Stratigraphy is the science of the layered character of rocks. The rocks can be sedimentary, volcanic, metamorphic or igneous. Sequence stratigraphy, a branch of sedimentary stratigraphy, uses the order strata accumulated in along with a framework of major depositional and erosional surfaces to interpret the depositional setting of clastic and carbonate sediments from continental, marginal marine, basin margins and down-slope settings of basins. The framework surfaces that bound and subdivide the strata were often generated during changes in relative sea level and formed during associated deposition and erosion. The template of surfaces, because the origin of the surfaces is understood, can be used to interpret the depositional origin and predict the heterogeneity, extent and character of the lithofacies. The interpretation is better and predictions of local and regional stratigraphy more accurate when the sequence stratigraphic framework is integrated with an understanding of:
The major problem with sequence stratigraphy is that the definition, terminology and interpretation of the surfaces of sequence stratigraphy is complex and sometimes contentious. In an attempt to circumnavigate this, the site places a heavy dependence on the explanation of terminology linked pop-up boxes so as to clarify the understanding and use of this discipline of stratigraphy.
Bounding surfaces of sequence stratigraphy
The key to sequence stratigraphy are the major bounding and subdividing surfaces of the sedimentary section (see the banner image above). These are commonly generated by the changing relative sea level. Since the oceans were first generated, their water volume and distribution across the globe has varied. Changes in ocean water volume are mostly in response to the presence or absence of continental glaciation and the changing temperature of the oceans. The consequent variation of the relative position the sea has been recorded in the marine sedimentary strata of the global stratigraphic section from the Pre-Cambrian to the Present. Relative rises in sea level may occur when eustasy, world wide sea level, rises or local crustal movement causes the substrate to subside. The associated transgressions cause the shore and the near-shore to be flooded forming transgressive surfaces (TS). When the rate of sea level rise reaches its most rapid change, the rate of sediment accumulating seaward of the shore slows but the pelagic and benthic organic matter continues to accumulate. These organics sequester radioactive elements in the water column. The result is the sediments have a strong radioactive signal on gamma logs with matching condensed sections of fossils that accumulated on a surface or in a thin zone which is known as the maximum flooding surface (mfs). In contrast, a drop in sea level may cause the shore and the near-shore to be eroded, forming sequence boundaries (SB).
sequence stratigraphic interpretative analysis thus involves identifying the subdividing surfaces enveloping discrete sediment body geometries of the sedimentary section. The interpreter then conceptually reverses the order of deposition by back-stripping the geometries from oldest to youngest and then reassembles these in order of accumulation, using as a template the subdividing surfaces, lithofacies geometry, and fauna to interpret the evolving character of depositional setting. The reassembly tracks the evolution of the sedimentary system, its hydrodynamic setting, and accommodation (see Basin Clastic Sequence Hierachies for such a reassembly).
The back-stripping and analysis is aided by the subdivision of the sequence stratigraphic section on the basis of major depositional and erosional surfaces alluded to above. There are a variety of elements subdivided by the surfaces and their hierarchy from low frequency to high frequency includes:
As these sediments are reassembled, the genetic character of the sequences, systems tracts, parasequences, and beds will be seen as products of changes in accommodation. A limit to this analytical strategy is often the extent of ones understanding of the inferred depositional setting. The advantage of the strategy is it considers new questions, leading to more realistic interpretations and enhanced predictions of lithofacies heterogeneity's. Thus the sequence stratigraphic framework is used to analyze and explain how sedimentary rocks acquire their layered character, lithology, texture, faunal associations and other properties. These properties in turn can be used to explain how the mechanisms of sediment accumulation, erosion and inter-related processes produced the current configuration of these rocks.
The sequence stratigraphic approach recommended on this web site for the interpretation of sedimentary rocks contrasts with:
Analyses based on sequence stratigraphy apply allostratigraphic models to interpret the depositional origin of these sedimentary strata but in contrast assumes, though this is not always stated, an implicit connection to relative sea level or base level change.
Neils Steno and Johannes Walther
Steno and Walther's contributions to sedimentary interpretation and so sequence stratigraphy were profound. Steno established the order and the way sediments were laid down. His principle of superposition recognized that older sedimentary layers underlie the newer layers. His principle of original horizontality recorded how sediments are deposited to fill a basal irregular surface enclosed above by a smooth surface. His principle of lateral continuity proposed layers of sediment are continuous till they pinch out, or a barrier prevents their further spread during deposition, or subsequent abrupt changes in the landscape break up the sediment layers. Walther then recognized that as depositional settings change their lateral position and fill accommodation, so the sedimentary facies of adjacent depositional settings succeed one another as a vertical sequence.
Stacking Patterns and Geometries
To establish the depositional setting of the sedimentary section, sequence stratigraphy uses the geometric arrangement of sedimentary fill, particularly the vertical succession of sedimentary facies geometries and their enveloping surfaces known as stacking satterns. The geometries and so stacking patterns of un-cemented carbonates and clastics are similar. This is because both respond to changes in base level, both can be subdivided by similar surfaces and both respond to wave and current movement similarly and may be transported.
Never the less major differences in the sequence stratigraphy of the two sediments exist. All clastics are transported to their depositional resting place while carbonates are produced and accumulate "in situ". Rates of carbonate production are linked to photosynthesis, so are depth dependent with rates greatest close to the air/sea interface. This means that carbonate facies and their fabrics are often used as indicators of sea level position. Additionally rates of carbonate accumulation often have a biochemical and physicochemical origin that is influenced by the chemistry of the water from which they are precipitated.
Stacking patterns of both sediments are expressed by geometric bodies that may be:
Stacking patterns for both clastics and carbonates that are the product of physical accommodation vary between:
- Unconfined by topography
- Confined within eroded topography.
Confined bodies represented by fill of Incised topography include
Channel fill and stacking of confining valleys, unconfined lobes and sheets may be expressed as:
- Organized bodies
- Disorganized bodies
Other Stratigraphic Tools Utilized with Sequence Stratigraphy
Prediction and interpretation improves not only when sequence stratigraphy is coupled to the Laws of Steno and Walther but when tied to indicators of deposition and time. Indicators of depositional setting include:
Chronostratigraphic markers include:
Use of the “Over Simplification” of time as it relates to sequence stratigraphy
Though the linkage between the sequence stratigraphy and the other sub-disciplines of stratigraphy can be ‘fuzzy’ these links are important to prediction and interpretation. A key problem to strengthing theses links is not only that the terminology of sequence stratigraphy carries connotations related to the interpretation of the surfaces used to interpret the stratigraphic section but also a consideration of sedimentology and chronostratigraphy. How the terminology is defined and used and/or fits preconceived classifications is tied to the character of the data and stratigraphic techniques used. In the end it is up to the user to consider their data, and the goals of their interpretations. They should be able to explain their choice of terms and then make their interpretation!
As is explained in the pages that follow, using the above approach geologists infer the processes responsible for that sedimentary rock and so interpret its origin. The sedimentary layering of a stratigraphic section has a vast array of dimensional hierarchies. These range from units millimeters thick that might be formed over seconds to thousands of feet thick and formed of millions of years. As much of the literature related to these surfaces indicates, it should be recognized that whatever the dimension of a layer is and whatever the time involved in its deposition, each layer is bounded by surfaces that transgress time (Wheeler, (1958); Middleton, (1973); Vail et al (1977); Galloway, (1989); Catuneanu, et al, (1998); Schwarzacher, (2000); Catuneanu, (2002); Embry, (2002); Cross, and Lessenger, (submitted)). This means an interpretation of the depositional setting for a section cut by these diachronous surfaces contravenes Walther's Law. Most interpreters accept and take into consideration that the layered units bounded by these surfaces formed at different times, and recognize that the subdividing surfaces are of a higher order frequency than the time envelope of the parasequence being considered. In other words the situation is simplified when the surfaces are taken to represent instances in time between which sediments continuously accumulated. Thus the surfaces of the layers transgress time and the sediments filling between these surfaces also transgress time while being continuously reworked through a series of geological events.
Thus it should be recognized that in sedimentary interpretation the application of Steno's principles and Walther's Law provide powerful and useful simplifications that assume the sediments packaged by surfaces accumulated within discrete moments of time. If one thinks about this, these simplifications don't contravene logic (which is literally Fuzzy) and aid in the interpretation of the sedimentary section. The above discussion provides a general introduction to the subdivision of the sedimentary section by the surfaces listed above and their relationship to base level change. For for a more complete and thorough discussion of this topic you should read Catuneanu (2002).
|Figure - Hierarchy of sedimentary structures:
A - Flaser structures from an intertidal flat setting in which the individual components probably accumulated over minutes but the whole section may represent tidal cycles over months (Bar Scale - 2 cm)
B - Cross bedded Ordovician carbonates from a beach or nearshore shallow shoal setting that probably represent accumulation and reworking over several years
C - Flat bedded Mississippian downslope siliciclastic fan deposits; each bed of which may have accumulated over a period of hours, though the whole section encompasses potentially hundreds of thousands of years (bar scale - 1m)
Introduction to the Web Site
This Web Site explains:
1. How to make sequence stratigraphic interpretations of sedimentary sections:
- Subdivide of these sections into sequences, parasequences &/or their associated systems tracts
- Determine their depositional setting
- Characterize and predict of the extent of their lithofacies, particularly when associated with hydrocarbon reservoirs, and aquifers.
2. The use of:
3. How the above surfaces have time significance and establish:
- a relative time framework for the sedimentary succession
- the inter-relationship of the depositional settings and their lateral correlation
- a compartmentalization of hydrocarbon reservoirs
In summary this web site explains how "Sequence Stratigraphy" can be used to study sedimentary rock relationships within a time-stratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformity
(Posamentier et al., 1988; Van Wagoner et al., 1988).
Using the sidebar menu you can select topics in sequence stratigraphy and access exercises related to this. You should be able to learn how to subdivide the sedimentary section into packages defined by bounding unconformities and internal surfaces. You will be able to see how sequences, parasequences and their associated systems tracts are the products of changes in relative sea level and rates of sedimentation. The various forms of sequence stratigraphic analyses outlined include the use of seismic cross-sections, well logs and outcrop studies of sedimentary rocks to infer changes of relative sea level and rates of sedimentation. You will be shown how to construct chronostratigrapic diagrams and also be show how to predict facies geometries and build depositional models using a variety of techniques!
Later in the section on the Basics of Sequence Stratigraphy you will be introduced to the details of how systems tracts respond to changing base level. However as a preview you can trace clastic systems tract evolution through time in the linked movie!
Sunday, February 24, 2013