Chapter 5 - Stratal units

Abstract

This chapter presents the building blocks of the sequence stratigraphic framework, which include sequences and component systems tracts and depositional systems. Sequences and systems tracts are units of sequence stratigraphy, whereas depositional systems are units of sedimentology. Parasequences have also been used as building blocks of seismic-scale systems tracts, but have become redundant with the advent of high-resolution sequence stratigraphy as high-frequency sequences that develop at parasequence scales provide a better and more reliable alternative for stratigraphic correlation. Sequences and systems tracts are defined by stratal stacking patterns and bounding surfaces, and not by their inferred controls, sedimentological makeup, age, time span, or physical scales. Within the nested architecture of the stratigraphic record, sequences, systems tract, and depositional systems can be observed at all stratigraphic scales, in relation to stratigraphic cycles of different magnitudes. At each scale of observation, stratigraphic cycles define sequences, which consist of systems tracts and depositional systems accumulated during the same cycle of change in stratal stacking pattern. At the smallest stratigraphic scales, systems tracts and component depositional systems consist of beds and bedsets (sedimentological cycles). At any larger scales, systems tracts and depositional systems consist of higher frequency (lower rank) sequences (stratigraphic cycles). This nested architecture of sedimentary cycles can be observed in both downstream- and upstream-controlled settings.