Accommodation and supply—a dual control on stratigraphic sequences

Abstract

It is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences. I suggest that environmental change be added as a third, autonomous control. Sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates —the rate of change in accommodation (space available for sedimentation) and the rate of sediment supply. Sea level has direct control on accommodation, but its influence on sediment supply is remote and easily overshadowed by environmental factors. For instance, the record of the most recent sea-level rise is a transgressive systems tract where supply is low; it is a prograding highstand systems tract in deltas where the supply is high. Examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins, changes in ocean currents, pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress. Furthermore, the stratigraphic sequences in fluviatile continental basins are physically removed from sea-level induced changes in accommodation and must have formed by changes in the rate and pattern of supply. Subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles. Other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences. Some third-order cycles (ca. 0.5–3 Ma duration) meet these criteria, others do not. Cycle-stacking patterns and the shifting facies belts on cratons indicate that many second- and third-order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid, shorter oscillations. Seismic data yield poor images of these gradational changes because they lack resolution. Seismic models of outcrops show that near the limits of resolution, the seismic tool tends to portray rapid changes in facies and bed thickness, i.e. lateral variations in supply, as unconformities. Seismic unconformities do not necessarily correspond to unconformities in outcrop. The succession of stratigraphic sequences is best considered a composite record of sea-level and environmental change. Separating the various controls requires carefully planned experiments. Work on the oxygen-isotope curve—another proxy for sea level—has set an example in this respect.