool box DOES NOT contain any tools for recognizing time surfaces in rocks. If Ole thinks it does, ISSC would appreciate receiving information on such a wondrous tool which would potentially revolutionize stratigraphy. 17) “the statement that "past attempts to develop methods and terminology for sequence stratigraphy have not honored these principles" (see above), is outright wrong” Ole takes this statement out of context by leaving out the key phrase “principles which govern the well-established, stratigraphic disciplines”. One of these principles for the well established, material-based disciplines is that only empirical, well characterized surfaces are used as boundaries (see NACSN Code). Thus abstract time surfaces are not used in biostratigraphy and lithostratigraphy etc and such conceptual surfaces are confined to chronostratigraphy. Past attempts to develop methods and terminology for sequence stratigraphy advocated the use of time surfaces, thus they clearly did not honor the principles used for the other material-based stratigraphies. That is the point that is being made by that statement and it is not wrong. 18) “it is absurd to state that all the other hundreds of publications on sequence stratigraphy by various authors including the group copied here are model-driven and not empirically based. “ I think Ole misunderstands what is meant by “model-driven” and we perhaps were not as clear on this point in the Report as we should have been. We now use the term deductive instead of model-driven (see Comments on ISSWG Report). Anyone who uses the surfaces and consequent units that result from the deductive approach to sequence stratigraphy would naturally be classed as being deductive (formerly model-driven ) in their approach to sequence stratigraphy. This seems obvious. As discussed in the Report, the deductive approach uses the two abstract time surfaces and the units they define (e.g. LST, FSST etc). Thus anyone who shoehorns their data into the deductive units is clearly using the deductive approach (model-driven in the Report). There is no doubt they are also using empirical data when they also recognize the empirical surfaces (SU, MFS etc) but it is this combination of empirical surfaces and abstract ones that characterize the deductive approach which Ole and others use. You can’t empirically delineate an LST because one of the defined boundaries of such a unit is an abstract time surface which is not material-based. 19) “On the contrary, creating a stratigraphic nomenclature based basically on one basin-fill succession and considering it to be universal can only be characterized as a model-driven approach.” The ISSC Report does not recommend such a procedure and Ole is constructing a fictitious strawman to knock down. The ISSC Report provides general, very flexible definitions of a sequence and systems tract and practitioners can adapt these to their own specific studies/needs. We provide examples of empirically defined sequences and systems tracts and state that others may well be defined in the future. If Ole can demonstrate that other material-based surfaces correlate with an SU then he can propose a variant of a depositional sequence. The key here is that any surfaces used to define and delineate units must be well characterized, empirical entities. That is the main constraint and it is difficult to see how anyone can disagree with such an obvious and necessary constraint. 20) “I certainly hope that this document will not be accepted by the ISSC as a standing document for sequence stratigraphy. It needs a major revision. Both the scientific content, the way it is presented and the one-sided nature of the debate need to be changed in order to merit publication, either by the ISSC or by any international journal.” ISSC has tried very hard to circulate the draft Report as widely as possible and to seek comment from many sequence stratigraphers. Over the summer it will be revised on the basis of all the comments received. Ole’s comments, for example, demonstrate we need to clarify various points which he misinterpreted. Such misinterpretations are undoubtedly in part due to our failure to express a concept as clearly as we might have. I don’t think the Report is one sided. The ISSC Report is perhaps the only document available which considers and discusses the relative merits of BOTH the deductive approach and the empirical approach to sequence stratigraphy. Most do not even consider the empirical approach and this indeed is one sided and the source of a lot of problems. We are still hopeful that practitioners like Ole, who subscribe to the deductive approach, will provide us with some data and reasoning for the use of abstract time surfaces in sequence stratigraphy. Ole has provided some criticisms but he did not provide any viable alternatives. If Ole wants the deductive approach to be advocated in the ISSC report, he and his colleagues need to provide a solid, scientifically based rationale for doing so. Our current assessment of the literature has suggested such an approach is not viable but solid data and reasoning can easily change our assessment on this. Ashton Embry AEmbry@nrcan.gc.ca +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On May 1st 2007 12:52 pm Ashton Embry wrote: Reply to Wolfgang Schlager’s Comments of April 27th Wolfgang’s measured and polite comments are much appreciated. My replies to specific statements are below. 1) “However, one crucial difference between the above methods and sequence stratigraphy is barely ever mentioned: All these established methods rely on one-dimensional sections as long as they obey Steno’s law of superposition. Sequence stratigraphy, on the other hand, was born as seismic stratigraphy of 2D sections, typically including land-to-sea transects of basin margins.” First of all I might remind Wolfgang that sequence stratigraphy was born back in 1949 and was not based on seismic data. It stood on the shoulders of numerous earlier outcrop observations including those of Barrell (1917), Weller (1930) and Wanless (1936). Like the other material-based stratigraphies Wolfgang listed, sequence stratigraphy depends on the ability to observe differences in a specific attribute of strata –depositional trend (see Embry (2002) and Catuneanu (2006) for an elaboration of this key point). Seismic data made a major contribution to sequence stratigraphy about 27 years after the discipline made its debut. That contribution included demonstrating how useful sequence stratigraphy was for subdividing the geological record into sequences and that such units were very convenient for interpreting facies relationships and consequent evolving paleogeography. Seismic data can be usefully seen as a detailed cross section with very many, closely spaced “logs” (seismic traces). Each trace represents a 1D section and correlation of a reflector, often which is interpreted in terms of one or more established sequence stratigraphic surfaces, proceeds from one trace to the next just like how a log cross section or a string of outcrop sections is correlated (i.e. two sections at a time). Such correlation depends on the recognition of a given surface, or reflector in the case of seismic data, at each 1D section and its correlation to an equivalent surface on the adjacent section. Two sections or 1000 sections (seismic traces), it doesn’t matter. An interpreter can only correlate between two sections at any one time. With seismic data, such section to section correlation happens very fast because there is little debate as to the proper correlation. Regardless, it is important to understand the same correlation process is used no matter what the data type. Thus sequence stratigraphy is not different from the other stratigraphic methods in regards to the reliance on 1D sections which are correlated with each other. 2) “Consequently, the geometry of sediment bodies and the lapout patterns of bedding are integral elements of the method” There is no doubt that such geometric relationships (e.g. lapout and truncation patterns) help to identify specific types of sequence stratigraphic surfaces and thus are indeed an integral part of sequence stratigraphy. Such geometrical relationships can be recognized on all types of data including those from outcrop, well log and seismic and are part of the characteristic features of various surfaces as presented in the ISSC report. The use of geometrical relationships of strata does not make sequence stratigraphy fundamentally different from the other material-based stratigraphies. It simply is one of the ways in which material-based surfaces are defined and delineated in sequence stratigraphy. 3) “The report virtually ignores them and thus arrives at some puzzling conclusions, for instance that highstand and lowstand systems tracts are “not acceptable sequence stratigraphic units”. The ISSC Report does not ignore the geometrical relationships of strata. In fact it emphasizes such criteria in the recommended definition of sequence stratigraphy. Notably Embry (2002) was the first to include geometrical relationships in the definition of sequence stratigraphy. Catuneanu (2006) followed Embry’s lead on this. Furthermore, the Report uses such geometric relationships in the defining characteristics of each surface of sequence stratigraphy. I agree completely with Wolfgang on the importance of stratal relationships in sequence stratigraphy and I laude his major and very important contributions on problems of false stratigraphic relationships being sometimes apparent on seismic data. As shown by Wolfgang, such vagaries of seismic data have led to the misidentification of various surfaces of sequence stratigraphy on seismic sections. Wolfgang mentions the ISSC recommendation that the LST and HST, as defined in the current literature, are not acceptable stratigraphic units. As I mentioned in my reply to Bruce Hart, unacceptable bounding surfaces result in unacceptable units. As discussed in detail in the ISSC Report, both the HST and LST have one unacceptable bounding surface. The HST has the BSFR as its upper boundary and the LST has either the BSFR (Posamentier and Allen, 1999 LST) or the CC (Hunt and Tucker, 1992 LST) as its lower boundary. Because neither the BSFR nor the CC were accepted as bona fide surfaces due to a lack of defining characteristics, the HST and LST automatically became unacceptable. If Wolfgang would like to see the HST and LST recommended as units of sequence stratigraphy, the ISSC TG would require proper documentation of the “realness” of such surfaces in well exposed strata. Only then can the HST and LST be separated out as acceptable units of sequence stratigraphy. I might point out that the ISSC Report does recommend the use of the adjectives highstand and lowstand for strata (e.g a lowstand submarine fan). There is a fundamental difference between such use as a modifier for strata and use in the definition of a stratigraphic unit which needs to have clearly defined boundaries which can be recognized with reasonable objectivity in a variety of geological settings. 4) “The authors undermine their own attempt to formalize principles for the practice of sequence stratigraphy in two ways. For one, they operate like the ancient Romans in Circus Maximus: thumbs up or down; there are no attempts to differentiate the judgement.’ I emphasize that the recommendations in the International Stratigraphic Guide are only guidelines, not formal rules that must be followed. I would hope Wolfgang has regarded the guidance in the current edition of the ISG in this way. I would be willing to bet that the vast majority of the members of ISSWG have never read the ISG. I like Wolfgang’s analogy and imagery when it comes to our decision making process. I don’t think the ISSC TG should be hailed as Caesar. There is no doubt we had to make choices when it came to our recommendations for units of sequence stratigraphy. And such choices do involve a “recommend or don’t recommend” decision. I don’t think it would be wise to say “maybe we recommend” a given unit as a bona fide unit of sequence stratigraphy. We did provide our reasons for acceptance or rejection of every proposed unit and there is not much else we could have done in this regard. If Wolfgang thinks we should have given a “thumbs up” to a unit which we gave a “thumbs down” to, I appeal to him to provide adequate data and argument to support his position. That is what this debate is all about. Most importantly the definitions provided in the ISSC report leave the door open to new types of sequences and systems tracts. All that is needed is the establishment of new, well characterized surfaces of sequence stratigraphy to act as boundaries of such new units. 5) “Second, the evaluation of current concepts and practices is presented in an unnecessarily dogmatic style. The language is replete with expressions such as “not scientifically valid”, “fundamental error”, “theoretically impossible”, “untenable” etc.” I would say we have been anything but dogmatic in that we have provided a detailed and well supported explanation for each of our recommendations. They were definitely not made in a dogmatic fashion but were done in an transparent, scientific fashion. Also our recommended definitions allow new types of sequences and systems tracts (or old, resuscitated ones) to be established. In regards to our critiques of past interpretations, we again provided logical arguments and data to support our critical views. If a stratigraphic relationship is theoretically impossible (e.g. the terminal end of a subaerial unconformity joining with the time surface at the start of base level fall), it is difficult not to call a spade a spade. Some current problems may stem from a past reluctance to criticize obvious flaws in past models. We will try hard to ensure the final draft does not have any criticisms which are not well documented and we will ensure such criticisms are free of value judgments. 6) “I am prepared to argue that the positions rejected in this way are tenable, albeit not necessarily best practice in sequence stratigraphy.” We would welcome such arguments from Wolfgang. In fact, we were hoping such arguments would be forthcoming in the first place. If we have referred to a unit, or surface or practice as untenable, and Wolfgang believes it is tenable, we would greatly appreciate hearing his arguments for such a position. We cannot improve the Report very much until we receive comments on specific points. 7) “The ISSC report as well as the reactions posted on the sequence-stratigraphy website indicate that sequence stratigraphy still evolves rapidly and several lines of synthesis are being pursued simultaneously. Attempts to formalize the method at this point probably are unnecessary and counterproductive” Sequence stratigraphy is a very young stratigraphic discipline compared with its cousins, lithostratigraphy and biostratigraphy. Thus it is evolving much more rapidly than those disciplines. However, ISSC wants to include some guidance on sequence stratigraphy and, given its widespread popularity and utility, I think its inclusion in the next edition of the ISG is warranted. Because it is evolving rather rapidly, it is recognized that whatever guidance is in the ISG will perhaps be a little dated but this cannot be avoided. But this is no reason, not to address sequence stratigraphy at all. It is a good reason however to be somewhat conservative in the guidance because such conservative recommendations will likely have a longer shelf life than much more contentious ones such as units defined by deduced, abstract time surfaces. Finally I would again emphasize that ISSC is not formalizing anything when it comes to sequence stratigraphy. It is only ISSWG that wants to formalize sequence stratigraphy (see the title of the ISSWG Report). In contrast to ISSWG, ISSC wants to provide some scientifically reliable, pragmatic guidance especially when it comes to unit classification. I hope Wolfgang will submit some more specific comments which will help us do the best job possible in this regard. Ashton Embry AEmbry@nrcan.gc.ca> +++++++++++++++++++++++++++++++++++++++++ On May 1st 2007 12:52 pm Ashton Embry wrote: Reply to Bruce Hart’s Comments of April 23 I thank Bruce for taking the time and effort to go through the various documents concerning the ISSC Report and for his comments. My reply is below. 1) ” whether or not they are correct, the recommendations in the report (r-t sequences) do not represent a consensus but rather one model for defining/analyzing sequences that is being proposed by a limited number of scientists. A broader consensus among leading sequence stratigraphers would be needed, and that is not apparent in the ISSC Report.” There is no doubt that the recommendations of the ISSC Report do not represent a consensus at this time. The main mandate of the ISSC Report is to recommend scientifically acceptable and pragmatic units for sequence stratigraphy that are applicable in a variety of geological settings. This will allow their use throughout the world and this will improve communication and understanding. I hope Bruce and others reading this agree that ill-defined units or those which are not applicable in most geological settings should not be recommended for use to the international stratigraphic community. Such units would only cause confusion and miscommunication as past experience has shown. ISSC has attempted to circulate the draft of the Report as widely as possible and we appreciate Chris Kendall’s help in this effort. We are seeking input from as many sequence stratigraphers as possible to allow us to revise the Report according to their comments and thus reach as much consensus as possible. We are naturally most interested in comments on the recommended units in terms of their definition and pragmatism. We are also looking for comments on the appropriateness of units which we have not recommended but which others may believe fit with the ISSC mandate. We can only achieve a reasonable consensus if sequence stratigraphers make their opinions known. Unfortunately most comments offered so far have been of a very general nature and thus have not been overly helpful for making specific changes to the recommendations regarding useful units. If Bruce or anyone else thinks a “falling sea level systems tract” should be recommended to stratigraphers throughout the world, we will need some solid data and arguments to support such a suggestion. We could not find adequate data in the literature to support such a recommendation and our personal experiences in a variety of basins and sediment types corroborated this. However, we recognize others may be in possession of data which might require us to reconsider our decision to not recommend such a unit. We cannot reach consensus on various units until those who do not agree with our recommendations, and favour the use of units such as the FSST, provide us with well supported alternatives. To sum up, ISSC is doing everything it can to reach consensus. We need specific and preferably detailed input from those who do not agree with the recommendations if such consensus, which people such as Bruce and Ole Martinsen claim they would like to see, is to be reached. Perhaps Bruce, Ole and other concerned sequence stratigraphers would like to lead the way in helping us achieve consensus on such issues as suitable units. As discussed in my Reply to the ISSWG Report, I discussed how ISSWG has not provided any suitable evidence to support their deductive, seven surface approach which includes various units not recommended by the ISSC Report. So far there has been a reluctance of those who favour a deductive approach and the accompanying units to offer well reasoned arguments to support their preferred choices. Perhaps they have had second thoughts and agree with the recommendations in the ISSC Report. I hope that those who would like to use the BSFR and CC as bona fide surfaces to define various units of sequence stratigraphy will provide ISSC with sufficient documentation which demonstrates the scientific reliability and pragmatism of units which depend on such surfaces for their definition and delineation. I am not sure where Bruce stands on this issue but now is the time for him to speak up in a scientifically suitable fashion if he doesn’t agree with the units recommended by ISSC. We are trying hard to reach some consensus but we need some more specific input from those who have different opinions. We can only take silence on a given topic as agreement. 2) “ there is insufficient documentation of the applicability/utility of r-t cycles in a range of depositional settings. more examples from different depositional settings (shallow-water carbonates, deep-water clastics, etc.) and using different types of datasets (seismic, logs, core, outcrop) would be needed” I hope Bruce agrees that the four surfaces we used to define the various units are all acceptable, well established surfaces which can be, and have been, recognized in many geological settings by many different geologists, including Bruce. Thus there can be no doubt as to the validity and applicability of sequence stratigraphic units which use such surfaces as boundaries. Valid bounding surfaces equal valid units and, as discussed in the ISSC Report, invalid bounding surfaces equal invalid units. We will be adding a few, real world carbonate examples (ramp and shelf/slope settings) in the final Report. We will also consider using a few seismic examples if space allows (one was dropped from the draft for this reason). We will also make an effort to reference numerous studies which have used such a methodology, knowingly or unknowingly, for unit delineation. The main question, which was stressed by Henry Posamentier in his comments, is whether or not we have recommended as fine a subdivision as possible, that is, is our classification scheme too simple or is it just right. Henry would claim it is too simple. This brings us back to the main problem of is the more detailed but scientifically questionable, deductive approach applicable or not. The question is not, are the recommended units applicable, but rather, do they represent the best possible (most precise) unit classification for sequence stratigraphy. 3) “more peer-reviewed papers need to be published that document the utility of the approach.” As discussed above, the fact that the surfaces which are used to define and delineate the recommended units are empirically based and widely accepted as bona fide surfaces, there can be no dispute about the scientific reliability of the units they define. For example, the R-T sequence, which is bound by maximum flooding surfaces, has been used by a number of practitioners and, given that MFSs are readily identifiable, I do not think we need any more documentation of the validity and usefulness of an R-T sequence (or whatever we finally decide to call it). A similar argument can be made for the depositional sequence of the ISSC Report. Other subtypes of a depositional sequence may be proposed but they will need to employ bona fide surfaces of sequence stratigraphy as correlative surfaces of an SU. In regards to a regressive systems tract, once again the MFS and MRS are empirical surfaces that have been recognized and mapped by countless workers. Thus a unit which uses the MFS as its basal boundary and the MRS as it upper boundary is valid, has applicability and needs no further documentation. The only question is can the RST be subdivided into acceptable sequence stratigraphic units such as a lowstand systems tract and highstand systems tract thus making the less precise RST of little relevance in many settings. We were not able to provide guidance on how an RST could be subdivided in a scientifically reliable and pragmatic fashion and thus we had to go with the less precise RST until methods for its subdivision are well documented. We are looking to Bruce to provide such documentation if he believes an RST can and should be subdivided. All we need are more bona fide surfaces of sequence stratigraphy to allow increased subdivision. 4) “there is indeed a confrontational feeling to much of the text. this may not have been intended, but perception is reality” I regard Bruce as reasonably objective when it comes to sequence stratigraphic practice so I have to take his perceptions seriously. It would be most helpful if Bruce would either contact me directly or post on this site some examples of statements he perceives as confrontational. I can assure him there was no desire to be confrontational and the ISSC TG would be most pleased to revise any statements which might be perceived in this way. Ashton Embry AEmbry@nrcan.gc.ca> +++++++++++++++++++++++++++++++++++++++++++++++ On May 1st 2007 8:59 am Maurice Tucker wrote: Dear All Sorry not been able to write earlier, before the end-April deadline. But perhaps having a short deadline for comments on the ISSC report is telling us something in itself !! For me the idea of having a set of rules and regulations for seq strat is premature and also misses the point of seq strat still being an evolving tool. Also of course the application of seq strat does vary thru geological time depending on icehouse-greenhouse, and for carbonates, the nature of the biota at the time. Having one model to apply to all sed rox of the geol record is not appreciating how the rock record varies thru time, or with facies. I do find a lot of the criticism in the report unwarranted – the subject of seq strat has been evolving for 30 years and still has a way to go. To criticise the early models is unfair and out of order in this report, and could have been done more tactfully. For me, seq strat is a practical way of describing and looking at seds and there is no need for hard and fast rules at this stage, as we are still discovering the natural variation and controls in sedimentary systems. It is incorrect too to suggest that the seq stat models were out of people’s heads – most seq stratigraphers were working with seds in the field and using that information to derive their models, not the other way round. Hunt & Tucker’s 93 approach was based on the Cretaceous carbonates of France which showed some features that did not fit in with the classic model – we did not sit down with a gamay looking for holes in the original model. Seq strat has developed from observations and is still doing so. I feel it is a pity at this point in time to draw up a concrete model we are all supposed to use – things don’t / wont work that way. I often feel that sedimentology / stratigraphy is more of an art than a science – lots of information from the outcrops/subsurface/microscope etc, not all pertinent, so that it becomes more of an art to make the most appropriate interpretations; experience helps a lot of course. I do believe that seq strat has settled down in the last few years in terms of approach, terminology and models, and that a consensus is emerging – but I don’t think that is what the ISSC report is presenting us with – it is advocating one scheme that has few adherents in the real world of practicing, field geologists, and it seems to be removing a lot of very useful concepts and terms, many of which are now household friends. With regards Maurice Tucker Durham University, England m.e.tucker@durham.ac.uk ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On April 30th 2007 Charles Henderson wrote: I sent this note to Henry Posamentier dated April 21, 2007 in response to his plea for further review. Henry: I have voiced my concerns to ICS colleagues that this manuscript is not representative of the entire sequence stratigraphic community, but have not reviewed it completely because of the length - I have too many things to review at present from GSSPs to theses. I am pleased to see that you have decided to give it a thorough review. It is important that ISSC ultimately produce a document that won't be quickly rejected by the majority of the community. Sincerely, Charles Henderson Additional Comments: The International Stratigraphic Guide produced by ISSC is a very important document and is an attempt to standardize stratigraphic practice (my interpretation). I hear many people express concerns about standardization, but in my view this is deterrent to Geoscience being accepted as a pure science in the same way as Chemistry and Physics. We should be pushing for a degree of standardization throughout our discipline, which still leaves the door open for creative and innovative approaches. Sequence Stratigraphy is very important to capturing the architecture of a basin through the recognition of surfaces, just as sedimentology describes the facies between these surfaces. The ongoing debate demonstrates that we do not yet have a consensus on what to call these surfaces or to the degree/resolution in which they can be recognized. Furthermore, the comments on model-driven versus data-driven are most unfortunate. Data appear in many forms and are fundamental to both of these “two schools of thought”. Until we are closer to a consensus, I believe it is premature for ISSC to add a Chapter on Sequence Stratigraphy to the International Stratigraphic Guide. There may never be a unanimous view, but there must be a greater consensus out there so that such an important chapter for stratigraphic geoscientists and geoscience students can be added to a revised guide in the future. Sincerely, Charles Charles Henderson Director, Consortia for Applied Basin Studies Chairman, Subcommission on Permian Stratigraphy of ICS Professor, Department of Geology and Geophysics University of Calgary 2500 University Drive, NW Calgary, Alberta, Canada T2N1N4 Phone: 403-220-6170 Fax: 403-284-0074 email: charles.henderson@ucalgary.ca Website: www.geo.ucalgary.ca/asrg +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On April 30th, 2007 10:51 am John Harper wrote: Good morning all: I read this discussion when it was first sent to me and have since allowed it to sit and marinate. Now that some of you have begun to offer your comments I will add mine as well. I think debate always is worthwhile and certainly on the subject of Sequence Stratigraphy that is true. However in this report I must say that the use of adversarial comments of disagreement do not encourage open discussion and are poor forms of discussion. Perhaps the usefulness of the Embry et al paper is to draw our attention to the myriad of problems facing the application of SS so we can devise an approach to clear up any muddle. In that regard the report does us a good service. I sum up my thoughts in this regard at the end of my comments. A great deal of effort has gone into this report but I had the sense that there was a biased tone of presentation. Disappointment was my immediate reaction. There was too much hypothetical discussion and not enough fact. Of the authors contributing to the paper one had 13 references, 2 had one each, 2 had none. There were many authors of much earlier work (eg Shaw and Curray) not even discussed. My own concerns about SS relate to the often non-critical application of the concepts, especially when applied to seismic. I have listened to researchers describe core and they force the data into the “SS models”. When queried on the justification of their ideas there is either resentment or bluster, not discussion of depositional process. I am concerned that we are leading our future scientists into a world of making data fit theory. In many regards our modern SS is really Seismic Stratigraphy. The interpretations are often based on recognition of geometric patterns which record long-understood depositional processes. Fundamentally, seismic boundaries are different from those seen in outcrop and in core, or on logs. As such often the interpretations applied to seismic boundaries are wrong or are deemed inconsistent with ground-truthing data. The problem increases when seismic interpretation deals with decreased density contrast with depth. Thus seismic data commonly offer low resolution response relative to the rocks themselves. It is here that resides the discomfort felt by many regarding SS. Many so-called SS interpretations are not readily acceptable. I certainly feel it is too early to stamp the discipline (if it is even such) as being concrete by initiating terminology formalization. As I went through the text I at times agreed with the authors and other times disagreed. Here are some of my comments, not in any particular order of significance: 1) The whole discussion of “surfaces “ bothers me – always has. For instance the authors state that “surfaces in biostratigraphy represent changes in fossil content”. These are not surfaces in the context of discreet boundaries as anyone who has used such data can attest. 2) Terminology has become a hodge-podge of words, many really meaningless. 3) I have to agree that the interpreted equivalence by many of SS and Chronostratigraphy haunts us even today in spite of the many indications that it such equivalence is not the case. 4) The use of “eustatic sea level change” has become common, yet “eustatic” and “sea level change” are the same so the phrase is redundant. The word “eustatic” needs a qualifier such as “glacio-eustatic” or ” tectono-eustatic” to clarify meaning. 5) The difference between base level falling or not falling below a shelf margin and defining each as a different “type” is surely one of degree. To change terminology on such a basis only adds to confusion, especially in areas where the shelf margin is not recognizable. 6) A “maximum flooding surface” is an erroneous concept. The idea of a point of maximum flooding is readily understandable, and there would obviously be a “surface” related to that point. However it is not possible to identify such a unique surface over a whole basin. The “surface” is really a hypothetical boundary to facilitate description of relationships changing from inundation to progradation. The fact that many workers equate a high-radioactivity kick on logs (the hot spike) to the MFS is not a completely correct approach either because it can be demonstrated in many basins that the interval of high radioactivity can simply be related to several progradations and inundations, not just one. Flooding (I prefer inundation) is by its nature diachronous and is therefore not a surface. It is simply the maximum extent to which a shoreline has been pushed landward before regression takes over. Nowhere to my knowledge is consideration given to that situation where once the maximum landward displacement of shoreline has occurred, that position is maintained over a long period of time. As such there is once again no “surface”. 7) Another point indicated in the report is the fact that introduction of new terminology can really mess up the process if it is not appropriate, but becomes accepted. I sense that SS has become fertile ground for individuals to create a plethora of terms which are not needed, when a simple description would work, and would perhaps be more accurate. A high percentage of SS users do not read or keep up with the literature. They apply the concepts they hear in talks and papers with out really understanding those concepts. The result is terminology chaos. Communication becomes a real issue for the health of any idea. 8) I am concerned by the fact that every time many workers encounter conglomerates they throw in a sequence boundary because they consider the base to be an erosional contact. This is simply incorrect as has been demonstrated in many modern sedimentology studies. Whereas some such contacts can be sequence boundaries, most are not. 9) The comment by the authors that subaerial unconformities are rarely preserved in carbonate strata is not accurate in my experience. If they were not preserved we would not recognize karst topography. 10) It is unfortunate that many SS discussions are based on hypothetical situations. Everyone loves the term “paradigm”, and it is used “ad nausem” as a proxy for fact. While such an exercise may help thought processes, for one to argue over the hypothetical out of context of data is a meaningless exercise. 11) Hypothetical schematics are great to play with when dealing with terminology but tend to be oversimplified and often commonly lack reality in fact. 12) The usefulness of the term “parasequence” is often diminished when applied because a parasequence in one well is often found to become a sequence in another well. If one were to take this report as gospel one would have to say that if ever there was a condemnation of SS this report says it. Summed up the issue is that we have fallen into a situation of there presently being very poor communication regarding the subject, the terminology, the application, the problems, the concerns. There is far too much hypothetical discussion without the facts being available to back up the ideas. Too many terms have been introduced without factual basis for use. There is far too much ambiguity in discussion, leading many workers to “go their own way”, with resultant confusion of terminology and disagreement over terminology. We have fallen into a quagmire of semantics to the point the concept has suffered damage. Our chore will be to begin to clear up the “mess” although I am aware that there are SS aficionados who do not feel there is a mess at all. I have to agree with Professor Tesson’s comments. If we cannot make the issue clear to our students then SS is in deep trouble. I deal with the application of the concepts by many of our young new employees and I am appalled at what they understand, and how they apply the concepts. So in spite of the antagonism raised by this report, and the many areas of disagreement it fosters, I believe it has served to stir the pot and behooves us to look beyond indignant reaction and to look back to the original intentions for SS, and perhaps clean up many of the concerns. John Harper John.Harper@conocophillips.com ++++++++++++++++++++++++++++++++++++++++++++++++++ On April 28, 2007 at 8:45 am Michel Tesson wrote: Dear Dr Kendall The ISSC report project to standardize terminology and methodology using the sequence stratigraphy approach has been a surprise for me, and not really a good surprise. But, along my long life I learned what is science and what are men. This is, thus, one more time, an attempt for men to use what they call « science » but is no more than « personal conviction », to close the mouths of 100’s and 1000’s of more anonymous researchers. I’m a researcher and particularly a man of « the field », and a teacher. As a researcher, I’ve nothing to expect from such standardisation and sentences. The objective of the ISSC, under this form, will not have any impact on my research because I’m able to jump over this kind of standardisation if it becomes « official ». We are numerous that don’t appreciate too much the « range “A” publishing system » and the very critical editing committee made of auto-assigned people. This added rigid frame is only one among the constraints we are subjected to. But now, with Internet, we have a free way to be heard….by those which have a true scientific interest. Internet in the future will be the way to be totally free. But it is not the same when I consider that I’m am a teaching professor. It is essentially why I’m written this in my poor English. The ISSC report is dangerous in at least two ways: - the - the picture, that the « selected senior researchers » involves in this mission are given of the impartiality, honesty and the serious that this position requires. For the students three things are essential : … these people must be one of the qualified best researchers in the topic. And I am sorry to say that, in sequence stratigraphy, the seniors researches (they are well known with 30 years of publications) are not in this commission. Why ? Why are they in the IWGSS and not here? There is not response to give to the students, and when they read the ISSC report they don’t understand. … some of the terms used in the report have excited very strong reactions coming from scientists of other disciplines. What is a « good science » based on « only real data » such as rocks. Mathematics, most of the Physic, Astronomy and so on are not sciences ( ???). Its is a pity. When the Sciences (?) of the Earth are so few estimated when we have to fight to get some influence everywhere, this kind of assumptions is catastrophic. And try to explain this to the students. It seems that we are fools. I just talk about it, but you know that in the point of view of Earth Sciences and stratigraphy there are in this report too much scandalously erroneous opinions. This is not my debate here. …we teach along our courses the most possible diverse schools and case study variability in the stratigraphic organisation under control of various factors, allocyclic or autocyclic, regional or local. This is the base of sciences teaching: diversity, controversy. We don’t want to teach students only one opinion, one way. All this is in books. We must learn criticism, questioning, reinterpretation. What should we do with dogmas? And don’t say that we have actually too much dogma. When they are numerous, they are not dogmas. This report is dogmatic. And false! It is time for me to conclude. Either the IWGSS position is the way to go and we have essentially to organize Conferences, Meetings and so on where scientists should discuss, modify or not their approaches, best fit them sometimes, and go forward, either this report of the ISSC commission is validated and the ISSC message will not be taken under consideration. At this step, the IWGSSC commission appears the best qualified to try to improve the situation if needed, and the money spent by the ISSC commission would be better used to organise conferences and invite young researches. This is my modest feeling. And please, excuse me for my English. Best regards Michel Tesson Professeur, Sédimentologie-Géophysique Laboratoire de Biophysique et Dynamique des Systèmes Intégrés (BDSI) tesson@univ-perp.fr gdargo66@aol.com +++++++++++++++++++++++++++++++++++++++++++++++++++ on April 26, 2007 Ashton Embry Replied to Posamentier’s April 21, 2007 Comments It is most gratifying to see that Henry Posamentier has jumped into the debate. Henry and I have been beating on other’s work since 1985 when he presented Mac Jervey’s concepts and model at the SEPM Research Conference on Sea Level Change. Thus I welcome his latest comments. Below, various statements from Henry’s comments are copied and put in quotations and are followed by my comments on them. 1) “with regard to nomenclature and methodology, the community tends to “vote with its feet”.” Periodic reviews and guidance on various stratigraphic disciplines have proven worthwhile. The various editions of the NACSN Code and ISSC International Stratigraphic Guide have responded to the need for such updates. ISSC is simply trying to bring some standardization to the practices and terminology of sequence stratigraphy. Henry was on the previous ISSC Working Group which attempted to do this but, because of the makeup of the WG, it went around in circles for almost 6 years and did not come up with a single recommendation. Notably ISSWG is also planning to recommend some standardized guidelines for sequence stratigraphic methods and terminology. Given that Henry belongs to ISSWG, it would seem he also sees the need for such an effort. 2) “In my opinion the present attempt to codify and formalize the terminology and methodology of sequence stratigraphy is both unnecessary as well as highly presumptuous” ISSC is not trying to “codify or formalize” terminology and methodology for sequence stratigraphy. It is simply trying to provide some scientifically acceptable guidance on these topics to match the guidance it provides on other stratigraphic practices. Such guidance helps stratigraphers throughout the world to communicate more effectively with each other when it comes to stratigraphic disciplines. Many people have struggled with the current terminology and methods for sequence stratigraphy and are looking to ISSC and NACSN for such guidance. Notably, ISSWG also sees the need for such guidance. It is just that ISSC and ISSWG have different perspectives on what type of guidance is worthwhile. We are not debating the need for guidance (a given) but the content of the guidance. 3) “Consequently the present proposal before us is highly flawed from the outset, in my opinion. It presumes to largely take apart the workings of a discipline that has already been embraced by the overwhelming majority of scientists, and replace it with an entirely new approach and methodology.” Actually our proposals follow most sequence stratigraphic practices and, as has been noted before, the only difference between our work and that proposed by the ISSWG (which includes Henry) is the use of two time surfaces (paleo-sea floors) for correlation and the delineation of units in sequence stratigraphy. Thus to characterize the ISSC Report has ‘an entirely new approach and methodology” is misleading and unsupportable. The ISSC Report simply questions one aspect of previous work and makes proposals on the basis of the rejection of the use of two time surfaces which have not been adequately characterized in the sequence stratigraphic literature. 4) “Even the title, wherein the manuscript represents itself as a “concrete” version of sequence stratigraphy, implying that what has come before is less than “concrete”, seems an attempt to capture a phantom moral high ground” First of all, I do not believe “a moral high ground, phantom or otherwise, exists for sequence stratigraphy. Thus I am somewhat puzzled by Henry’s suggestion that ISSC is trying to capture such a thing. Anyone aware of the problems which have beset various stratigraphic disciplines over the last hundred years (mainly lithostratigraphy and biostratigraphy in the first half of the 20th century) can appreciate the difference between a material-based stratigraphic discipline (one based on observable features in the strata) and a conceptual stratigraphic discipline (one based on highly interpretive, abstract concepts, usually time related). For many years both lithostratigraphy and biostratigraphy combined both methods and this resulted in considerable confusion and problems for terminology and methodology in these disciplines, not to mention lots of arguments (sound familiar?). Starting with the classic Schenk and Muller paper of 1941 and followed up by the legendary efforts of Hollis Hedberg, both lithostratigraphy and biostratigraphy were eventually liberated from abstract time concepts and were made material-based stratigraphic disciplines. The time-based, conceptual stratigraphy was separated out as a specific stratigraphic discipline called chronostratigraphy. Material-based stratigraphies are also known as “concrete” stratigraphies and the terms material-based (preferred by NACSN) and concrete are interchangeable. Sequence stratigraphy, as described in various publications mentioned by Henry (Posamentier et al. (1988), Van Wagoner et al. (1990), Emery and Meyers (1996), Posamentier and Allen (1999), Coe et al. (2003), and Catuneanu, 2006), as well as the current ISSWG Report, combine both material-based (concrete), empirically-based surfaces such as the MFS and the SU with conceptual, abstract, time-based surfaces such as the BSFR and CC. It is one of the main theses of the ISSC Report that many of the problems associated with sequence stratigraphy stem from this mix of concrete and abstract surfaces and that most problems can be resolved by making sequence stratigraphy a concrete discipline (i.e. dropping the time-based surfaces which belong in the realm of chronostratigraphy) 5) “lithostratigraphy, biostratigraphy, magnetostratigraphy, chemostratigraphy [are] “concrete” stratigraphies [that] contrast with chronostratigraphy which is a “highly interpretive” stratigraphic discipline.” I find it rather surprising that Henry regards this statement as “contentious”. To me it is a simple declarative sentence that essentially states the obvious. I can now understand why he finds many statements in the ISSC Report to be “contentious”. In the ISSC Report it was necessary to critically appraise past work in order to extract useful concepts, practices and nomenclature and to reject those that were not supported by adequate science. I can only challenge Henry to provide alternative views for all the statements he finds contentious. Perhaps he thinks chronostratigraphy is a concrete (material-based) discipline. No problem. Just let us have the reasoning behind such a view. 6) “What the Embry report purports to do is to supplant the body of work that has come before, which has described and shaped a scientific discipline, i.e., sequence stratigraphy, with the opinions of a very small minority of the scientific community. Though changes and modifications of this discipline will and should occur in the future, these earlier works nonetheless have largely stood the test of time in the sense of scientific acceptance”. This is a gross overstatement of what the ISSC Report does. It follows most established sequence stratigraphic practice and simply points out the lack of acceptable empirical evidence to support the recognition and use of two time surfaces for correlation and unit definition. On the basis of the accepted practices and the rejection of the time surfaces, the Report makes a series of recommendations. The lack of widespread criticism in the past of the problems associated with the delineation and correlation of abstract time surfaces in sequence stratigraphy is a curiosity. The understandable inability of many practitioners to recognize and correlate the time surfaces forced them to shoehorn their data into the model. This usually entailed the use of diachronous facies contacts (e.g. base shallow water high energy facies) as ill-conceived proxies for the time surfaces and this often resulted in the interpretation of unrealistic stratigraphic relationships as has been described in the ISSC report. Overall, the sequence stratigraphic model as presented in such books as Posamentier and Allen (1999) and Catuneanu (2006) has not really stood the test of time. It has become more and more contorted such that it needed a critical review and revision. The ISSC Report provides such a review and required revisions. 7) “The undue emphasis on transgressive and regressive cycles is in my opinion a step backwards. It represents a return to a lithostratigraphic approach in that these cycles are so dependant on sediment supply factors.” I think it is important to understand the difference between lithostratigraphy and sequence stratigraphy. As discussed by Embry (2002) and Catuneanu (2006), each concrete stratigraphic discipline is characterized by a specific type of change in a material-based property (e.g. fossil content, magnetic characteristics etc). Lithostratigraphy is characterized by changes in lithology whereas sequence stratigraphy is characterized by changes in depositional trend. These are very different criteria and, despite this, there continues to be some confusion between these two very different stratigraphic disciplines as evidenced by Henry’s comment and by Van Wagoner et al’s definition of a parasequence (a lithostratigraphic unit). I suspect this stems from the fact that the two types of surfaces can sometimes coincide (e.g. a formation contact coinciding with an SU). 8) “It is common that along a given stretch of coastline one part can be marked by transgression because of relatively low sediment supply whereas just down the coast, regression may be occurring because of relatively high sediment supply” This is well accepted by everyone and is the reason surfaces such as the MRS and MFS are NOT time surfaces. No material-based (concrete) surface from any stratigraphic discipline is a time surface! We can only make somewhat speculative interpretations of time relationships based on the time transgressive, material-based surfaces we map, including those of sequence stratigraphy. Anyone who thinks they can recognize and map time surfaces either is scientifically far ahead of anyone currently practicing stratigraphy or is delusional. 9) “ In the absence of a chronostratigraphically based approach, the R-T cycles of these settings would be very similar and their correlation would lead to erroneous interpretations of paleogeography. Chronostratigraphy, either based on paleontology, magnetostratigraphy, well-log correlations, or seismic data, would be essential for properly constructing a stratigraphic framework.” There is no doubt that the MFSs and MRSs are somewhat diachronous and thus the contacts of R-T and T-R “cycles” would be diachronous. Unfortunately biostratigraphic and magnetostratigraphic contacts are also diachronous although in some cases less diachronous than MFSs and MRSs. We simply can’t delineate time surfaces in rocks, only approximations (i.e. low diachroniety surfaces of various concrete stratigraphies). That is the prime reason for using sequence stratigraphy (as emphasized in the ISSC report). It provides us with low diachroniety surfaces in many cases where such surfaces from other disciplines are not available. The implication in Henry’s statement that surfaces picked on well log and seismic (i.e. lower resolution data sets) are time surfaces is unfortunate and is basically an “urban myth”. One cannot recognize time surfaces on such data sets anymore than one can in rock. Once again, one picks low diachroniety “surfaces” on seismic and logs which in most cases are related to low diachroniety MFSs and MRSs. 10) “The R-T approach would not distinguish between deposits of so-called “normal” regression and deposits of overlying “forced” regression. Losing this distinction by lumping these deposits would be most unfortunate inasmuch as forced regressive deposits are associated with significant sediment bypass of the shelf and have the potential to be significantly different from normal regressive deposits” It is true that the ISSC Report does not recommend a methodology for separating forced regressive deposits from normal regressive deposits. If one is to draw a boundary (on a X section or an outcrop) between normal regressive deposits (ie base level is rising) below and forced regressive deposits (base level is falling) above, one has to be able to recognize a time surface which represents the start of base level fall (start forced regression). Henry implies in his statement this can be routinely done but unfortunately this is not so. Henry himself has never been able to draw such a boundary in real world rock data and nor has anyone else. The reason for this is simply that there are no criteria for doing so. And that is why the ISSC TG rejected such a ‘surface” to define one or more units in sequence stratigraphy. It would be great to be able to delineate such a time surface for the reasons Henry states but we need more than wishful thinking based on a deductive model. I again ask Henry and all members of ISSWG to provide the required criteria and examples of their methods for recognizing such a surface in strata in a variety of geological settings. 11) “In general, the R-T approach represents a gross over-simplification of a potentially very complex stratigraphic architecture. Such attempts to propose a simple template in lieu of a more complex methodology does little service to the science.” I agree it is important to be able to subdivide the stratigraphic succession in as much detail as possible. Henry implies that the seven surface/four system tract model of ISSWG is an established methodology which can be readily applied in most geological settings. If this was so, we would not be having this debate. The ISSC Report takes the position that the five surface/two systems tract method is as detailed as the data allow and that the additional subdivisions proposed by previous authors, and now ISSWG, cannot be achieved because of the lack of criteria to delineate the two time surfaces. Given that Henry has never been able to demonstrate in rocks the type of stratigraphic subdivision he claims is possible, his statement that a “more complex methodology” is available cannot be taken seriously at this time. Clearly if he disputes this, he knows what has to be done to support his position. 12) “The Embry report also largely discounts the utility of multichannel seismic data with respect to identification of key bounding surfaces and sequences “ The ISSC TG does not dispute that the five empirical surfaces of sequence stratigraphy can be interpreted on seismic data. Most of us have done this and there are innumerable published examples of this. The problem comes down to identifying the proposed time surfaces on seismic. The severe problems of doing this are discussed in detail in points 32 and 33 in my comments on the ISSWG report. Also it is important to emphasize that any proposed surface of sequence stratigraphy has to be characterized on the basis of data from rocks. Once this is done and a surface is well characterized, it can then, and only then, be potentially interpreted to be inside a given seismic reflector. Given the BSFR and CC are not yet characterized, any attempt to interpret them on a seismic line is illogical. You can’t interpret the presence of something which has no defining characteristics in the first place! 13) “This is where regional data, such as seismic data or long well-log correlation panels, have the advantage over outcrop data. It is through the use of just such data that surfaces of onlap, toplap, and truncation, as well as correlative conformities can be recognized.” As discussed previously and in detail in my comments on the ISSWG report, the two time surfaces (the correlative conformities in the above statement) cannot be identified on seismic because they have not yet been defined and characterized in rock. Notably no one (including Henry) has ever been able to recognize them on detailed log cross sections because they have no characteristics and thus an interpreter has no idea how to recognize them on a log. Of course if Henry has examples of “long well-log correlation panels” which illustrate the regional correlation of such surfaces I would appreciate it very much if he could send them to me or provide the reference they are in. 14) “Seismic data is generally accepted as being a proxy for chronostratigraphically significant time lines and therefore seismic reflections are reasonable approximations of time horizons.” This statement is very ambiguous. What is “a proxy for a chronostratigraphically significant time line”? Seismic reflectors are created by material-based changes in the rocks not by conceptual time surfaces. Such material based changes are both lithologic changes and depositional trend changes. Seismic reflectors usually parallel MFSs and MRSs which for the most part have a low diachroniety. Thus many seismic reflectors have a relatively low diachroniety (i.e. time lines pass through them at a low angle). 15) “I can think of very few instances where an onlapping geometry is obvious in outcrop data. Such stratigraphic terminations, which were the backbone of early sequence stratigraphy, are essential for defining stratigraphic discontinuities, are far easier to observe using seismic data. Outcrop data, in its limited availability, can yield such observations only rarely with the exception of beds truncated by channels or other erosive processes.” Such terminations do not apply to the identification of the two time surfaces, only to the five empirical surfaces which are not in dispute. These empirical surfaces (e.g. a subaerial unconformity) can be identified in outcrop, on logs and on seismic. 16) “There are numerous technical issues that I could take to task within the report, but that lies beyond the scope of this response” It is unfortunate that Henry decided not to comment on some of the more contentious technical issues of the ISSC Report. His comments would be very helpful in this respect. We are not going to be able to eliminate purported erroneous interpretations unless people like Henry speak up and criticize specific points. The general comments Henry has made are of some use and will allow us to emphasize and clarify a few points. However, they will have no impact on the 20 recommendations because of their very general nature. I can only encourage Henry to send us some more specific criticisms. 17) “My general suggestion is that this report not be published. The mandate for the group, the makeup of the group, and the technical thrust of the report, are aspects that I find unacceptable” I can understand why Henry would not want a paper which disagrees with his past work to be published. However that is not a bona fide reason to not publish a paper. Similarly, the mandate and makeup of the TG have nothing to do with the quality of the science and the resulting recommendations. If Henry disagrees with specific scientific points we would love to have his comments on them. We are completely prepared to revise the report to accommodate worthwhile and insightful critical comments which are well supported by scientific data. 16) “Any attempt to impose or dictate a highly simplified scientific doctrine as this paper purports to do, is in my opinion unacceptable.” First of all ISSC does not dictate or impose stratigraphic methods and terminology. Through the ISG it provides guidance on such topics. I again ask Henry, if he thinks a more complex methodology/terminology is warranted for sequence stratigraphy, to send us the empirical evidence which supports such a practice. We need more than hypothetical deductions to provide reliable guidance to stratigraphers throughout the world. 17) “.I have participated on enough committees and in enough similarly-themed conferences with highly regarded and pre-eminent scientists involved in the application of sequence stratigraphic concepts to know that attempts to formalize the terminology and methodology are largely exercises in futility. Through time, the self-regulatory nature of the scientific publication process has been highly effective in the past” I expect the scientific publication process will indeed continue to be a highly effective filter for proposals for sequence stratigraphic practice and nomenclature. The ISG is just one part of the publication process when it comes to stratigraphic practice. It is certainly not the end all and be all when it comes to stratigraphy. Ashton Embry: AEmbry@nrcan.gc.ca +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On 27th April 2007 7:32 am Wolfgang Schlager wrote Prof. M.B. Cita Dear Maria, Below please find some comments on Embry et al. 2007 (draft) Sequence stratigraphy as a “concrete” stratigraphic discipline. The report represents an interesting, often very perceptive analysis of concepts and practices of sequence stratigraphy along with extensive recommendations for future application of the method. Despite its strong points, I cannot see how this document can serve as a general (and formal) guideline for practicing sequence stratigraphy. My main concerns are as follows: ---- The authors take measure from established stratigraphic methods such as lithostratigraphy, biostratigraphy, magnetostratigraphy and chemostratigraphy. However, one crucial difference between the above methods and sequence stratigraphy is barely ever mentioned: All these established methods rely on one-dimensional sections as long as they obey Steno’s law of superposition. Sequence stratigraphy, on the other hand, was born as seismic stratigraphy of 2D sections, typically including land-to-sea transects of basin margins. Consequently, the geometry of sediment bodies and the lapout patterns of bedding are integral elements of the method. The report virtually ignores them and thus arrives at some puzzling conclusions, for instance that highstand and lowstand systems tracts are “not acceptable sequence stratigraphic units”. --- The authors undermine their own attempt to formalize principles for the practice of sequence stratigraphy in two ways. For one, they operate like the ancient Romans in Circus Maximus: thumbs up or down; there are no attempts to differentiate the judgement. Second, the evaluation of current concepts and practices is presented in an unnecessarily dogmatic style. The language is replete with expressions such as “not scientifically valid”, “fundamental error”, “theoretically impossible”, “untenable” etc. In most instances, I am prepared to argue that the positions rejected in this way are tenable, albeit not necessarily best practice in sequence stratigraphy. --- The ISSC report as well as the reactions posted on the sequence-stratigraphy website indicate that sequence stratigraphy still evolves rapidly and several lines of synthesis are being pursued simultaneously. Attempts to formalize the method at this point probably are unnecessary and counterproductive. Sincerely yours Wolfgang Schlager <wolfgang.schlager@falw.vu.nl> Emeritus Professor of Sedimentology Faculty of Earth and Life Sciences Vrije Universiteit Amsterdam +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On 26th April 2007 at 8:15 am Ole J. Martinsen wrote Dear Dr. Cita, I have read with interest through the document titled "Sequence Stratigraphy as a "Concrete" Stratigraphic Discipline" written by the ISSCtask group on sequence stratigraphy led by Ashton Embry. My views I think are representative for a vast group of people, both working in industry and academia, who use sequence stratigraphy as a working tool and method to analyse sedimentary successions and the fill of basins. My credibility, along with that of the other members of the IWSCC (copied on this e-mail) come from publication of many tens of papers, books, editorship and decades of experience in working with sedimentary systems. Thus, I have considerable empirical basis to say that I find the report to be unbalanced, one-sided and flawed in many ways. I must say I do not see the need for a fomalization of the method. Such a fomalization, whatever principles it is based on, will barely be used, and if a fomalization is based on the current document, it will be used by very few. The learning from working and publishing on sequence stratigraphy over the last 15-20 years is that pragamatism and not formalism is needed to capture the immense variability of sedimentary system, clastic or carbonate. It is a paradox to me that the authors of the report claim to be emprically based and pragmatic and everyone else is model-driven, when what they do is attempt to squeeze the enormous variability of sedimentary systems into a couple of systems tracts and separated by four surfaces. It is also a paradox that the template suggested in the report is based on only one basin or basin complex (Sverdrup Basin) and one geologic time period (Triassic). I think it is common knowledge that geologic variability cannot be captured in a template created from the stratigraphic expression of one basin and one time period in that basin. Thus, the authors are model-driven to the utmost extent themselves by forcing the stratigraphic variability of all other time periods, sedimentary systems and basins into a template erected from one basin. I will focus the further discussion on a few specific examples, particularly from the introduction sections of the document to point out what I think are some of many key issues: (1) Choice of model (as written in abstract) The authors state: "Our systematic evaluation of these proposals has revealed that only four surfaces are appropriate for bounding units in sequence stratigraphy. These are a subaerial unconformity, an unconformable shoreline ravinement, a maximum regressive surface and a maximum flooding surface." My point here is how one can make such a template universally valid when the formation of the four surfaces they have chosen vary immensely in time and space (e.g. compare rift systems with passive margins), are dependent on the type of sedimentary system, are diachronous in relation to one another (the three first ones are not genetically related by definition; i.e. they subarial unconformity does not form at the time that the maximum regressive "surface" forms). The maximum regressive "surface" is not a physically recognizable surface anyways in most instances: it is a conceptual break between regressive and transgressive stacking patterns (see Helland-Hansen and Martinsen, 1996; Martinsen & Helland-Hansen 1995) (2) Sequence definition The authors state: "A sequence is the main unit of sequence stratigraphy and it is defined in a generic fashion as a unit bounded by a specific type of unconformity and its correlative surfaces. Two distinct types of sequences have been recognized. A depositional sequence has a subaerial unconformity as its primary unconformity and an unconformable shoreline ravinement and a maximum regressive surface as the correlative surfaces. An R-T sequence has maximum flooding surfaces as its boundaries. Both of these sequence types can be divided into two systems tracts, a transgressive systems tract and a regressive systems tract." A very simple point here, partly touched on above: by choosing these boundary surfaces to redefine a depositional sequence the whole concept that sequences should have chronostratigraphic significance breaks down because the subaerial unconformity, the shoreline ravinement and the max regressive "surface" do not form a surface with chronostratigraphic significance as they are likely to form at different times and they separate various units. If this concept is chosen and formalized the whole point about sequence stratigraphy disappears. Moreover, I think it is common knowledge that erosion somewhere leads to deposition elsewhere: thus one should not search for correlative surfaces everywhere, rather deposits, condensed or expanded, that correspond to the surfaces. I think this is something everyone has learnt over the last 10-15 years and a major learning from initial models, Sloss' to Exxon's to Galloway's to anyone else's. I would almost feel better if the authors actually stated that sequence stratigraphy does NOT work based on what they say instead of making a new definition... (3) Introduction to theme The authors state: "During the last 30 years sequence stratigraphy has been discussed in dozens of books and thousands of scientific papers. It also has become the most commonly used stratigraphic discipline for developing a correlation framework within a sedimentary basin because of the low costs associated with such an analysis as well as its applicability in many cases to a well log and seismic data base in addition to cores and outcrop (Embry, 2002). Despite such popularity, considerable confusion and various misconceptions are associated with the methods and terminology (e.g. unit definition) for sequence stratigraphy." Reference to your own work and not to the overwhelming existing literature (a simple search in Geoscience World will reveal the imbalance), that in fact says something else than the lead author's and the group's opinion, in a document that is intended to be universally accepted is not standard scientific practice, and will never be accepted in major journals (I am an Associate Editor of Sedimentology and an editor of several books). The entire document is full of argumentation such as "we are right and they are wrong". Very few use this type of argumentation: in fact many of us (copied on the e-mail) believe that the observations in the Canadian Arctic are a very valuable contribution, but it is one example from one basin and one particular type of stratigraphy. It is not universal and the only empirical approach-a statement like that is very provocative towards the rest of the sedimentary geology community who have spent hundreds of months in the field or working the subsurface in other basins looking at other types of expressions of sequence stratigraphic variability. We have all learnt that sequence stratigaphy is NOT ONE universal model, but a method that directs our thinking, makes us make predictions and guides interpretations. Consider what sedimentary geology was before sequence stratigraphy: Walther's Law made us force interpretations into models where everything co-existed in sub-environments side by side. Sequence stratigraphy guided us towards understanding stratigraphic breaks, missing sections and made us understand how we could predict what we could expect updip, downdip or laterally to our window to the world. (4) Validation of report The authors state: "This document is the initial report of the ISSC Task Group on Sequence Stratigraphy and it provides numerous recommendations for the methods and terminology of sequence stratigraphy. These recommendations will bring sequence stratigraphy on a par with the aforementioned, concrete stratigraphic disciplines. These recommendations rest on a foundation of a few basic principles which govern the well-established, stratigraphic disciplines. As will be demonstrated, past attempts to develop methods and terminology for sequence stratigraphy have not honored these principles (e.g. Posamentier et al, 1988; Van Wagoner et al, 1990; Emery and Meyers, 1996; Posamentier and Allen, 1999; Coe et al, 2003; Catuneanu, 2006) because they were formulated primarily on the foundation of a deductive model rather than on empirical observations." I am not sure I follow the argumentation for concrete stratigraphic disciplines, and I am uncertain how dedicated the authors are either by putting "concrete" in inverted commas on the front page. Do they mean that sequence stratigraphy really is not as concrete as they would like it to be? But are any of the other stratigraphic disciplines that concrete? Both biostratigraphy, lithostratigraphy and chronostratigraphy have large uncertainties, and terminology is many times controversial and not followed in a strict sense. Perhaps this is a weakness of geology, but for sequence stratigraphy it guides our way of thinking, varies as mentioned before from basin to basin and from stratigraphic unit to others for some very simple reasons: both the bounding surfaces and the enclosed units' expression depend of sedimentary processes, accommodation and sediment supply which vary locally. Thus, most of the geological community is content with sequence stratigraphy as a "tool box" by which we can interpret our sedimentatary section or basin fill, and we should not limit its application to strict rules and formalism but rather appreciate the variability. And we have enough "tools" by which to do that. And the statement that "past attempts to develop methods and terminology for sequence stratigraphy have not honored these principles" (see above), is outright wrong. How can the authors use the stratigraphy of the Sverdrup Basin and some observations from the Cenozoic of Spitsbergen as the proof that their principles are universal and will honor the same principles in all other basins globally? This is not a scientifically valid argument. (5) Model-driven vs. data-driven hierarchies or the value of an empirical approach This point was commented on above, but it is absurd to state that all the other hundreds of publications on sequence stratigraphy by various authors including the group copied here are model-driven and not empirically based. On the contrary, creating a stratigraphic nomenclature based basically on one basin-fill succession and considering it to be universal can only be characterized as a model-driven approach. I certainly hope that this document will not be accepted by the ISSC as a standing document for sequence stratigraphy. It needs a major revision. Both the scientific content, the way it is presented and the one-sided nature of the debate need to be changed in order to merit publication, either by the ISSC or by any international journal. With very best wishes, Ole Martinsen ------------------------------------------------------------- Ole J. Martinsen, Dr.sc. Geology Manager Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Ole.Martinsen@hydro.com Office phone: 47-55996937 Fax: 47-55995704 Mobile phone: +47-95996274 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ Reply to ISSWG Report from Ashton Embry 4:44 pm 25th April 2007 Dear ISSWG members, Please find attached my critical comments on the ISSWG Report which is posted on the USC webpage. As emphasized in the comments, the deductive approach of ISSWG contrasts sharply with the empirical approach of ISSC. I am somewhat surprised that some of the ISSWG members, who have done some excellent empirical studies, subscribe to such an approach which demands the recognition and correlation of conceptual time surfaces (BSFR, CC). Perhaps with these comments and some good input from the various members who have lots of experience with doing sequence stratigraphy in rocks in a variety of stratigraphic settings, an improved ISSWG report will emerge. I encourage all of you to come to grips with the necessity of being able to reliably and consistently identify those time surfaces (BSFR, CC) in well exposed rock in a variety of geological settings if the overall approach of ISSWG is going to have any practical applicability. ISSC wants to provide reliable guidance on how to recognize various types of sequence stratigraphic units in a variety of stratigraphic settings in the real world. Guidelines on boundary recognition is of course key to this. Right now we have no idea how to provide such guidelines for the recognition of the two time surfaces advocated by ISSWG. Any input anyone can provide on this will be gratefully received. Ashton Embry: AEmbry@nrcan.gc.ca +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On 24 April, 2007, 11.34am. Guy Plint wrote SEQUENCE STRATIGRAPHY AS A ‘CONCRETE’ STRATIGRAPHIC DISCIPLINE Report of the ISSC Task Group on Sequence Stratigraphy The massive scale of this report precluded a detailed commentary; there were very many points that could have been debated, but I was unwilling to devote the time to this, primarily because I am of the firm opinion that it would be time utterly wasted. I was also dismayed at the unnecessarily combative, even snide tone of the discussion. This is not scientific discourse, but a rant. I am keenly aware of the strongly-held viewpoint of at least some members of the committee. We are all products of our experience, for better or for worse, and the individual experiences of the committee are evident from their comments. It is my firmly-held opinion that a process of Darwinian selection will inexorably dictate what elements of sequence stratigraphy survive, and which wither through lack of use. This process is already evident. Therefore, time spent on this sort of geological legislation I see as largely wasted effort. The complexity and variability of the natural world make efforts to cast a set of ‘concrete’ rules such as this quite futile. A fundamental contradiction is built into the central point of the recommendation, which states that a sequence “... is defined in a generic fashion as a unit bounded by a specific type of unconformity and its correlative surfaces”. The authors state that this bounding surface will consist of a subaerial unconformity, a shoreline ravinement surface, and a maximum regressive surface. The implication is that these three surfaces, each of which are of quite different origin, and age range, are somehow ‘correlative’. These surfaces certainly may be mappable, but in no way are they ‘correlative’, and I am quite sure that a sequence bounded by such a hodge-podge composite ‘surface’ will provide little information about the detailed depositional history of a basin. Notwithstanding their vigorous criticism of various well-used sequence stratigraphic surfaces as being unacceptably diachronous, the authors of the report propose a sequence bounding ‘surface’ that is nothing but a chimera, of no temporal significance wwhatsoever. From this point onward (i.e. the Abstract), the credibility of the report, in my opinion, became very low indeed. With reluctance, I feel compelled to comment on three points that drew my attention: 1. Use of ‘shoreline ravinement unconformity’ as an important sequence-bounding surface: Although the diachroneity of transgressive surfaces is acknowledged, it is severely downplayed. It would appear to me that the transgressive surface of erosion and the regressive surface of marine erosion are potentially of equal diachroneity. This is especially true of foreland basins, and possibly rift basins, where accommodation increases landward. 2. The ‘maximum regressive surface’ is also of very doubtful utility. It is influenced by way too many local controls, particularly local sediment supply, and slope. Its recognition in terrestrial clastic strata seems especially problematical. 3. The ‘Data Driven Hierarchy’ “....of boundaries and units is established on the basis of the interpreted (my italics) relative magnitude of the boundaries.” There is immediate scope for operator bias here! The magnitude of base level shift may actually not be at all obvious from the stratigraphic record, - one only has to think of a coastal plain that is subsiding at the same rate that it is aggrading - where the base level change can be large, yet the facies offsets are negligible. A similar situation could be envisaged for a marine setting where accommodation and supply are matched. Using interpreted scales of base level change as a means of ranking sequences seems hazardous indeed. While I applaud the effort and ‘public spirit’ displayed by the authors of the report, I can see no way that its recommendations will help me do a better job of correlating rocks and surfaces, and working out the detailed depositional history and paleogeographic evolution of sedimentary basins. The one point I was relieved to see was #20, that sequence stratigraphic units not be formalized. The current set of allostratigraphic rules work just fine! With tongue in cheek, I suggest that as they rest from their labours, the authors of the Report peruse a splendid little book: The Dorbot of Vacuo, or How to Live with the Fluxus Quo, by Patrick Woodroffe; Dragon’s World Books, Limpsfield, Surrey, 1987, 79 p. The moral of the story is “Learn to live with loose ends” Guy Plint UWO, London, Ontario Canada. 24 April, 2007 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Bruce Hart wrote Apr 23 I have gone through many/most of the documentation (reports, comments, replies) concerning the issc task group's recommendations (embry et al. 2007). It is clear that those recommendations should not be accepted by the issc at this time. There are three major problems: 1) there is obviously no consensus among leading sequence stratigraphers on the utility or applicability of the concepts presented in the report. sure, other people have turf to defend and will resist new ideas. sure, there are problems with "exxonian-style" (wendte's terminology) sequence stratigraphy. however, and whether or not they are correct, the recommendations in the report (r-t sequences) do not represent a consensus but rather one model for defining/analyzing sequences that is being proposed by a limited number of scientists. a broader consensus among leading sequence stratigraphers would be needed, and that is not apparent in the issc report. 2) there is insufficient documentation of the applicability/utility of r-t cycles in a range of depositional settings. more examples from different depositional settings (shallow-water carbonates, deep-water clastics, etc.) and using different types of datasets (seismic, logs, core, outcrop) would be needed. this means that more peer-reviewed papers need to be published that document the utility of the approach. this absence might help to explain the lack of consensus. Alternatively, it may turn out that attempts to apply the r-t approach to other datasets highlight shortcomings of this method. 3) despite assertions to the contrary, there is indeed a confrontational feeling to much of the text. this may not have been intended, but perception is reality and that perception is shared by many of the readers who have posted comments. Bruce Hart, Associate Professor Earth and Planetary Sciences McGill University 3450 University St. Montreal, QC CANADA H3A 2A7 hart@eps.mcgill.ca http://www.eps.mcgill.ca/~hart 1-514-398-3677 (tel) 1-514-398-4680 (fax) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On 4/21/07, 5:18 pm. Henry Posamentier <hposamen1@gmail.com> wrote: I have read with interest the report of the ISSC Task Group on Sequence Stratigraphy, chaired by Ashton Embry. According to the report, the group was tasked with “making recommendations regarding practical and scientifically valid methods and terminology for sequence stratigraphy.” Before I comment on the substance of the manuscript, I would like to backtrack a bit and touch on the group’s mandate as well as the make-up of the working group. I have long believed that the scientific community has a built-in regulatory system of checks and balances. Through the process of peer review involving 100’s of active geoscientists over many years the science moves forward and refines itself very effectively. In this way, the community tends to police itself. And, in essence, with regard to nomenclature and methodology, the community tends to “vote with its feet”. In my opinion the present attempt to codify and formalize the terminology and methodology of sequence stratigraphy is both unnecessary as well as highly presumptuous. A quick look at the technical literature of the past 30 years will reveal that thousands of technical papers that employ sequence stratigraphic concepts have been written. And all this has been accomplished without a formalized or standardized nomenclature in place. The science is and will be dynamic. New terminology has been proposed through the years, while other terminology has been rejected. New concepts or refinements of old concepts have emerged, while others have fallen by the wayside. This is the essence of science. For a small group to undertake to codify the discipline is consequently a highly ambitious task that is likely to have few adherents especially if it is contentious. Consequently the present proposal before us is highly flawed from the outset, in my opinion. It presumes to largely take apart the workings of a discipline that has already been embraced by the overwhelming majority of scientists, and replace it with an entirely new approach and methodology. Science should not proceed by edict as the working paper purports to do. Moreover, though I have no doubt that the members of Embry’s working group are well-intentioned and respected individuals, they do not at first glance seem to represent the disparate thoughts of the stratigraphic community at large. In fairness, no group of five individuals could fairly represent such a wide-ranging discipline. As for the manuscript itself, the tone is combative and confrontational right from the outset. Even the title, wherein the manuscript represents itself as a “concrete” version of sequence stratigraphy, implying that what has come before is less than “concrete”, seems an attempt to capture a phantom moral high ground. But the title notwithstanding, the first 10 pages or so is filled with strong contentious comments like “Proposed systems tracts, which require unrecognizable time surfaces or highly diachronous surfaces as part of their boundaries, are not scientifically valid.” or “considerable confusion and various misconceptions are associated with the methods and terminology for sequence stratigraphy.” or “lithostratigraphy, biostratigraphy, magnetostratigraphy, chemostratigraphy [are] “concrete” stratigraphies [that] contrast with chronostratigraphy which is a “highly interpretive” stratigraphic discipline.” In fact, the entire document is laced with such comments. In conclusion, the Embry report states that “These recommendations [found in this report] rest on a foundation of a few basic principles which govern well-established, stratigraphic disciplines. …past attempts to develop methods and terminology for sequence stratigraphy have not honored these principles.” The “attempts” to which the Embry report specifically refers include Posamentier et al. (1988), Van Wagoner et al. (1990), Emery and Meyers (1996), Posamentier and Allen (1999), Coe et al. (2003), and Catuneaunu, 2006. Regardless of whether one agrees or disagrees with these authors, these papers have been referenced widely over the past 20 years. What the Embry report purports to do is to supplant the body of work that has come before, which has described and shaped a scientific discipline, i.e., sequence stratigraphy, with the opinions of a very small minority of the scientific community. Though changes and modifications of this discipline will and should occur in the future, these earlier works nonetheless have largely stood the test of time in the sense of scientific acceptance. I have grave reservations about numerous technical aspects of this paper. Rather than nitpicking the entire report I will focus on few key issues. The undue emphasis on transgressive and regressive cycles is in my opinion a step backwards. It represents a return to a lithostratigraphic approach in that these cycles are so dependant on sediment supply factors. The processes of transgression and regression records the direction of migration of the shoreline through time and are responses to the balance between sediment supply and space available for sediment to fill – i.e., accommodation. It is common that along a given stretch of coastline one part can be marked by transgression because of relatively low sediment supply whereas just down the coast, regression may be occurring because of relatively high sediment supply. This is the situation along the eastern coast of Borneo, where the coastline in the vicinity of the Mahakam delta is in a regressive phase, whereas the coastline in the vicinity of the Tarrakan delta is in a transgressive phase. This contrast likely is in response to a significantly higher sediment supply associated with the Mahakam River than the Tarrakan River. In the absence of a chronostratigraphically based approach, the R-T cycles of these settings would be very similar and their correlation would lead to erroneous interpretations of paleogeography. Chronostratigraphy, either based on paleontology, magnetostratigraphy, well-log correlations, or seismic data, would be essential for properly constructing a stratgigraphic framework. Were sea level to begin a significant fall tomorrow, both systems would be characterized by forced regression. The R-T approach would not distinguish between deposits of so-called “normal” regression and deposits of overlying “forced” regression. Losing this distinction by lumping these deposits would be most unfortunate inasmuch as forced regressive deposits are associated with significant sediment bypass of the shelf and have the potential to be significantly different from normal regressive deposits. In general, the R-T approach represents a gross over-simplification of a potentially very complex stratigraphic architecture. Such attempts to propose a simple template in lieu of a more complex methodology does little service to the science. The Embry report also largely discounts the utility of multichannel seismic data with respect to identification of key bounding surfaces and sequences. On the face of it, this seems ironic inasmuch as the modern incarnation of sequence stratigraphy has its roots in the analysis of seismic data, and only later migrated into other data sets including well log, outcrop, and core. The argument raised by the Embry report is that “imaginary time surfaces” such as correlative conformities have no outcrop expression and therefore cannot be objectively identified. Consequently, such surfaces in their opinion are unacceptable as sequence boundaries. The Embry report is, indeed, correct that given a small “window to the world” such surfaces cannot be objectively identified. But this is not to say that such surfaces do not exist; in fact, they do. This is where regional data, such as seismic data or long well-log correlation panels, have the advantage over outcrop data. It is through the use of just such data that surfaces of onlap, toplap, and truncation, as well as correlative conformities can be recognized. Seismic data is generally accepted as being a proxy for chronostratigraphically significant time lines and therefore seismic reflections are reasonable approximations of time horizons. The resolution of seismic data is continually improving such that the resolution on conventional seismic data is in some instances less than 3-5 m. High-resolution, shallow-penetration seismic data may have resolution of less than 1 m. Taken even further, ground penetrating radar can have resolution in the 10’s of cm. Such stratigraphic characteristics as onlap and toplap or truncation, as well as downlap, are far more apparent using seismic data than using outcrop data. I can think of very few instances where an onlapping geometry is obvious in outcrop data. Such stratigraphic terminations, which were the backbone of early sequence stratigraphy, are essential for defining stratigraphic discontinuities, are far easier to observe using seismic data. Outcrop data, in its limited availability, can yield such observations only rarely with the exception of beds truncated by channels or other erosive processes. There are numerous technical issues that I could take to task within the report, but that lies beyond the scope of this response. The technical issues upon which I have touched represent merely the tip of the iceberg. My general suggestion is that this report not be published. The mandate for the group, the makeup of the group, and the technical thrust of the report, are aspects that I find unacceptable. Any attempt to impose or dictate a highly simplified scientific doctrine as this paper purports to do, is in my opinion unacceptable. I have participated on enough committees and in enough similarly-themed conferences with highly regarded and pre-eminent scientists involved in the application of sequence stratigraphic concepts to know that attempts to formalize the terminology and methodology are largely exercises in futility. Through time, the self-regulatory nature of the scientific publication process has been highly effective in the past and I have no reason to believe that this will not be the case in the future. Henry Posamentier P.S. Please note that my office email address has changed. I have left Anadarko Petroleum and am now working for Chevron Corporation; my new office email address is Henry.Posamentier@chevron.com. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Comment in support of Embry's Report from "Jack Wendte" <JWendte@nrcan.gc.ca> Apr 2 2006 Dear Mother Kendall, I couldn't pass up an opportunity to present my views on the February, 2007 submission by Embry, et al., to the ISSC Task Group on Sequence Stratigraphy. As you are aware of, I have used a form of sequence stratigraphy, since our days at the Exxon Research Lab in the 1970's, to build a number of three-dimensional models of either Western Canadian Devonian age reef reservoirs (Judy Creek, Redwater, Golden Spike) or a regional scale model of a Devonian mixed carbonate shelf/basinal shale-limestone succession from the east to the west side of the Central Alberta Basin (the Beaverhill Lake succession, September 2005 issue of Bulletin of Canadian Petroleum Geology). All these models are based on the examination of thousands of feet of core and the use of wireline logs. I believe that this qualifies me as creditable reviewer of the manuscript by Embry et al. Just so that readers of your website are aware of my bias, I wish to point out that I do not refer to myself as a "sequence stratigrapher". I refrain from doing so because of the faith-based tenets and model-driven conclusions that many forms of sequence stratigraphy are based on. I would rather be referred to as a carbonate sedimentologist who employs an actualistic mode of sequence stratigraphy in order to build depositional and reservoir models. I find that the submission by Embry et al. addresses the key, but mainly overlooked or ignored, tenets of the various forms of sequence stratigraphy. To evaluate the creditability of the various forms of sequence stratigraphy, I believe that two basic questions need to be addressed. Firstly, what is it being used for? And, secondly, what is the scientific basis for accomplishing the first goal? In this discussion, I will contrast the form of sequence stratigraphy that Embry et al. espouses (T-R Sequence Stratigraphy) and the (unfortunately) more widely used form of sequence stratigraphy that has been referred to by some as"Exxonian-style Sequence Stratigraphy" or by Catuneanu (2002) as being based on the "Depositional Sequence". In addressing my first question, I view sequence stratigraphy purely as a methodology that allows for the correlation of depositional surfaces or boundaries, either in outcrop or in the subsurface. As most subsurface practioners recognize, this is an absolutely necessary step in building three-dimensional, depositional or reservoir models. Unfortunately, it has been my experience that many academicians do not appreciate this point. As an additional caveat, I would stipulate that a creditable form of sequence stratigraphy must provide the means of correlating from shallow-water or shelfal successions to deep-water or basinal successions. The second question is then what is the scientific basis for correlating using the various forms of sequence stratigraphy? Where the tops of the major successions are conformable, there is no significant difference between T-R and "Exxonian-style" sequence stratigraphy. Both forms of sequence stratigraphy correlate a T-R cycle boundary that is either referred to as the "transgressive surface" or as the "maximum regressive surface". The difference between the two methodologies occurs where the tops of the shallow-water or shelfal successions are unconformable. The significant difference between T-R and "Exxonain-style" sequence stratigraphy is the nature of the more basinal "correlative conformity" that is used to correlate to either the subaerial unconformity or the ravinement surface (where it is formed by shoreline erosion of the subaerial unconformity during transgression). In T-R sequence stratigraphy, the correlative conformity corresponds to the transgressive surface (or the maximum regressive surface of Embry et al.). By using this surface, I was able to correlate the major subaerial unconformity in the Beaverhill Lake succession (R4) from shelfs on both the east and west sides of the Central Alberta Basin into and throughout most of the intervening basin. One practical outgrowth of this correlation was the subsequent identification of the stratigraphic position of a siliciclastic sand in a marginal position of my study area, which is the current target for exploration. In "Exxonian-style sequence stratigraphy", the correlative conformity corresponds to a hypothetical surface for which there is no scientific basis for identification. In core or outcrop, this correlative conformity corresponds to some unknown position within an upward-shoaling sequence of facies. In reality, the selection of the position of such a correlative conformity is based more on faith than science and leads ultimately to model-driven conclusions. In my experience in Devonian strata of the Western Canada Basin, this approach has utterly failed to provide a creditable means of correlating the subaerial unconformity to its correlative conformity. In one recent paper employing "Exxonian-style sequence stratigraphy", correlations away from the subaerial unconformity are off by two or more conodont zones. As a scientist, I find it difficult to rationalize as how such an unacceptable form of sequence stratigraphy (Exxonian-style sequence stratigraphy) can be so blindly accepted and embraced by so many sedimentologists and stratigraphers. The Embry et al. submission is a first step in correcting such a faith-based approach to correlation. Your lost son, Jack Wendte Geological Survey of Canada-Calgary +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Reply to Jack Neal from Ashton Embry (ISSC) 10:52 am 27th March 2007 ISSC TG First of all we are most grateful to Jack Neal for taking the time and effort to provide numerous, in-depth comments that criticize ONLY scientific topics addressed in the ISSC Report on Sequence Stratigraphy. His comments will be helpful for our revision of the Report and allow us the opportunity to expand on some topics and to more fully explain others. The only thing we regret is that Jack chose not to address the key critical question of the validity of the two disputed surfaces, the BSFR and the CC. Perhaps he will wrestle with this "elephant in the living room" in a later post. Below, we first reply to Jack's Abbreviated Comments . We have broken Jack's Expanded Comments into 22 points and we present our views on each of them. Jack's comments will be in italics and quotations and will be followed by our reply. Abbreviated Comments: 1) "Concrete" and "Model-driven" vs. "Data-driven" discussions are opinion and not a foundation for objective standardization" These concepts are much more than opinion. They are well accepted concepts that have significance for understanding the evolution of sequence stratigraphy, for appreciating the "realness" of various entities and for seeing how sequence stratigraphy fits with the other stratigraphic disciplines. We will review these very important concepts in a later, separate post. 2) No compelling case is made to modify the Mitchum et al. (1977) definition of a depositional sequence. The need to clarify the term sequence and specific types of sequences comes from Van Wagoner et al's (1988) redefinition of a depositional sequence and from Galloway's (1989) introduction of an entirely different type of sequence. Notably, in our recommendations, we have only slightly modified the Mitchum et al definition and have been able to maintain the meaning of the depositional sequence of Van Wagoner et al and of the "genetic stratigraphic sequence" of Galloway. Furthermore our recommendations allow for new types of sequences to be easily integrated into sequence stratigraphy. The option of status quo (1977) as suggested by Jack was simply not realistic or practical given the later developments and potential future ones. 3) Insufficient perspective is displayed in this proposal to make it widely applicable, including the use of seismic and other subsurface mapping data and the need to make a framework applicable for environments other than ramp or distally-steepened ramp We would argue the surfaces we propose for use and the units which are based on them are applicable to all depositional settings, from ramp to platform/slope/basin, to both carbonates and siliciclastics and to outcrop, well logs and seismic. No other proposed classification system for sequence stratigraphic surfaces and units has such wide application while maintaining scientific integrity. We challenge anyone to demonstrate otherwise. Expanded Comments 1) "lithostratigraphy, biostratigraphy, magnetostratigraphy and Chemostratigraphy ... are interpretive, potentially non-unique, and, excepting lithostratigraphy, not based on observable physical features (component characteristics, yes, but physical surfaces, no). Only sequence stratigraphy is based on physical surfaces in the rock record" Jack misses an important point here. We agree that concrete stratigraphic disciplines like biostratigraphy and magnetostratigraphy are not based on physical surfaces but rather their boundaries (ie stratigraphic surfaces in that discipline) are based on interpreted horizons of property change. Importantly these interpreted changes are based on observable, empirical data. There is no doubt that SOME of the surfaces used in sequence stratigraphy are real physical surfaces that represent a cessation of sedimentation. However other surfaces such as the MFS and MRS are in part interpreted horizons of property change just like those in the other disciplines. In many cases there is no actual physical surface at the change from a coarsening upward succession to a fining upward (MRS) but the stratigraphic surface/boundary/horizon is still placed at that interpreted change in depositional trend in the rock record as can be best determined with the data available. Galloway and Sylvia (2002) provide a nice discussion of these different types of surfaces in sequence stratigraphy. The bottom line is all these surfaces/horizons are based on observable, reproducible data. 2) "sequence stratigraphy is the only discipline that explicitly integrates all available data in the effort to determine "relatively conformable succession of genetically related strata". Given that the whole idea of sequence stratigraphy is to subdivide the column into units, each of which exhibits a "relatively conformable succession of genetically related strata" we have no disagreement here. Of course the big question is how sequence stratigraphy accomplishes this task. As we discuss in the report, it accomplishes this by the determination of surfaces (horizons) that represent changes in depositional trend. Every time you have a change in depositional trend you have a new unit of relatively conformable, genetically related strata. In short, such boundaries separate genetically unrelated strata (for the most part). 3) "The entire "concrete" discussion is opinion and not scientifically valid." The separation of "concrete" stratigraphic units, which are based on observable properties, is absolutely critical and is the feature that links the five stratigraphic disciplines. This is clearly expressed in the NCASN Code in the discussion of "Material categories based on content or physical limits". Such categories are what we call the "concrete" stratigraphic disciplines, a fundamental category of stratigraphic units to which sequence stratigraphy clearly belongs and must belong if we are going to properly establish surfaces and units. Such units and their boundaries are based on observable, reproducible data. This is why they are considered "concrete"(solid basis). 4) "Depositional sequence is clearly defined in the literature (Mitchum et al. 1977) and any deviation from this definition or revision should clearly demonstrate that the existing definition is insufficient". Only if this was so. As discussed in the Report, the Mitchum et al, 1977 definition of a depositional system was an important advance. A major change was introduced by Van Wagoner et al (1988) who declared that a subaerial unconformity was the only type of unconformity worthy of being called an unconformity. This then changed the definition of the deposition sequence into a unit bound by subaerial unconformities or their correlative surfaces. This restricted definition of a deposition sequence stands in contrast to the more general one of Mitchum et al. We think our solution to the problem is by far the best way to go. We use a slightly modified version of Mitchum et al for the definition of the general term sequence and then use a slightly modified version of Van Wagoner et al for the more specific depositional sequence. As a bonus this allows Galloway's concepts and proposed sequence type (genetic stratigraphic sequence) to be integrated and keeps the door open for other types of sequences. Just like Mitchum et al was an improvement over Sloss et al 1949, we feel our definitions allow a much better understanding of the two terms "sequence" and "depositional sequence" and provide a flexible system for further developments 5) "The question "Can the two time surfaces (clinoforms/paleoseafloors), which represent the initiation of base level fall and the initiation of base level rise, be delineated and correlated in a scientifically acceptable manner in a variety of geological settings?" misses the point made above. Nowhere is base level fall or rise invoked in the definition of a depositional sequence. It is defined on physical stratigraphy of genetically related strata. The boundary is the physical surface and its correlative conformity across which strata above are not genetically related to strata below". This is a most important comment and we agree entirely. Our main premise is built on such thinking. As you say, the boundaries of a depositional sequence have to be real surfaces/horizons that subdivide the stratigraphic succession into units of "genetically related" strata. As discussed above, the ONLY way to accomplish this is to recognize REAL surfaces of change in depositional trend. That is why the discussion of the "realness" of the BSFR and CC is right on the point. We claim they are not real surfaces which can be empirically recognized in the rock record in most settings. It would have been helpful to hear Jack's take on this. 6) "Base level rise after a base level fall can produce this relationship." We agree that such a change can produce surfaces of change in depositional trend - an SU or SR-U on the basin margin and an MRS farther basinward. We discuss this in depth in the Report. That is why those surfaces are used as the boundaries of a depositional sequence. Importantly the change from base level rise to base level fall DOES NOT produce a horizon/surface of change in depositional trend over much if not all the marine portion of a basin. Regression (coarsening-upward) in the marine area simply continues and the marine strata deposited just below the time surface of start BL fall are genetically related and essentially identical to those directly above such a time surface. That is why the concept of a BSFR has no theoretical or empirical support (cannot be seen!) for use as a unit boundary in sequence stratigraphy. 7) "Marine above continental sediment is one scenario at one point along a depositional profile. If transgressive erosion removes all evidence of coastal onlap- look updip! Somewhere the evidence will be preserved (transgressive deposits have updip equivalents as shown in ISSC figure 25 & 26 - that is coastal onlap), often times with coastal plain on coastal plain distinguished by paleosol types." This is all fine and what you are describing is simply the creation of an SR (sometimes an SR-U and sometimes an SR-N). Remember the boundary between continental strata below and marine above is in most cases an SR. In many cases it is a highly diachronous SR-N which develops INSIDE the TST. Van Wagoner et al's placement of a systems tract boundary at such a highly diachronous boundary cannot be supported and numerous authors have pointed this out including Catuneanu. 8) A bigger perspective is required to fully interpret local observations - this is a limitation to a framework based largely on outcrop exposures as is the current ISSC proposal. There is no doubt that the more data one has, the more reliable and more precise the sequence stratigraphic analysis. Unfortunately in many instances seismic data are not available or, if they are, the resolution is such that they contribute little, especially if well control is abundant. An organization like ISSC has to make recommendations which are primarily rock-based and which can be implemented when only outcrop data are available. Any other path would be irresponsible and not realistic. Any recommendations, which are NOT applicable to outcrop, have to be rejected. This is non-negotiable. We are trying to produce guidelines for stratigraphers throughout the world and many of these primarily use outcrop data. Of course it is important that our recommendations can also be applied to the subsurface and clearly ours can be, and have been, in many cases. The Catuneanu et al 1997 is a terrific example of our recommendations being used for a comprehensive dataset and revealing some new tectonic insights. We define our surfaces and units on the basis of rock data and then apply them to the less precise, subsurface data sets such as logs and seismic. This is the only way to proceed. 9) "A key component to the definition is the phrase "relatively conformable succession of genetically related strata", which is data resolution-dependent but allows the existing definition to be correctly applied even to today's high resolution data. The ISSC draft report does not discuss this key concept." As discussed earlier, our methodology for surface and unit recognition agrees with this entirely although we clearly have to be more explicit on this topic. It is critical to understand that "relatively conformable successions of genetically related strata" are separated by horizons/surfaces of changes in depositional trend - the surfaces of sequence stratigraphy!. We have determined that the BSFR and CC, as described most recently by Catuneanu (2006), are NOT horizons/surfaces of change in depositional trend and thus DO NOT separate, in most situations, one unit of genetically related strata from a different unit of genetically related strata. There is no change across these boundaries and that is why we cannot see them in rocks and that is why we have rejected them for use as unit boundaries in sequence stratigraphy. At the risk of repeating ourselves, if you or anyone else do not agree with this, we urge you and your colleagues to produce the required data that shows the BSFR and CC are recognizable horizons/surfaces of change in depositional trend in most geological settings. Then, and only then, can they be considered for use as unit boundaries. Remember ISSC is all about classifying strata and we use only valid surfaces/horizons to accomplish this in sequence stratigraphy. 10) "Sequence stratigraphy started as seismic stratigraphy with the true breakthrough of Vail et al. (1977), that seismic reflections follow geologic time lines", We cannot agree with these statements and we hope most stratigraphers would also not agree. First of all, sequence stratigraphy officially started in 1949 some 28 years before Vail et al (1977)(see our Report - The Early Years). Many people practiced sequence stratigraphy in those 28 years including myself. A line from Embry and Klovan (1974) which describes the Devonian clastic wedge of the Arctic Islands demonstrates this "Three depositional sequences comprise the wedge". Back then we used the term deposition sequence for an unconformity-only bounded unit. There is no doubt that Vail et (1977) made major contributions to sequence stratigraphy as well described in our report (Revitalization with Seismic Data) but he definitely did not start it. From the perspective of today, he arrived at half time As discussed in the Report, seismic reflectors do NOT follow time lines, especially parallel to strike. They parallel stratigraphic surfaces which have a low diachroniety (mainly MFSs, MRSs). There is a big difference between these two concepts. 11) "The ISSC draft report specifically excludes seismic stratigraphy from the discussion by dismissing the tool resolution, effectively excluding a large percentage of the user population to whom the report is aimed and the vast majority of subsurface problems for which the method is needed". This is somewhat of an overstatement and the relationship between seismic and sequence stratigraphy is discussed in the report. We do agree that such a discussion has to be expanded and this will not be difficult. However when it comes to defining specific surfaces to be used in sequence stratigraphy, rock data have to be used. If you cannot see a surface in most cases in well-exposed rocks then it has no value as a surface of sequence stratigraphy or for unit definition. This seems self-evident. Once the surfaces and consequent units are defined through rock-based data, then they can be applied to seismic data. Thus seismic is not critical for a discussion of unit classification in sequence stratigraphy because the surfaces and consequent units have to be defined with rock data. There is no doubt seismic is helpful for pointing researchers to what surfaces MIGHT be useable, but the final decision of applicability rests in the rock data. As discussed, there are insufficient rook data to support the existence and use of the BSFR and CC in sequence stratigraphy and thus all proposed units which require such conceptual entities for boundaries are rejected. We have no doubt that all seismic interpreters can use our recommendations. It just comes down to how they interpret given seismic reflectors. Interpreting them in terms of undocumented, hypothetical surfaces is not a way to go. We have provided the types of sequence stratigraphic surfaces that can be found "inside" seismic reflectors. 12) "Portrayal of everything derived by "Exxon" researchers as "model-driven" in contrast to the "data-driven" alternative is not scientifically valid. By inference, anyone interpreting depositional sequences from seismic data or other methods not espoused by the ISSC report is doing misguided work." You are putting words in our mouth here and are not portraying our position fairly. First of all it is clearly stated in the Report that the work of Vail et al (Mitchum et al) was inductive and thus data-driven (p.19). Secondly, the work of Posamentier et al is clearly model-driven. Importantly there is nothing inferior about a model-driven approach compared to a data-driven approach. We will be elaborating on this most important subject later this week in a separate post. When it comes to defining real surfaces and real units, a data-driven approach is required and a model-driven approach, which can be extremely valuable in other contexts, is not appropriate or valid for this specific endeavour. Finally we have never said or even implied that interpreting depositional sequences on seismic data is misguided. This is valid, data-driven interpretation procedure as long as the seismic reflectors are interpreted in terms of real, well-defined, data-driven (ie rock-based) surfaces of sequence stratigraphy. Making up new types of sequence stratigraphic surfaces on the basis of seismic is NOT data-driven but is model-driven. This can be of value if such surfaces prove to be "real' and valid through subsequent proper inductive research on the rock record. 13) "The efforts to contrast past studies in a belittling light is counter-productive." Again this is not a fair representation of our work. We do not belittle a model-driven approach as a scientific endeavour. Furthermore to criticize a past study ( a very necessary scientific endeavour) does not entail belittling it. We certainly do not feel your criticisms of our report have belittled it. As discussed above, a model-driven approach is just not the proper approach to use if one wants to define real surfaces and units that can be employed by geologists throughout the world. For this you need a data-driven approach. We have only pointed out that the BSFR and CC are model-driven surfaces and are not appropriate for defining units of sequence stratigraphy. All we ask is that enough data be presented so that such surfaces can be considered to be data-driven surfaces which would then make then appropriate boundaries for units of sequence stratigraphy. Until then, our hands are tied. 14) "Onlap, and the distinction between coastal onlap vs. submarine onlap is entirely missing from the discussion. Understanding of this difference is the key to determining "relatively conformable" from unconformable in a sequence stratigraphic framework." We have discussed coastal onlap as one of the key properties of an SU, a surface which is part of a depositional sequence boundary. Submarine onlap is a property of an SR-U as discussed and can be a property of a submarine unconformity (shelf/ slope/basin). We have no problem with seeing onlap as a feature of an unconformable surface and the type of strata onlapping the surface and the physiographic setting often determine the specific type of unconformity one is dealing with. This is well accepted and we are in agreement. Of course when the onlap is based on seismic data, resolution becomes a key factor and the onlap/downlap can well be an artifact of the data set. Again this is all well accepted and we are in agreement. 15) "Order" is a time term, not a rock term. Definitions of order in the literature vary significantly (see discussion by Schlager, 2004). Use of order terminology must be accompanied by time scale data (e. g. biostratigraphy or radiometric age dating) and referenced to a published scale. Magnitude is different from order. Large magnitude events can be short duration and small magnitude can be long duration as discussed in the proposal. A hierarchal approach is preferred with the depositional sequence as the basic unit, nesting into sequence sets and composite sequences and so on as described in Mitchum and Van Wagoner (1991)." Order is simply a classification term which allows sequences to be assigned to different classes (orders). The reasons why this has to be done are clearly laid out in our report. The question becomes how to best to do this. As discussed, boundary frequency is not a valid way because one has to know what a 3rd order boundary is before its frequency can be determined. The method of nested orders or named classes is the same as the one we propose. Identify the large ones, which you perhaps call super sequences or super sets or whatever, and then work your way down. This is an exercise in classes identified on the basis of magnitude as we have also advocated (as has Catuneanu). We just call the various classes "orders" which keeps it simple. It seems you want to give each class a specific name. We, like many other workers, but unlike you, see a depositional sequence as scaleless (see Catuneanu for a good discussion of this) and this is why we use the term order. It gets very tricky to try to implement what you propose. What scale is deserving of the term depositional sequence with all others being called something else. We rejected such a complex scheme as basically unworkable. Finally we must note that such an ordering scheme works because big events which generate big boundaries are rare and little events which generate small boundaries are very common. If the opposite was true we could not have much of a hierarchy. We suspect that depositional sequence boundaries have a fractal distribution as suggested by Schlager (2005) but the data are not sufficient yet to test this. 16) "Definitions based on depositional trend is too vague (does delta lobe switching count?)" You have validated such an approach by emphasizing that sequence stratigraphy is the practice of dividing the succession into units of genetically related strata. The only way to do this is to identify surfaces/horizons of change in depositional trend. This is not vague. It is the only way to go!! 17) "Shoreline ravinement and maximum regressive surface are within-trend surfaces and dependent on shoreline configuration, point source deposition, subsidence variations, initial depositional profile, and position along that profile and are therefore inappropriate choices for universal key bounding surfaces as opposed to the depositional sequence boundary and the maximum flooding surface." A shoreline ravinement is an erosional surface (change in depositional trend from deposition to non-deposition) so is a surface of sequence stratigraphy and not one of lithostratigraphy. The MRS is clearly a change in depositional trend (CU to FU) just as the MFS is (FU to CU). The generation of such surfaces at a given point does depend on the factors you mention to an extent but you cannot escape this. We have noticed you have not shared your preference for what surfaces combine to form a through going depositional sequence boundary. We all agree the SU and SR-U form the main unconformable surface. How do you suggest we extend the SB basinward? What do you consider to be "universal key bounding surfaces"? If you don't like our recommendation of the MRS for extending a depositional sequence boundary basinward, what surface do you prefer? As long as it is a rock-based surface which represents a change in deposition trend and has characteristic properties to allow its identification, we will definitely consider it. So please let us all know what your preferred alternative is and we can weigh it against our proposal. That is what this debate is all about. 18) Maximum flooding surface is not an unconformity and workers basing sequence boundaries on maximum flooding surfaces, recognizing that internal unconformities exist within, already call this method Genetic Sequence Stratigraphy (Galloway, 1989) There is little doubt that in many instances at least part of an MFS is an unconformity (mainly non-depositional) and everyone portrays this on Wheeler diagrams including papers by Exxon authors. We have discussed the use of MFSs as sequence boundaries. The name of such an entity is open for discussion. Galloway proposed genetic stratigraphic sequence and we have suggested R/T sequence for the same unit. The name we end up agreeing on is not a big deal. What is your preference? Our methods at least allow the integration of such a sequence type into sequence stratigraphy. As far as we can tell Exxon workers do not accept the validity of such a sequence type which clearly fits the Mitchum et al (and ours) definition of a sequence and is well supported by empirical data. 19) "Revisions to systems tract terminology are moot until the depositional sequence definition is settled. Following that, definitions would ideally not rely on sea-level position terminology, but rather accommodation/sediment fill rate relations expressed in objective physical stratigraphic stacking". Clearly we had to make recommendations on systems tracts. If we had not, I am sure we would have been strongly criticized for such a huge omission. We hope you agree that, the type and number of systems tracts that are potentially definable, depend on the number of valid sequence stratigraphic surfaces that are defined on rock-based data. Catuneanu in his position paper advocates for 7 surfaces which yields 4 systems tracts. We argue for only 5 valid surfaces with only 4 being suitable for systems tract definition. This gives us only two systems tracts. It all comes down to the validity of the BSFR and CC. We have to emphasize that systems tracts like other sequence stratigraphic units are defined on the basis of surfaces of sequence stratigraphy. Your suggestion of the use of "stacking patterns" has some applicability in some settings but not in many others. In the settings where small scale sequences are recognizable, their stacking pattern is simply another way of saying what the grain size or water depth trend is. Given such "stacking pattern"-defined trends (eg progradational versus retrogradational), it is straightforward to delineate a horizon/surface of change in depositional trend (eg a MRS at the change from progradational to retrogradational). This is standard procedure for determining various horizons/surfaces which then define various units. We can see we will have to me much more explicit when it comes to the use of stacking patterns for defining specific surfaces of sequence stratigraphy which are then used to define specific units of sequence stratigraphy. Note the degree of separation of stacking pattern from eventual unit definition and delineation. This is often not appreciated. 20) "Parasequences are a defined unit and the ISSC report has not demonstrated that they are invalid" We have to disagree with this assertion. We have clearly shown that the boundaries of a parasequence as defined by Van Wagoner et al (1988, 1990) are NOT surfaces of sequence stratigraphy but are lithostratigraphic surfaces. Thus parasequences are not valid sequence stratigraphic units BUT are valid lithostratigraphic units. Just imagine if those authors had defined a parasequence on the basis of changes in fossil content rather than on changes in lithology. Parasequences would then be biostratigraphic units. We hope you can see that the nature of the property change which defines a horizon/surface/boundary defines the type of stratigraphic discipline that the consequent unit belongs to. If you want parasequences to be valid sequence stratigraphic units they must be bound by one or more surfaces of sequence stratigraphy. As discussed in the Report, suggested revisions of a parasequence boundary have made it redundant with other previously defined units of sequence stratigraphy. If you want to stick with your statement that parasequences are valid sequence stratigraphic units, we need some logical arguments not just an assertion 21) "Order should be reserved for time units and appropriately referenced to a published framework with supporting geologic time data." This has been discussed above and in the Report. The term Order with an accompanying number is a convenient name for different classes of sequences recognized on the basis of magnitude. Your suggestion of using specific names for each magnitude class is extremely unwieldy and basically unworkable. It was considered as an option but was rejected. 22) "Hierarchies of stacking (e.g. Mitchum and Van Wagoner, 1991) are preferred to recognize long and short-term trends with varying magnitude events" The existence of nice, well behaved hierarchies of stacking is a model-driven ideal which has not been confirmed by empirical data. In fact our experiences with real world strata demonstrate that such well behaved and, dare we say, well stacked successions are not the norm in many situations. This is to be expected given the vagaries of sediment supply. Summary In summary, Jack's very thoughtful and in-depth comments have shown us where we have to "beef-up" our Report and we are deeply grateful to him for this. None of his arguments have any notable effect on our recommendations. We are still looking for substantial comment on the validity of the BSFR and CC conceptual surfaces. As discussed in other posts, this is the main difference between our recommendations and those of Catuneanu in his position paper. Once this is settled, everything else is minor. Jack, for whatever reason, did not address this critical issue. We are hopeful he will in the future when he shares what specific surfaces he recommends to use as a boundary of a depositional sequence. As noted in our initial distribution of our Report, we are most interested to hear alternative recommendations for those that are deemed unsuitable. If not the MRS for extending the depositional sequence boundary basinward, then what documented, rock-based surface do you recommend? This is the crux of the matter and this cannot be dodged if we are going to resolve key issues. References Most of the references are well known ones, can be found in our report and are not repeated here. The two new ones are below. Catuneanu, O, Sweet, A. and Miall, A., 1997, Reciprocal architecture of Bearpaw T-R sequences, uppermost Cretaceous. Bulletin of Canadian Petroleum Geology, v. 45, p. 75-94 Embry, A. and Klovan, J.E., 1974, The Devonian clastic wedge of the Canadian Arctic Archipelago. In Abstracts with program, GSA 1974 Annual Meeting, Miami, FL. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ jack.e.neal@exxonmobil.com - am 26th March 2007 Following the suggestion of Christopher Kendall (and the use of his excellent website as a forum) to keep the debate scientific, below are some specific comments on the ISSC draft report: Abbreviated Comments: * "Concrete" and "Model-driven" vs. "Data-driven" discussions are opinion and not a foundation for objective standardization * No compelling case is made to modify the Mitchum et al. (1977) definition of a depositional sequence. * Insufficient perspective is displayed in this proposal to make it widely applicable, including the use of seismic and other subsurface mapping data and the need to make a framework applicable for environments other than ramp or distally-steepened ramp Specific comments on Recommendation are found below Expanded Comments: * The draft proposal of the ISSC to distinguish "concrete" stratigraphies ("one in which the recognized surfaces in that discipline, which are used for correlation and bounding specific unit types, can be defined and delineated on the basis of observable physical features) using lithostratigraphy, biostratigraphy, magnetostratigraphy and chemostratigraphy as examples is fundamentally flawed. All these methods are interpretive, potentially non-unique, and, excepting lithostratigraphy, not based on observable physical features (component characteristics, yes, but physical surfaces, no). Only sequence stratigraphy is based on physical surfaces in the rock record and is the only discipline that explicitly integrates all available data in the effort to determine "relatively conformable succession of genetically related strata". * The entire "concrete" discussion is opinion and not scientifically valid. Depositional sequence is clearly defined in the literature (Mitchum et al. 1977) and any deviation from this definition or revision should clearly demonstrate that the existing definition is insufficient. The ISSC draft report does not make this case. * The question "Can the two time surfaces (clinoforms/paleoseafloors), which represent the initiation of base level fall and the initiation of base level rise, be delineated and correlated in a scientifically acceptable manner in a variety of geological settings?" misses the point made above. Nowhere is base level fall or rise invoked in the definition of a depositional sequence. It is defined on physical stratigraphy of genetically related strata. The boundary is the physical surface and its correlative conformity across which strata above are not genetically related to strata below. Base level rise after a base level fall can produce this relationship. Marine above continental sediment is one scenario at one point along a depositional profile. If transgressive erosion removes all evidence of coastal onlap - look updip! Somewhere the evidence will be preserved (transgressive deposits have updip equivalents as shown in ISSC figure 25 & 26 - that is coastal onlap), often times with coastal plain on coastal plain distinguished by paleosol types. A bigger perspective is required to fully interpret local observations - this is a limitation to a framework based largely on outcrop exposures as is the current ISSC proposal. * A key component to the definition is the phrase "relatively conformable succession of genetically related strata", which is data resolution-dependent but allows the existing definition to be correctly applied even to today's high resolution data. The ISSC draft report does not discuss this key concept except in reference to parasequence boundaries, where it has no relevance. * Sequence stratigraphy started as seismic stratigraphy with the true breakthrough of Vail et al. (1977), that seismic reflections follow geologic time lines, demonstrated in integrated datasets (those that resolve physical stratigraphy represented by seismic reflection or parasequence boundary terminations and biostratigraphy or other means to establish chronostratigraphic correlation). The ISSC draft report specifically excludes seismic stratigraphy from the discussion by dismissing the tool resolution, effectively excluding a large percentage of the user population to whom the report is aimed and the vast majority of subsurface problems for which the method is needed. * Portrayal of everything derived by "Exxon" researchers as "model-driven" in contrast to the "data-driven" alternative is not scientifically valid. By inference, anyone interpreting depositional sequences from seismic data or other methods not espoused by the ISSC report is doing misguided work. All interpretations of data are "model-driven" to some extent (e. g. facies models). The efforts to contrast past studies in a belittling light is counter-productive. * Onlap, and the distinction between coastal onlap vs. submarine onlap is entirely missing from the discussion. Understanding of this difference is the key to determining "relatively conformable" from unconformable in a sequence stratigraphic framework. Numerous erosional surfaces can be identified (e.g. distributary channel scours, marine ravinement, slope failure, etc.), that may or may not be related to allocyclic forcing. Coastal onlap surfaces are produced above a depositional sequence boundary, observable on subsurface data and mappable. Marine onlap (or downlap) can occur throughout a depositional sequence and may or may not be unconformable given sufficient data resolution * "Order" is a time term, not a rock term. Definitions of order in the literature vary significantly (see discussion by Schlager, 2004). Use of order terminology must be accompanied by time scale data (e. g. biostratigraphy or radiometric age dating) and referenced to a published scale. Magnitude is different from order. Large magnitude events can be short duration and small magnitude can be long duration as discussed in the proposal. * A hierarchal approach is preferred with the depositional sequence as the basic unit, nesting into sequence sets and composite sequences and so on as described in Mitchum and Van Wagoner (1991). Since chronostratigraphy is dismissed in this proposal, so too should be order. Regarding ISSC Recommendations * Definitions based on depositional trend is too vague (does delta lobe switching count?) * Shoreline ravinement and maximum regressive surface are within-trend surfaces and dependent on shoreline configuration, point source deposition, subsidence variations, initial depositional profile, and position along that profile and are therefore inappropriate choices for universal key bounding surfaces as opposed to the depositional sequence boundary and the maximum flooding surface. This invalidates the proposed acceptance of the T-R sequence as separate from a depositional sequence. * Maximum flooding surface is not an unconformity and workers basing sequence boundaries on maximum flooding surfaces, recognizing that internal unconformities exist within, already call this method Genetic Sequence Stratigraphy (Galloway, 1989) * Revisions to systems tract terminology are moot until the depositional sequence definition is settled. Following that, definitions would ideally not rely on sea-level position terminology, but rather accommodation/sediment fill rate relations expressed in objective physical stratigraphic stacking * Parasequences are a defined unit and the ISSC report has not demonstrated that they are invalid * Order should be reserved for time units and appropriately referenced to a published framework with supporting geologic time data. Hierarchies of stacking (e.g. Mitchum and Van Wagoner, 1991) are preferred to recognize long and short-term trends with varying magnitude events Jack Neal CORP-GP8-429 ExxonMobil Exploration Co., 233 Benmar 77060 Ph: 281-654-1655, Cell: 713-299-6671 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Octavian Catuneanu to Maria Cita pm 25th March 2007 Dear Maria Many thanks for your message and your encouragement for open scientific debate. I support the statement in your newsletter document (attached - see page 5 for relevant discussion) that a correct and fair "integrated stratigraphic scenario" should be placed under the umbrella of the ISSC. It is for this reason that I have been the unofficial "spokesperson" speaking for a segment of the sequence stratigraphic community that sees the ISSC task group formed by Ashton Embry as representing the exclusive interests of a minority group in the sequence stratigraphic world. Unfortunately the position of the group I represent ( i.e., all other schools = the majority of people applying sequence stratigraphy) has been misrepresented while none of the major contributors to, or founders of the various sequence stratigraphic "schools" have been included in the ISSC task group. The "scientifically incorrect implications" mentioned in your letter are another important consideration. As I pointed out in my comments to the preliminary ISSC report significant inaccuracies are present and in my opinion, serious peer-review is required before its publication should take place. In order to ensure fairness and a balanced perspective, leaders of the various sequence stratigraphic schools should be involved in this review process. This would lend strength to final ISSC recommendations and would mean that many world-class practitioners of sequence stratigraphy would have access to a balanced format to work from. The International Working Group on Sequence Stratigraphy has tried to make our group inclusive and includes 20 world-class stratigraphers who belong to different schools of thought (I have not included myself on that list), and our document on the standardization of sequence stratigraphy will be made available to you later this year. Best regards, Octavian P.S.: we did meet about 3-4 years ago at one of the NACSN annual meetings, and I look forward to meeting you again."
Illustration to Zoran Kunštek's text message