Sequence stratigraphy – concepts

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Contents

• Introduction• Sedimentology – concepts• Fluvial environments• Deltaic environments• Coastal environments• Offshore marine environments

• Sea-level change• Sequence stratigraphy –

concepts• Marine sequence stratigraphy• Nonmarine sequence

stratigraphy• Basin and reservoir modeling• Reflection

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Sequence stratigraphy – concepts

• Sequence stratigraphy highlights the role of allogenic controls on patterns of deposition, as opposed to autogenic controls that operate within depositional environments• Eustasy (sea level)• Subsidence (basin tectonics)• Sediment supply (climate and hinterland tectonics)

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• Accommodation is the space available, at any given point in time, for sediments to accumulate; accommodation is created or destroyed by RSL changes

• Water depth is controlled by changes in accommodation as well as sedimentation

• Base level is the horizontal surface to which subaerial erosion proceeds; therefore it corresponds to sea level

• Base level is a principal control of accommodation, and, hence, whether erosion or deposition is likely to occur at any given location; attempts to extend the concept landward are controversial

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• Allostratigraphy is a relatively new approach to stratigraphic subdivision, and is based on the separation of strata based on unconformities or other discontinuities (e.g., paleosols)

• Sequence stratigraphy is the analysis of genetically related depositional units bounded by unconformities and their correlative conformities

• A depositional sequence is a stratigraphic unit bounded at its top and base by unconformities or their correlative conformities (=allostratigraphic unit), and typically embodies a continuum of depositional environments, from updip (continental) to downdip (deep marine)

• The subtle balance between RSL and sediment supply controls whether aggradation, regression (progradation), forced regression, or transgression (retrogradation) will occur

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• A RSL fall on the order of tens of meters or more will lead to a basinward shift of the shoreline and an associated basinward shift of depositional environments; commonly (but not always) this will be accompanied by subaerial exposure, erosion, and the formation of a widespread unconformity known as a sequence boundary

• Sequence boundaries are the key stratigraphic surfaces (high-order bounding surfaces) that separate successive sequences and are characterized by subaerial exposure/erosion, a basinward shift in facies, a downward shift in coastal onlap, and onlap of overlying strata

• Parasequences are lower order stratal units separated by (marine) flooding surfaces; they are commonly autogenic and not necessarily the result of smaller-scale RSL fluctuations

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• Systems tracts are contemporaneous, linked depositional environments (or depositional systems); they are the building blocks of sequences and different types of systems tracts represent different limbs of a RSL curve• Falling-stage (forced regressive) systems tract (FSST)• Lowstand systems tract (LST)• Transgressive systems tract (TST)• Highstand systems tract (HST)

• The various systems tracts are characterized by their position within a sequence, by shallowing or deepening upward facies successions, or by parasequence stacking patterns

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• Systems tracts are contemporaneous, linked depositional environments (or depositional systems); they are the building blocks of sequences and different types of systems tracts represent different limbs of a RSL curve• Falling-stage (forced regressive) systems tract (FSST)• Lowstand systems tract (LST)• Transgressive systems tract (TST)• Highstand systems tract (HST)

• The various systems tracts are characterized by their position within a sequence, by shallowing or deepening upward facies successions, or by parasequence stacking patterns

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• Maximum flooding surfaces form during the culmination of RSL rise, and maximum landward translation of the shoreline, and constitute the stratigraphic surface that separates the TST and HST

• In the downdip realm (deep sea), where sedimentation rates can be very low during maximum flooding, condensed sections may develop

• LSTs are separated from overlying TSTs by transgressive surfaces; transgression is further characterized by coastal onlap

• An alternative approach to sequence analysis uses genetic stratigraphic sequences that are bounded by maximum flooding surfaces

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• Maximum flooding surfaces form during the culmination of RSL rise, and maximum landward translation of the shoreline, and constitute the stratigraphic surface that separates the TST and HST

• In the downdip realm (deep sea), where sedimentation rates can be very low during maximum flooding, condensed sections may develop

• LSTs are separated from overlying TSTs by transgressive surfaces; transgression is further characterized by coastal onlap

• An alternative approach to sequence analysis uses genetic stratigraphic sequences that are bounded by maximum flooding surfaces

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• In a very general sense, RSL fall leads to reduced deposition and formation of sequence boundaries in updip areas, and increased deposition in downdip settings (e.g., submarine fans)

• RSL rise leads to trapping of sediment in the updip areas (e.g., coastal plains with a littoral energy fence) and reduced transfer of sediment to the deep sea (hemipelagic deposition; condensed sections)

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• Seismic stratigraphy is based on the principle that seismic reflectors follow stratal patterns and approximate isochrons (time lines)

• Reflection terminations provide the data used to identify sequence-stratigraphic surfaces, systems tracts, and their internal stacking patterns

• Technological developments have been prolific:• Vertical resolution improved to a few tens of meters• Widespread use of 3D seismic

• Seismic data should preferably always be interpreted in conjunction with well log or core data

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• A better understanding of stratigraphic sequences can be obtained by the construction of chronostratigraphic charts (‘Wheeler diagrams’); these can subsequently be used to infer coastal-onlap curves

• Variations in sediment supply can produce stratal patterns that are very similar to those formed by RSL change (except for forced regression); in addition, variations in sediment supply can cause stratigraphic surfaces at different locations to be out of phase

• In principle, sequence-stratigraphic concepts could be applied with some modifications to sedimentary successions that are entirely controlled by climate change and/or tectonics (outside the realm of RSL control)

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• A better understanding of stratigraphic sequences can be obtained by the construction of chronostratigraphic charts (‘Wheeler diagrams’); these can subsequently be used to infer coastal-onlap curves

• Variations in sediment supply can produce stratal patterns that are very similar to those formed by RSL change (except for forced regression); in addition, variations in sediment supply can cause stratigraphic surfaces at different locations to be out of phase

• In principle, sequence-stratigraphic concepts could be applied with some modifications to sedimentary successions that are entirely controlled by climate change and/or tectonics (outside the realm of RSL control)

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• The global sea-level curve for the Mesozoic and Cenozoic (inferred from coastal-onlap curves) contains first, second, and third-order eustatic cycles that are supposed to be globally synchronous, but it is a highly questionable generalization• Conceptual problems: spatially variable RSL change due to

differential isostatic and tectonic movements undermines the notion of a globally uniform control

• Dating problems: correlation is primarily based on biostratigraphy that typically has a resolving power comparable to the period of third-order cycles

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Sequence stratigraphy – concepts

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