Sequence Stratigraphy

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  • Topic: Stratigraphy, Geology, Sequence stratigraphy
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  • Published : April 5, 2011
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Sequence stratigraphy
Sequence stratigraphy is a method of stratigraphic interpretation that combines the chronological order of the accumulation of sediments, their stratal architecture and the geometric relationships of their facies to determine depositional setting and predict stratal continuity. The workflow of sequence stratigraphic analysis first identifies the genetic units and bounding surfaces that compose the stratigraphic section in outcrop, core, well-log and seismic data. The framework formed by the genetic units and bounding surfaces is tied depositional models that explain the recurring character of sequence stratigraphic units and surfaces and provides a common terminology for each of these models, whatever the tectonic setting, depositional setting, sediment types (siliciclastics, carbonates, evaporites) data set that is available for analysis (e.g., seismic data versus well logs or outcrop). It is generally assumed that many of the strata and surfaces are related to changes in base level driven by eustasy. The sequence stratigraphic methodology describes the order in which bodies of sediment were laid down and their geometric relationship to each other. It guides the interpretation of the origins of sedimentary rocks enabling the mapping and interpretation of single and multiple cycles of sedimentary rocks. The cycles are often in the form of vertically stacked associated sedimentary facies whose repeated occurrences often have similar geometric organization. These geometries and their bounding surfaces are closely associated with changes in accommodation (space available for sediments to fill). Thus the simple sequence stratigraphic framework is based on surfaces of erosion and non-deposition (sequence boundaries), and flooding (trangressive surfaces and/or maximum flooding surfaces [mfs]). In addition there is an implicit but sometimes unstated connection between the external and internal surfaces of a sequence and base level change driven by eustasy. As indicated above the surfaces and encompassed strata are assumed to have time significance so provide a relative time framework for the sedimentary succession. This latter is the reason that sequence stratigraphy is considered to be a branch of allostratigraphy. The technique of subdividing sedimentary rock on the basis of surfaces leads to a better understanding the inter-relationship of the depositional settings and their lateral correlation. Hence the definition that "sequence stratigraphy" is the study of rock relationships within a time-stratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformities (Posamentier et al., 1988; Van Wagoner et al., 1988). In conclusion sequence stratigraphy is more than a matter of "surface definitions", but a powerful strategy that, in conjunction with biota and sedimentary structures, enables the conceptual modeling of the different combinations of "sedimentary processes" including eustasy responsible for the hierarchies of geometry or elemental packaging exhibited by the lithofacies. Allostratigraphy versus sequence stratigraphy

In terms of the original definitions of “sequence stratigraphy” this methodology is applied when changes in the accommodation of sedimentary rocks are interpreted to have been the products of allocyclic change, i.e. base level change, caused by changing rates of eustatic and tectonic movement, and changing rates of sedimentation (Vail et al, 1977; Posamentier, et al, 1988; Van Wagoner et al, 1988; Van Wagoner, et al, 1990). However as sequence stratigraphy has become more widely used so the order of these layers and the surfaces that bound them are also often tied to the sequential packaging of autocyclic processes that are the result of point migration and mass wasting events, rather than base level change. This latter is because most petroleum and other stratigraphers, including academic geologists,...
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