Upwelling in Cretaceous Western Interior Seaway: Sharon Springs Member, Pierre Shale

Judith Totman Parrish, Donald L. Gautier

Upwelling zones are characterized by the following features: (1) facies distribution, which is related to oxygenation of bottom waters, of laminated, organic-rich sediment, phosphate, and glauconite; (2) abundant organic-rich fecal pellets; (3) abundant marine biota at all trophic levels; (4) high organic carbon accumulation rates, with predominance of marine organic matter; and (5) a distribution of the anoxic facies that is not controlled by bathymetry. By contrast, stratified stagnant basins characteristically (1) lack significant phosphate; (2) show little or no evidence of high biologic productivity; (3) have high organic carbon accumulation rates with significant influx of terrestrial organic matter; and (4) have an anoxic facies that is confined to the deepest part of the basin. The Sharon Springs Member of the Pierre Shale, usually described as a stagnant-basin deposit, shares more features in common with upwelling zones than with stratified stagnant basins. Specifically, the overall facies distribution of the Sharon Springs includes a basinward zone of laminated, highly organic-rich strata bounded on the west by phosphate, which is, in turn, bounded at the western shoreline by glauconitic sandstone. The same facies distribution might be expected on the eastern shoreline, but the Sharon Springs is truncated there by erosion. The organic matter in the Sharon Springs is almost entirely contained in fecal pellets and is predominantly marine in origin, organic carbon accumulation rates are comparable to those of upwelling zones. Finally, the Sharon Springs is noted for high concentrations of bones of marine reptiles. These bones occur in a band that may mark the zone of highest biologic productivity.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.