Abstract: Paleoceanographic Model for Ordovician Organic-Rich Facies in Eastern North America: Local or Global Causes?

Mark E. Patzkowsky, Steven M. Holland

Organic-rich limestones and shales occur at several stratigraphic horizons in Middle and Upper Ordovician strata of eastern North America. Upper Whiterockian and Lower Mohawkian (Llandeilian-Lower Caradocian) organic-rich deposits are restricted to the eastern margin of North America and occur in deeper parts of the Taconic foreland basin. Upper Mohawkian and Cincinnatian (Middle Caradocian-Lower Ashgillian) organic-rich shales are more widespread and are concentrated at three stratigraphic horizons: middle Mohawkian, basal Cincinnatian, and middle Cincinnatian. The organic-rich facies are deposited during sea level rises and occur within middle to upper parts of transgressive systems tracts and the lower parts of highstand systems tracts.

The widespread deposition of organic-rich shales is related to a major shift in paleoceanographic conditions across eastern North America. Siliciclastics shed from the Taconic orogen spread westward as tropical carbonate conditions retreated. Widespread phosphate deposition indicates an increased nutrient flux to the basin, possibly provided by freshwater runoff from the Taconic highlands and/or by upwelling of nutrient-rich waters. The combination of eustatic rise, thrustloading, and upwelling led to an influx of siliciclastics and permitted nutrient-rich, oxygen-poor waters to spread across eastern North America.

Extensive organic-rich deposits also occur in strata of similar age in Estonia and Australia indicating the widespread nature of organic carbon burial. Global oceanic circulation patterns conducive to upwelling, combined with eustatic sea level changes may have served as ultimate controls on the deposition of organic-rich facies.

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