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Carbonate
system tract boundaries, hierarchies & stacking patterns
Differences between carbonates and clastics
Carbonates have some similarities to clastics but major differences in the sequence
stratigraphy of the two sediments exist. While both respond to changes
in base level and both can be subdivided by similar
surfaces ,
the difference in the sequence stratigraphy of these sediment types
is related to carbonate accumulation tending to be "in situ
production" while clastics are transported to their depositional
resting place.
Rates of carbonate production are linked to photosynthesis and so
are depth dependent and greatest close to the air/sea interface.
This favors carbonate facies and their fabrics as clear indicators of sea level position .
Additionally carbonate sediments often have a biochemical origin
and are influenced by the chemistry of the water from which they
are precipitated. Thus the character of a carbonate sediment can
change as the plate tectonic configuration of the depositional setting
of the basin responds to pale-climate change, and/or changes in
paleogeography related to isolation or access to the open sea. This
means that carbonates can be used as indicators of depositional
setting that, when combined with sequence stratigraphy, make carbonate
facies analysis a powerful tool for the interpretation of the geological
section and lithofacies prediction away from data rich areas.
Subdividing Surfaces
As with clastic rocks carbonates can be subdivided on the basis
of bounding
and internal surfaces into sequences,
parasequences
and/or truncated carbonate cycles. These can include
It should
be emphasized that, as has been shown by Fischer
(1964), Pomar & Ward (1999), Goldhammer,et
al, (1990), and D'Argenio
et al (1997), that though shallow cycles of carbonate are composed
of a relatively conformable succession of genetically related beds
or bedsets
these cycles are often truncated and incomplete so that maximum
flooding and trangressive surfaces can be missing. This means that
these cycles are not, in the strictest sense, a match for the clastic
models of parasequences described by Van Wagoner et al, (1999).
However as one analyzes the cycles we argue that they can be used
like parasequences to
propose and build process/product oriented depositional models.
However should they exhibit truncated cycles and miss the sediments
of an initial transgression or maximum flooding event one should
consider them as high frequency carbonates cycles, not parasequences.
Carbonate
Response to Relative Sea Level Change - Links to Movies, Exercises & .pdf files.
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Carbonate
system tracts & enveloping surfaces; responses to sea
level change. |
Sea level rise & fall induce onlapping ramp, and forced
regression of offlapping system tract (OST) (Pomar,
1991) |
Lower
rates of carbonate accumulation of interior shelf produce
lagoon which gives up in response to sea level rise. |
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Early
Cretaceous Shaybah Formation, UAE, carbonate margin responds
to sea level change (Kendall et al, 2000) (below). |
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Upper
Devonian Judy Creek carbonate build up responds and "Gives
up" with sea level change (Scaturo,
et al, 1989), |
Late
Jurassic of Nequen Basin mixed carbonate & clastic system
tracts with enveloping surfaces; responding to sea level change
(Mitchum and
Uliana, 1985). |
Sedpak
simulation of evolving carbonate and clastic geometries and
cross section responding to changing base level.
Link to full explanation |
The Carbonate
cycle of a Sigmoid verus Parasequence explained
The basic reefal accretional unit of the Miocene reef complex of
Mallorca is the "sigmoid ".
This is bounded by clear erosion surfaces (the product of sea level
lowering and erosion with a matching correlative surface downdip)
but has no obvious marine flooding surfaces . Updip and landward the sigmoid is represented by a
horizontal lagoonal bed that basinward passes in sigmoidal bedded
reef-core lithofacies belt and seaward into clinoform bedded forereef
slope beds and sub-horizontal basinal lithofacies. The boundary
over the lagoonal and reef-core lithofacies of the sigmoid is formed
by an erosional surface that basinward becomes a correlative conformable
surface in the reef slope and basin lithofacies. Notably, the coral-morphology
zonation within the reef-core facies of the sigmoid migrates seaward,
aggrades vertically, or moves landward over the bounding erosional
surfaces. This enables the sigmoid (like system's tracts)
to be tied to specific segments of the sea-level curve. Consequently
the sigmoid can be considered "genetically" as a depositional
sequence, though not exactly fitting the original definition of
a parasequence. This is because the sigmoid, like the
parasequence ,
is composed of a relatively conformable succession of genetically
related beds
or bedsets .
Also the geometric patterns shown by stacked sigmoids
can be used, along with their position within a sequence, like the
patterns of stacked parasequence
sets, to define system-tracts ,
while within lower order depositional sequences there
are sigmoid sets, sigmoid cosets and megasets.
In the interests of keeping the sequence stratigraphic literature
from becoming over complex it is argued here that during the time
interval between the development of the erosional surface on the
underlying sigmoid
and the deposition of sediment marking the boundary of the overlying
sigmoid, sea level dropped to be followed by a trangressive flooding
event and the development of a maximum flooding surface. However
since no sedimentary fill has been recognized that records these
events, the sigmoid cannot be inferred to be equivalent to the parasequence,
or vice versa! Similarly this "simplification" should
not be applied to a shoaling upward carbonate cycle missing transgressive
or maximum flooding sediments. In this case the Transgression surfaces
(TS)
and maximum flooding surfaces (mfs)
are not equivalent to erosion surfaces initially
produced by a sea level fall, since the missing sediments mean that
one cannot establish how the erosion surface was modified on the
following transgression. Clearly the truncated high frequency carbonate
cycle may have different genetic elements to a parasequence and
should not be considered to be one! It should be noted that because "modern" type of reefal systems are able to build rigid
frameworks, resistant to wave energy, this depositional system has
the capacity to record even the highest-frequency sea-level cycles.
Thus some sigmoids appear to record 7th order sea-level cycles that
represent a periodicity of few-thousand years! Other depositional
systems that have not produced this "rigid framework" to the sea level are not able to record such high-frequency cycles
of sea level and parasequences may form.
Pomar (personal communication,
2004) proposes that parasequences form in response to sea-level
para-cycles (rise and stillstand of sea level), commonly as a response
of sea-level cyclicity when subsidence equals or exceeds the amount
of sea-level fall, OR when the sedimentary systems are dominated
by loose grains. In this latter case, lowering of base level (related
to the fall in sea level) would increase basinward shedding of sediment
and these erosional processes onto a granular seabed would not be
recorded as an erosion surface. This could be the reason that higher-frequency
sequences (simple sequences in Vail's definition) at the most commonly
record up to 5th-order cycles of sea level. These high frequency
carbonate cycles that have the genetic elements of the parasequence
are "of course" carbonate parasequences.
Carbonate Parasequence
Geometries - Tools of the Interpretation of Depositional Setting
The sequence stratigraphy of the carbonate sections is commonly
determined from a combination of 2 and 3 D Seismic data (providing
a comparatively low frequency resolution), well logs (providing
a comparatively high frequency resolution), cores (providing very
high frequency resolution) and outcrops (with best access to a combination
of high frequency resolution and low frequency resolution).
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thumbnail to access the large images |
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The analysis of
the sequence stratigraphy of carbonates is improved by applying
the recent realizations of Larue et al (1995); Sprague et al, (2002);
Sprague et al, (2003); and Sprague et al, (2004) for depositional
systems and integrating:
- The bounding surfaces
to their system tracts
- The component lithofacies
- The hierarchies in
the resulting geometries of the strata
- Their stacking patterns
Thus, as with clastic sediments (Sprague et al, 2002),
at a general level the physical stratigraphy of the carbonate strata can be broken down into a hierarchical framework.
This framework ties together genetically related
architectural elements
and their associated boundaries .
The hierarchy of these elements
is independent of the thickness and the time invovled in their accumulation. A top down breakdown of the architectural
elements shows a progressive decrease in scale from the complex facies geometries of the basin margin to single
tidal flat cycles or beds that accumulated on shallow shelves or in shallow lagoons. As Sprague et al, (2002) showed
for clastics, these carbonate hierarchical elements are directly relatable to stratal units defined on the basis of sequence
stratigraphy. Biostratigraphic data tied to the stratal units enable the direct comparison between shallow-marine and deeper
marine carbonate sequences and their related units with the potential of correlation of the carbonate cycles to
base level rise and fall.
All are the combined
products of base level change .
This is particularly true of shallow water carbonate
accumulations which are depth dependent, a response to the paleo-oceanography,
and processes of the depositional setting. The result of such an
analysis creates a "powerful" framework of parasequence
and high frequency cycle geometries that can be used to explain,
assess and predict reservoir and aquifer quality better independent
of thickness and time.
This approach even applies in deepwater settings.
For instance, using the Tamabra Formation of the Poza Rica Field Area of Mexico as an example, Loucks, et al (2006) have
demonstrated that deeperwater mass-transport carbonate deposits are carried by gravity flow and suspension processes into
deepwater basinal settings downslope from margins tied to shallow-water carbonate platforms. So while reefal and grain-rich
debris accumulate on the shallow platform carbonate debris wedges extend into the deeperwater basin.
The architecture of this debris wedge is
related to the availability of source material during changes in relative sea-level
(Loucks, et al., 2006). During sea-level lowstands and transgressions or during early highstands when the platform rapidly aggrades, debris and mud flows
composed of platform and slope carbonate mud, sand, and clasts generally accumulate. In contrast during
highstands of sea level when the platform is flooded and shedding, density-flow and turbidite deposits
composed of carbonate sand and lesser amounts of lime mud collect.
Click thumbnail to access the
large images and click on the larger image to see them full size!
Over
Simple
Dip Section |
Falling
Stage & Lowstand
System Tracts |
Transgressive
System
Tract |
High
Stand
System
Tract |
Normal
Marine Setting
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Stacking Patterns
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Geometries of
Carbonate Strata
The geometries of carbonate strata
are products of the shape of the
depositional surface
, changing base level and sediment accumulation. They
are defined by the underlying and overlying surfaces. These surfaces
may be the products of deposition and/or erosion and can coincide
with the depositional event or proceed or follow this. Physical
erosion, burrowing, boring, dissolution (Clari et al, 1995; Lukasik & James, 2003), and/or cementation may have modified them. Whatever
their origin, these surfaces provide a convenient means to subdivide
the carbonate section. From the perspective of sequence stratigraphy
these surfaces are used to determine the order in which strata are
laid down and define the geometries that they enclose.
Over
Simple
Dip Section |
Carbonate
fill, a response to base level change - Murray Basin |
Carbonate
fill, a response to base level change - Murray Basin |
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Normal
Marine Setting
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Stacking Patterns |
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As with the products
of other sedimentary depositional systems carbonate strata exhibit
a hierarchy of scales that include at the small-scale end (
beds ,
bed sets
, and bed cosets) and at the larger spatial scales reef complexes,
basin margin and slope complexes etc. These strata can be expressed
as unconfined sheets, unconfined but localized build ups (reefs,
banks and islands), unconfined but localized
sigmoids
(reef cores, Pomar 1991), bank margins etc.,) and confined incised
channels (tidal channels and the products of flood events). What
ever the final geometry this is the product of both accumulation
(aggradation
) and erosion.
Carbonate
Sequence Stratigraphy Exercises
Click on the highlighted title above to access the exercises that
are available on this site to examine the hierarchy of scales expressed
by carbonate strata. These may be the lower frequency subdivisions
that can be interpreted from seismic,
or higher frequency subdivisions outcrop
and well logs.
These consider facies or more complex lithofacies assemblages from
the perspective of sequence-stratigraphic concepts (
systems tracts,
parasequences,
sequences
and their response to seal level rise (
TST),
still stand (HST)
fall (FSST)
and lowstand (LST)
and their response to the paleo-oceanography and processes of the
depositional setting. The exercises are intended to develop skills
that can be used to establish direct relationships between the nature
of the carbonate bodies, the sequence stratigraphic architecture,
reservoir connectivity, reservoir characterization and prediction.
This would involve the use of systematic hierarchical relationships,
integration of seismic, well, and core data with outcrop and subsurface
analogs. From this you will gain a better understanding of how to
predict accurate net-to-gross, continuity, architecture, and reservoir
extent. If you are able to integrate biostratigraphy with your studies
this will provide an independent time framework correlation made
to cycles of base level rise and fall.
To conclude, carbonate
depositional facies hierarchy provides a framework for the systematic
description and comparison of carbonate deposits that is based on
the physical relationships of strata and their bounding surfaces.
The recognition of genetically related stratigraphic elements, is
independent of the lithofacies assemblage of carbonate, and is applicable
at all scales.
Carbonate
Response to Sea Level - Papers
Link
to a page that lists some of the literature on the carbonate sedimentary
record and how its sequence stratigraphic character varies in
response to base level change, usually eustasy and gain access to
.pdf files of these papers.
Bosence,
D.W.J., Pomar, L., Waltham, D.A. Lankaster, T., 1994. Computer
modeling a Miocene carbonate platform, Mallorca, Spain.
Arnerican Association of Petroleurn Geologists Bulletin, 78:247-266.
Kendall, Christopher G. St. C., Abdulrahman. S. Alsharhan, Kurt
Johnston and Sean R. Ryan; 2000; "Can The Sedimentary
Record Be Dated From A Sea-Level Chart? Examples from the Aptian
of the UAE and Alaska". In A. S. Alsharhan and R.
W. Scott, Eds, Society of Economic Petrologists and Mineralogists
(SEPM) Special Publication 69 on the Jurassic/Cretaceous Platform-Basin
Systems; Middle East Models; p 65-76 (Reference for the simulation
of Shaybah formation above)
Loucks, R. G., Charles Kerans, and Alfredo Marhx, 2006, "Origin and Organization of Mass-Transported Carbonate Debris in the Lower Cretaceous (Albian) Tamabra Formation, Poza Rica Field Area, Mexico", SEPM Research Symposium: The Significance of Mass Transport Deposits in Deepwater Environments II, AAPG Annual Convention, April 9-12, 2006 Technical Program
Pomar, L. and Ward, W.C. 1994. Response of a Miocene carbonate
platform to high-frequency eustasy. Geology, 22:131-134.
Pomar, L. and Ward, W.C, 1995. Sea level change, carbonate
production and platform architecture, in B. Haq ed., Sequence
stratigraphy and depositional response to eustatic, tectonic and
climatic forcing, Kluwer Academic Press. p. 87-112.
Pomar, L. 2001, Ecological control of sedimentary accommodation:
evolution from a carbonate ramp to rimmed shelf, Upper Miocene,
Balearic Islands. Palaeogeography, Palaeoclimatology, Palaeoecology,
175:249-272.
References
on a Hierarchical Approach to Sequence
stratigraphy
D. K. Larue, A. R. Sprague,
P. E. Patterson , J. C. Van Wagoner (1995): Multi-Storey
Sandstone Bodies, Sequence Stratigraphy, and Fluvial Reservoir Connectivity,
Bulletin AAPG, (Abstract) AAPG Annual Meeting, 79, 13, 54
Sprague,A. R., P. E. Patterson, R.E. Hill, C.R. Jones, K. M. Campion,
J.C. Van Wagoner, M. D. Sullivan, D.K. Larue, H.R. Feldman, T.M.
Demko, R.W. Wellner, J.K. Geslin1 (2002), The Physical Stratigraphy
of Fluvial Strata: A Hierarchical Approach to the Analysis of Genetically
Related Stratigraphic Elements for Improved Reservoir Prediction,
(Abstract) AAPG Annual Meeting
Sprague, A. R., M. D. Sullivan, K. M. Campion, G. N. Jensen, F.
J. Goulding, T. R. Garfield, D. K. Sickafoose, C. Rossen, D. C.
Jennette, R. T. Beaubouef, V. Abreu, J. Ardill, M. L. Porter, and
F. B. Zelt, (2003), The Physical Stratigraphy of Deep-Water
Strata: A Hierarchical Approach to the Analysis of Genetically Related
Stratigraphic Elements for Improved Reservoir Prediction,
AAPG Bulletin, (Abstract) AAPG Annual Meeting,.87, 10 p
Sprague, A. R., P.E. Patterson, M.D. Sullivan, K.M. Campion, C.R.
Jones, T.R. Garfield, D.K. Sickafoose, D.C. Jennette, G.N. Jensen,
R.T. Beaubouef, F.J. Goulding, J.C. Van Wagoner, R.W. Wellner, D.K.
Larue, C. Rossen, R.E. Hill, J.K. Geslin, H.R. Feldman, T.M. Demko,
V. Abreu, F.B. Zelt, J. Ardill, and M.L. Porter (2004), Physical
Stratigraphy of Clastic Strata: A Hierarchical Approach to the Analysis
of Genetically Related Stratigraphic Elements for Improved Reservoir
Prediction, ABSTRACT, AAPG 2003-04 Distinguished lecturer
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