6 Parasequence Sets and Depositional Sequences

Abstract

ABSTRACT This chapter presents definitions, recognition criteria, and examples of parasequence sets, depositional sequences, and their key defining surfaces to be applied within the overall sequence-stratigraphic workflow. This scale of stratification is most useful for correlating and mapping across large areas of basins and for understanding and predicting the character and distribution of rock properties. Hydrocarbon play-element properties vary systematically within depositional sequences in many settings. It is also typically the scale at which ties can be made to traditional lithostratigraphic formations. The depositional sequence is the fundamental stratal unit for basin-scale sequence-stratigraphic analysis. It has great practical utility for many reasons: its sequence boundaries are distinctive, widespread surfaces of chronostratigraphic significance that form independently of the sediment supply rate. (In contrast, transgressions and regressions are strongly controlled by rates of sediment supply and, for that reason, can be regionally diachronous. There are also several major and many minor flooding surfaces within each depositional sequence that can potentially be miscorrelated regionally, especially among widely spaced control points.) The regionally significant erosion and onlap associated with a sequence boundary exert a strong influence on facies distribution. The distinct break in deposition and basinward shift in facies across the unconformable part of a sequence boundary make it a quite useful surface for separating relatively conformable facies packages above and below. Systems tracts occur predictably within the depositional sequence and have characteristic relations to the sequence boundary. In contrast, considering only facies, without recognizing the hierarchy of the chronostratigraphic stratal surfaces within which such facies change, can lead to inaccurate conclusions about rock-property distribution or contemporaneity of the facies. The time-transgressive nature of lithofacies units makes it essential to use physical surfaces rather than lithologic units for correlation.