Abstract: Cretaceous Platform Margins and Extended Lowstands of Sea Level

W. Schlager, A. J. Everts, B. Fouke, J. Stafleu

The standard model of systems tracts assumes that stratigraphic sequences of the third order are generally bounded by lowstand unconformities. In contrast, the stacking patterns of higher-order cycles commonly show that third-order cycles are rather symmetrical and have gradational boundaries. Well-exposed margins of carbonate platforms can provide critical evidence in this debate because extended subaerial exposure postulated by the systems-tract model will produce basin-restricted lowstand tracts that form when sea level stands below the bank top. Conversely, in gradually prograding and retreating margins that lack long-term exposure events, the basin-restricted wedges will be absent.

We evaluate these contrasting interpretations for the Cretaceous Vercors platform of the French Alps. Basin-restricted wedges and long-term exposure have been postulated from the study of large-scale stratal patterns. High-resolution stratigraphy has been established with the quantitative variations in sediment composition and by physical tracing of beds. This evidence argues against the existence of basin-restricted wedges and for a pattern of gradual progradation and retreat. Diagenetic studies support this view as they indicate frequent exposure events in the fourth or fifth order domain rather than few, long-lasting events in the third-order domain. The third-order sequence boundaries seem to represent bundles of closely spaced exposure surfaces that belong to the higher-order cyc es. Seismic models of the two lithologic interpretations are virtually indistinguishable at standard industry frequencies.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994