Depositional bias and environmental change—important factors in sequence stratigraphy

Abstract

Differences among depositional systems, here called depositional bias, strongly influence sequence patterns. Siliciclastics and shallow-water carbonates, for instance, shed most of their sediment during opposite phases of a sea-level cycle (lowstand shedding and highstand shedding, respectively). Furthermore, the two systems generate their own, system-specific relief on the sea floor, disperse their sediment load along different avenues and differ in the way they are deactivated: reefs and carbonate platforms can be drowned, whereas siliciclastic deposition can be shut off and renewed at any depth. As a consequence of these differences, pronounced unconformities (drowning unconformities) develop where carbonate platforms are terminated and buried by siliciclastics (the siliciclastic-to-carbonate transition tends to be more gradual). Drowned platforms and drowning unconformities appeared world-wide in great abundance in the Miocene, Cretaceous (Valanginian-Turonian), Jurassic (Toarcian) and Devonian (Frasnian/Famennian). Examples of drowning unconformities interpreted as sequence boundaries, include those of the Early Cretaceous platforms off New Jersey and off Morocco, the mid-Cretaceous unconformity in the Gulf of Mexico and Miocene unconformities on top of reefs in the Far East.The eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms: either the rates of rise are an order of magnitude lower than the growth potential of platforms, or cycle amplitudes are too small, or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone. The critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems. Thus, drowning unconformities demonstrate the importance of environmental change as a control on sequence development. Other examples of dominantly environment-controlled sequences include slope deposits shaped by shifting currents (e.g. Florida and Blake Plateau) and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland (e.g. Gulf of Mexico).Environmental change must be considered as a third, independent factor that competes with eustatic and tectonically driven regional changes of sea-level for control of sequences. The definition of sequences and sequence boundaries should be broad enough to include the possibility of non-sea-level controls.