Distribution and origin of organic carbon in the Upper Cretaceous Niobrara Formation and Sharon Springs Member of the Pierre Shale, Western Interior, United States

Abstract

The Upper Cretaceous Niobrara Formation and the overlying Sharon Springs Member of the Pierre Shale are two of several organic-carbon (OC) rich units deposited in the Western interior seaway. A key to assessing the validity of models proposed to account for OC enrichment in these units is understanding the three-dimensional (3D) distribution of OC within these units. Within the study area abundant subsurface data in the form of geophysical well logs are available. These logs were used to divide the Niobrara and Sharon Springs into regionally correlatable chronostratigraphic horizons and to estimate OC-content. By combining these elements a 3D picture of the OC-distribution was obtained. This distribution was compared to changes in lithology, sedimentation rate, and tectonic activity. The Niobrara and Sharon Springs contain local and regional disconformities which indicate water depths were near an estimated storm-wave base depth of 100 m. Local disconformities in the Niobrara are indicative of basinal tectonic activity that is linked to Sevier thrusting to the west. Large-scale Niobrara chalk/marl cycles also appear to be linked to Sevier thrusting. There is a regular vertical pattern of OC enrichment in the Niobrara, but no pronounced regional patterns are evident. Estimated paleoproductivities are moderate, except to the southeast where higher productivities may have been a consequence of upwelling. Small-scale chalk/marl cycles result from alternation of high productivity during periods of fairly vigorous circulation (chalks) with low productivity during periods of more sluggish circulation (marls). The regional diachrony of the Sharon Springs facies was a result of clastic dilution associated with progradation from the west and the paleobathymetry. Paleoproductivities were moderately high during Sharon Springs deposition, but there is no conclusive evidence of upwelling. Both units were deposited beneath bottom waters that were on average dysoxic, but oxygenation levels varied intermittently from fully oxic to anoxic. These changes were climatically modulated. At short time scales they may have been seasonal and at longer time scales they may have been driven by Milankovitch cyclicity.