16 Controls on Stratal Record: Mechanisms and Contingencies Affecting Sediment Supply and Accommodation

ABSTRACT This chapter addresses controls on the stratigraphic record: the mechanisms, processes, and contingencies affecting sediment supply and accommodation and the resulting stratal surfaces and units. Although it is not necessary to know the forcing mechanisms of sequence formation to construct a sequence-stratigraphic framework and map the distribution of rock properties, it is commonly useful to incorporate one’s understanding of key processes to provide predictive capabilities away from sample control. Many factors influence the development and expression of parasequences and depositional sequences. These factors can be grouped usefully into two main categories: processes (sediment supply and accommodation) and contingencies (inherited and coeval factors that condition the effects of those processes). The main components of sediment supply include detrital, biogenic, and authigenic processes as well as lateral and temporal changes thereof; the components of accommodation include those processes that affect the upper and lower boundaries of sediment accumulation. Contingencies exert a significant influence on the expression of sequence-stratigraphic surfaces and units because they affect when, where, and how the processes of sediment supply and accommodation operate. Four main contingencies affect all depositional settings and generally do not change significantly during a depositional sequence: (1) geological age, (2) plate-tectonic setting, (3) paleolatitude, and (4) paleogeography at the continental and basinal scale. Other contingencies tend to be specific to particular types of depositional settings or change significantly during accumulation; these include (1) inherited and evolving bathymetry, (2) climate mode, and (3) ocean chemistry. Ultimately, it is difficult to uniquely identify causal mechanisms because of the many influences on accommodation and sediment supply and the commonly convergent effects of those influences (i.e., similar stratal patterns can result from various combinations of influences). Knowledge of mechanisms is not, however, an essential part of the sequence-stratigraphic approach (and is potentially not possible in many circumstances—especially not from the stratal patterns alone). Sequence stratigraphy allows construction of a comprehensive and useful stratigraphic framework based on a single criterion—the physical relations of the strata themselves—that reveals genetically related rocks.