accommodation

accommodation succession

allocyclic

allostratigraphy

autocyclic

base level

bed

bedding plane

boundaries

depositional elements

depositional sequence

depositional system

eustasy

genetic sequence

hierarchical boundaries

hierarchical elements

lamina

maximum flooding surface (mfs)

transgressive-regressive sequence

transgressive surface

sequence

sequence boundaries

sequence stratigraphic debate

sequence stratigraphic models

Stokes surfaces

vertical facies successions

 

 

Sequence stratigraphy provides a powerful framework for sedimentary interpretation that, in conjunction with biota and sedimentary structures, enables the conceptual modeling of the different combinations of "sedimentary processes", including eustasy, responsible for the hierarchies of geometry or elemental packaging exhibited by the lithofacies. The definition, terminology and interpretation of the "surfaces" of sequence stratigraphy is complex and sometimes contentious. This site and its heavy dependence on the interpretation of terminology, as in this menu box, is an attempt to clarify the understanding and use of this discipline of stratigraphy.

The sequence stratigraphy method of stratigraphic interpretation combines the stratal architecture (geometric relationship) of the sedimentary facies within a framework of geometries assumed to be the result of the repeated fill of accommodation by successions (Neal and Abreu, 2009) of sedimentary strata and the chronological order of their accumulation to enhance the interpretation of depositional setting and prediction of stratal continuity (Catuneanu, et al 2011). The analytical steps of sequence stratigraphic first identify the genetic units and boundaries that compose the stratigraphic section in outcrop, core, well-log and seismic data. The framework formed by the genetic units and boundaries is tied depositional models that explain the recurring character of the sequence stratigraphic units and surfaces and provides a common terminology for each of these models, whatever the tectonic setting, depositional setting, sediment types (siliciclastics, carbonates, evaporites) data set that is available for analysis (e.g., seismic data versus well logs or outcrop). It is generally assumed that many of the strata and surfaces are related to changes in the position of relative sea-level driven by eustasy.

The sequence stratigraphic methodology establishesd the order in which bodies of sediment were laid down and their geometric relationship to each other. It guides the interpretation of the origins of sedimentary rocks enabling the mapping and interpretation of single and multiple cycles of sedimentary rocks. The cycles are often in the form of vertically stacked associated sedimentary facies whose repeated occurrences often have similar geometric organization. These geometries and their boundaries are closely associated with changes in accommodation (space available for sediments to fill).

The basic sequence stratigraphic framework is formed by surfaces of erosion and non-deposition (sequence boundaries), and flooding (trangressive surfaces and/or maximum flooding surfaces [mfs]). There is an implicit but sometimes unstated connection between the external and internal surfaces of a sequence to relative sea-level change driven by eustasy. As indicated above the surfaces and encompassed strata are assumed to have time significance and so provide a relative time framework for the sedimentary succession. This latter makes sequence stratigraphy a branch of allostratigraphy. The technique of subdividing sedimentary rock on the basis of surfaces leads to a better understanding the inter-relationship of the depositional settings and their lateral correlation. Hence the definition that "sequence stratigraphy" is the study of rock relationships within a time-stratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformities (Posamentier et al., 1988; Van Wagoner et al., 1988).

Allostratigraphy versus sequence stratigraphy
In terms of the original definitions of “sequence stratigraphy” this methodology is applied when changes in the accommodation of sedimentary rocks are interpreted to have been the products of allocyclic change, i.e. base level change, caused by changing rates of eustatic and tectonic movement, and changing rates of sedimentation (Vail et al, 1977; Posamentier, et al, 1988; Van Wagoner et al, 1988; Van Wagoner, et al, 1990). However as sequence stratigraphy has become more widely used so the order of these layers and the surfaces that bound them are also often tied to the sequential packaging of autocyclic processes that are the result of point migration and mass wasting events, rather than base level change. This latter is because most petroleum and other stratigraphers, including academic geologists, analyze local fields, plays, or local geologic facies geometries and are unable to resolve “regional” versus “local” surfaces and sediment bodies. In the interest of the simplification of this problem these same stratigraphers avoid the confusion of what “regional” versus “local” surfaces ("autostratigraphic", "allostratigraphic" versus "sequence stratigraphic") by considering these surfaces and bodies as all sequence stratigraphic.

In spite of this simplification, or because of it, these same earth scientists working with local bodies of carbonate and clastic bodies are able to build accurate predictive models of the facies geometries for their locality by identifying repeated cycles of sediment bound by correlatable, but local, surfaces. These same geologists then often maintain they are using sequence stratigraphy to make their analysis, basing their fallacious assertion on the fact that their predictions are "spot on". To the contrary the "pure" sequence stratigrapher argues that of most sedimentary stratigraphers though they have made accurate lithofacies predictions, have “blurred” the meaning of sequence stratigraphy and their approach has become more allostratigraphic and autostratigraphic than sequence stratigraphic.

This more general lumping of sequence stratigraphy with allostratigraphy, sedimentology and/or sedimentary stratigraphy means that a student’s understanding of sequence stratigraphy may become confused. Additionally this “fuzzy” application of sequence stratigraphy has probably aided the evolution of the different viewpoints of the various sequence stratigraphic models (Depositional, Genetic, or T-R sequences) and leads to the major points of their disagreement. Some of these differences of opinion are expressed in the sequence stratigraphy terminology debate of this web site. Other problems of confusion associated with sequence stratigraphy are tied to the origins of the real and sometimes hypothetical surfaces associated with the stratigraphic column. Expressly are these surfaces confined to the area study or do they extend beyond it? If the answer to this question is unknown but assumed, this should be stated up front or emotional words are likely to be exchanged?

So as you become more familiar with the literature or the geology of the terrains you study you will often find that only the most obvious low frequency sequence stratigraphic layers can be identified. This means that sequence stratigraphy frequently only supplies a low frequency allocyclic template to the high frequency autocyclic processes within small-restricted areas, often only a few kilometers across at the most. These sequences may be identified from deep to shallow water settings, and even may include continental settings. The products of base level changes for continental settings are not quite the same as those of coastal settings, or a marine shelf or the deeper down slope. Facies assemblages and subdividing surfaces will be found to change with setting, be they associated with aeolian, glacial, fluvial, alluvial fan, deltaic barrier, margin and slope and distal slope and basin settings? For instance the Stokes surfaces of aeolian dune fields are different to channel boundaries, or those associated with evaporite basin shape, etc. Some of the framework of surfaces and sedimentary bodies from these different settings can be tied to base level change and other sedimentary elements are tied to autocyclic change. The causes of these allocyclic and autocyclic events may be difficult to impossible to separate, so when the geological interpreter decides to arbitrarily separate these they should state that this is what they are doing!

References
Catuneanu, Octavian, William E. Galloway, Christopher G. St. C. Kendall, Andrew D. Miall, Henry W. Posamentier, André Strasser, and Maurice E. Tucker, 2011, sequence stratigraphy: Methodology and Nomenclature, Newsletters on stratigraphy, Vol. 44/3, 173–245 Stuttgart (November 2011 50 figures and 1 table)

 Neal, J., and Abreu, V., 2009, Sequence stratigraphy hierarchy and the accommodation succession method, Geology, v. 37, p. 779-782
Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988, eustatic controls on clastic deposition. I. Conceptual framework. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes––An Integrated Approach, vol. 42. SEPM Special Publication, pp. 110– 124. 
Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S., Hardenbol, J., 1988, An overview of sequence stratigraphy and key definitions. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes––An Integrated Approach, vol. 42. SEPM Special Publication, pp. 39–45.
Van Wagoner, J.C., Mitchum, R.M., Campion, K.M., Rahmanian, V.D., 1990. Siliciclastic sequence stratigraphy in Well Logs, Cores, and Outcrops: Concepts for High-Resolution Correlation of Time And Facies. AAPG Methods Explor. Ser., 7.