Concepts of Depositional Sequences
Peter R. Vail, Robert M. Mitchum, John B. Sangree, 2002. "Concepts of Depositional Sequences", Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories, John M. Armentrout, Norman C. Rosen
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The depositional sequence was defined in 1977 as “a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities.” This definition has proven singularly robust in any number of sedimentary and tectonic settings.
The depositional sequence model evolves naturally from recognition of stratal surfaces in rocks as geological time lines and from the time-significance of unconformities and their correlative conformities as sequence boundaries. Seismic reflections follow stratal boundaries, and within limits of seismic resolution, represent time lines.
Sequence boundaries have been found to be of the same age in basins worldwide, and it has been postulated that global sea level changes are a major control on the stratigraphic record, along with local basin tectonics and rates of sediment supply. Eustatic cycle charts plot the ages and magnitudes of global eustatic cycles. High-resolution biostratigraphy is essential to dating sequences and determining paleo-environments.
The cyclic sequences bounded by these unconformities have a basic pattern of deposition which results from a relative fall and rise of sea level. This pattern can vary widely from basin to basin, depending on variations in basin tectonics and sediment supply. The accommodation model consists of lowstand, transgressive, and highstand systems tracts formed in response to various stages of the sea-level cycle.
The maximum flooding surface marks the maximum transgression of marine facies on the shelf, and is an important correlation point. It is not a sequence boundary in our model because strata bounded by maximum flooding surfaces would include two disparate units separated by a significant unconformity, which is our sequence boundary.
Several levels of sea level cyclicity may occur in a hierarchy that allows higher frequency cycles to be superposed or stacked into lower frequency cycles. The stacking of parasequences (fourth- or fifth-order cycles) into third-order (sequence) cycles is an example.
Examples of stratigraphic sections from the Paleozoic of the Arabian plate and the northern Gulf of Mexico show applications of sequence stratigraphy to both extremes of the Phanerozoic spectrum. They also contrast characteristics of the best-known models of sedimentary response to cyclic depositional control.