CONTRASTING EXPRESSIONS OF THE STRATIGRAPHIC PALEOBIOLOGY OF MARINE AND NONMARINE SYSTEMS

Owing to how sediment is supplied and deposited in a sedimentary basin, marine and nonmarine fossil records of the same event can differ markedly but predictably. Here, I simulate the stratigraphic record of a foreland basin with initially rapid subsidence and low sediment supply during thrust loading, followed by a protracted decline in subsidence rates as sediment supply increases, reflecting tectonic quiescence and erosion of the fold and thrust belt. The simulation is based on a conservation of sediment volume in which the nonmarine and marine systems are each constrained to maintain a concave-up depositional profile of differing concavity. In addition, species are distributed along a primary ecological gradient, water depth in the marine system, and elevation in the nonmarine system. In marine settings, the stratigraphic expression of this two-phase basin history is a pre-event period of slow shallowing, followed by elevated subsidence rates that create a hiatal marine downlap surface recording pronounced deepening. This is covered by thick offshore deposits, in turn overlain by facies recording rapid shallowing during progradation. The fossil pattern is marked by a surface displaying a cluster of last occurrences of primarily shallow-water taxa and a cluster of first occurrences of mainly deeper-water taxa, overlain by progressive faunal replacement by increasingly shallow-water taxa. In downdip nonmarine areas, no downlap surface forms during the period of high subsidence rates. Instead, the coastal plain rapidly aggrades, accompanied by decreasing elevation as the shore transgresses. As subsidence rates decrease, accumulation rates decrease, and elevation increases as the shore regresses. The fossil pattern in downdip nonmarine areas consists of an expanded interval with elevated last occurrences of inland taxa and elevated first occurrences of coastal taxa, followed by a prolonged return to inland taxa. In updip nonmarine areas, the stratigraphic and fossil patterns are far more muted, with gradual and likely undetectable changes in elevation and fossil assemblages. These results underscore the need to recognize the implications of stratigraphic architecture, especially during biotic events such as mass extinctions.