ABSTRACT: Facies Differentiation in Carbonate Shelf Strata: Does Stratigraphy Dictate Sedimentology?

MCDONOUGH, KATIE JOE, and STEPHEN D. BLASKOWSKI, Colorado School of Mines, Golden, CO

Shelf facies in a Cretaceous carbonate ramp-to-platform setting, southern Vercors National Park, France, show remarkable contrasts in textures, allochem types, and facies associations, despite evidence that the sediments were deposited in identical water depth ranges and geomorphic settings. Rather than strictly recording depositional environment, facies types are strongly associated with the positions of progradational units within larger scale depositional sequences. These observations suggest that sedimentation processes and constituents of carbonate facies are responding to the same forcing functions that control stratigraphic architecture, and that carbonate facies are recording changes in ramp/shelf width and gradient, and balance between hydromechanical energy and biological pr ductivity through time, as well as depositional environment.

Discrete, 8-30 m thick, progradational units mapped in the Combau Valley are arranged in landward- and seaward-stepping geometries. In vertical section, each is an asymmetrical shoaling cycle bounded by a sharp flooding surface. Sedimentary structures indicate the sediments were deposited in water depths that oscillated between a few meters below storm wave base to a few meters above fairweather wave base. Shelf facies in landward-stepping units are well sorted, very fine to fine grained, micritized skeletal grain, peloid packstones, and minor grainstones with a few percent of quartz sand. Those in seaward-stepping units are poorly sorted, coarse grained to granule, nonmicritized bioclastic grainstones with rare quartz. These two facies associations do not occur as lateral equivalents within a single progradational unit. Rather, they are temporally separated and restricted to progradational units that occupy different positions within the geometrical stacking pattern.

Up to 800 m of these shelf strata occur without stratigraphic interruption in repeated shallowing cycles, each deposited within the same narrow range of water depth. Concurrent with the base-level cycles recorded by vertical oscillation of facies on the shelf, temporally equivalent shorelines must have been alternately prograding and flooding. The two facies tracts are stratigraphically decoupled, diametric to the strong coupling between shoreface and shelf facies tracts of siliciclastic systems. In this view, the shelf is a dynamically maintained surface, approximately at base level, representing the position where there is an energy balance between the biologic production of sediment and the continuous shaving and redistribution of sediment by the efficient "wave-base razor." Becaus this balance is maintained through long time and thick intervals of strata, it further suggests the operation of multiple feedback process/response systems that keep the shelf in dynamic equilibrium. This would account for the segregation of successions of either platform cycles that shoal only to fairweather wave-base, or of peritidal/littoral cycles that virtually never deepen beyond shallow subtidal. The notion that there is a paradox between high rates of carbonate sediment production in the Recent and calculated low accumulation rates in ancient shelf strata may have to be shelved.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)