12.3: Sequence Stratigraphy
- Page ID
- 20390
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Sequence stratigraphy is based on the recognition and correlation of stratigraphic surfaces that represent changes in depositional trends (Embry, 2001). Remember that this is different from lithostratigraphy which is when we subdivide rocks into lithologic formations based on their physical characteristics.
Figure \(\PageIndex{1}\): Diagram showing the major types of lateral and vertical lithologic contacts and lithostratigraphic units; a sequence stratigraphic subdivision of the same units is provided along the right margin of the diagram (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).
Parasequences
Relative relative sea-level (RSL) curves are the combined product of eustatic and tectonic curves. The RSL curve that we made is a smooth theoretical one; in reality these curves are very complex and composed of a series of small-amplitude, short term oscillations in RSL and sediment supply (caused by things like delta-lobe switching, etc).
Parasequences are the sedimentary record of the small amplitude changes described above. They are small scale, shallowing upward succession of beds bounded by flooding surfaces (abrupt, nearly instantaneous increases in water depth). The flooding surfaces are overlain by deposits that record progressive shallowing. Parasequence thickness is extremely variable and can range from a few tens of centimeters to several meters. Laterally, parasequeces are often tracable over tens to thousands of square kilometers.
Figure \(\PageIndex{2}\): Typical facies successions in common types of clastic parasequences (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).
Figure \(\PageIndex{3}\): Typical facies successions in common types of carbonate parasequences (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).
Parasequence Sets
When stacked atop one another, parasequences collectively show long-term changes in shoreline position. We can recognize three different parasequence stacking patterns (parasequence sets).
Progradational parasequence sets – form when sedimentation rates outpace the creation of accommodation which results in each parasequence being shallower than the previous one, progressive shallowing, and an overall seaward shift in shoreline position (regression).
Aggradational parasequence sets – happen when there is no net shift in shoreline position or average water depth.
Retrogradational parasequence sets – form when accommodation rates are outpacing sediment supply rates. The result is that each paraequence represents deepening relative to the previous one and an overall landward shift in shoreline position (transgression).
Figure \(\PageIndex{4}\): Parasequence set architecture (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)
Systems Tracts, Bounding Surfaces, and Sequences
Sequences are the fundamental unit of sequence stratigraphy. They are composed of a package of genetically-related strata deposited during one cycle of sea-level rise and fall. In many basins, they are bounded by unconformities and their correlative conformities.
Systems tracts are parts of a sequence that are deposited during a given phase of the sea-level cycle; each has a distinctive parasequence architecture. There are a variety of models for sequence architecture; the simplest schemes have just two systems tracts and the most complex ones have four. Each systems tract is separated from the next via a stratigraphically important surface.
Figure \(\PageIndex{5}\): Overview of sequence stratigraphic nomenclature relative to a sea level cycle (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)
Highstand systems tract (HST)
The highstand systems tract forms when sea-level is still rising, but rising slowly enough that it is being overpowered by sediment supply. The result is an aggradational to progradational parasequence set architecture, overall (normal) regression, and the position of the shoreline to build both up and out to sea. It is composed of progradational to aggradational parasequence sets. It is bounded below by the maximum flooding surface and above by the basal surface of forced regression and/or the start of unconformity development.
Basal Surface of Forced Regression (BSFR)
The basal surface of forced regression occurs at the moment that sea-level starts to fall and negative accommodation is generated. As the name implies, the resulting regression is forced because there is no option but for the shoreline to shift both seaward and topographically down. That stands in contrast to a normal regression which can happen under a positive accommodation regime any time that sediment supply outpaces the generation of accommodation.
Falling Stage Systems Tract (FSST)
The falling stage systems tract is only preserved in basins that are experiencing a high subsidence rate and/or in deepwater parts of basins that are not subaerially exposed. Sea level is falling throughout and the basin experiences a massive forced regression under a negative accommodation regime. It is composed of downstepping progradational parasequnce sets. The shoreline is progressively moving out and down and exposed sediments are eroded or pedogenically overprinted as unconformity development occurs across an ever increasing percentage of the basin. It is underlain by the BSFR and overlain by the sequence boundary or its correlative conformity.
Sequence Boundary (SB)
The sequence boundary represents the maximum extent of the unconformity across the sedimentary basin. In parts of the basin that were never subaerially exposed, it passes laterally into the correlative conformity. Remember that the unconformity is a diachronous event, it starts sooner, ends later, and lasts longer in updip parts of the basin.
Lowstand Systems Tract (LST)
The lowstand systems tract forms in the early phases of sea-level rise while sediment supply is still outpacing the generation of accommodation. Regression continues, but its no longer forced and although the position of the shoreline continues to build out into the basin it can also start to build topographically upward. It is composed of aggradational to progradational parasequence sets.
The LST is underlain by the sequence boundary and overlain by the transgressive surface.
Transgressive surface (TS)
The transgressive surface marks the start of transgression and occurs at the moment when transgression begins because accommodation overtakes sediment supply. Some authors will refer to this surface as the maximum regressive surface. At this time the shoreline reaches its most basinward position.
Transgressive Systems Tract (TST)
The transgressive systems tract occurs during the most rapid phase of sea level rise and is the only systems tract where A>S which causes a transgression. It is composed of retrogradational parasequence sets formed as the shoreline moves up and in a landward direction.
It is underlain by the transgressive surface and overlain by the maximum flooding surface.
Maximum flooding surface (MFS)
The maximum flooding surface marks the end of transgression, the most landward position of the shoreline, and the deepest water deposition in the basin.
Figure \(\PageIndex{6}\): Cross-sections sketch of a shelf edge showing sequence stratigraphic stacking patterns (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)
Lithostratigraphy vs. Sequence Stratigraphy
Figure \(\PageIndex{6}\): Comparision of sequence stratigraphic correlation versus lithostratigraphic correlation (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)
Alternate Sequence Architectures
The sequence stratigraphic model described above represents the most complex and complete expression of a sequence. Its important to remember that not all systems tracts are going to be preserved at all locations and/or in all sedimentary basins.
ExxonMobil invented sequence stratigraphy and their scheme does not identify the FSST, instead they lump these sediments together with lowstand deposits.
In the “Genetic sequence” model, it’s the maximum flooding surface that defines the start and end of a sequence. This model works well in rapidly subsiding basins where the flooding event is widespread/well preserved and the regression and sequence boundary is suppressed.