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:
- Steno's Laws of sediment accumulation
- Walther's Law of the vertical and lateral equivalence of sediments
- The chronology of the section
- Sedimentary structures
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.
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:
The sequence stratigraphic approach recommended on this web site for the interpretation of sedimentary rocks contrasts with:
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.
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:
- Sequences
- Systems tract
- Parasequences and/or cycles
- Bedsets
- Beds
The sequence stratigraphic approach recommended on this web site for the interpretation of sedimentary rocks contrasts with:
- Lithostratigraphic analysis which maps lithofacies independent of subdividing external and internal boundaries
- Allostratigraphic analysis that identifies and includes as time markers bounding discontinuities including erosion surfaces, marine flooding surfaces, and markers that include tuffs, tempestites, and/or turbidite boundaries etc., but considers these as independent of any model of base level change
Niels 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".
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:
- Unconfined by topography
- Confined within eroded topography.
- Unconfined sheets that:
- Prograde (step seaward) as
- Retrograde (step landward)
- Aggrade (build vertically)
- Unconfined prograding carbonate sheets that are the product of physical accommodation are further subdivided below into:
- Low angle ramps of fine shallow-water carbonate that in deeper-water pass to gravels
- Homoclinal ramps of fine shallow-water carbonate
- Distally steepened ramps of shallow-water grain-dominated carbonate
- Unconfined carbonate platform sheet geometries formed with ecological accommodation form
- flat-topped open shelves with moderate shallow-water ecological accommodation
- Reef-rimmed platform with highest shallow-water ecological accommodation
- Massive steep to cliffed margins with maximum shallow-water ecological accommodation
- Subaerial incised valleys
- Submarine incised valleys
- Organized bodies
- Disorganized bodies
- Multi-storied
- Amalgamated
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:
- Ichnofacies and fossils
- Sedimentary structures
- Volcanics
- Storm layers or tempestites
- Sequence stratigraphic boundaries
- Fossils
- Magneto-stratigraphic
- Radioactive markers or gamma ray log signal Radioactive markers or gamma ray log signal
- Radiometric markers
Terminology
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 strengthening 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!
Curled from: Society for Sedimentary Geology.
