Concepts of Depositional Sequences

Peter R. Vail: Rice University, Houston, TX; Robert M. Mitchum, Consultant, Houston, TX; and John B. Sangree: Consultant, Corpus Christi, TX

Abstract

The depositional sequence is defined 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". The model evolved from recognition of stratal surfaces as geological time lines and of 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.

Because sequence boundaries appear globally synchronous, it was postulated that global sea level changes are a major control on the Stratigraphic record. Eustatic cycle charts were developed to plot the ages and magnitudes of global eustatic cycles. Their latest revision was in 2000. High-resolution biostratigraphy is essential to dating sequences.

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 varies widely, depending on variations in basin tectonics and sediment supply. Lowstand, transgressive, and highstand systems tracts form in response to various stages of the sea-level cycle. Interpretation of seismic facies of these units is an important tool for recognizing basic lithofacies.

Several levels of sea-level cyclicity may occur in a hierarchy that allows higher-frequency cycles to be superposed or stacked into lower-level cycles. The stacking of parasequences (fourth- or fifth-order cycles) into third-order (sequence) cycles is an example.

Similarities and differences of the depositional sequence with other stratigraphic models will be discussed.