Print this page3-D Patterns of Organic Carbon in the Cretaceous Western Interior Seaway: Implications for Paleoceanography
PARRISH, JUDITH TOTMAN, and GLEN S. TANOK
Limited geochemical data on the distribution of organic matter in the Cretaceous Western Interior Seaway (WIS) has meant that paleoceanographic models of organic sedimentation have been poorly constrained. Abundant well logs, however, make the WIS ideal for estimating total organic carbon (TOC) from well logs and thus determining the distribution of organic carbon in detail. Combined with detailed stratigraphic information, TOC estimates give a picture of the 3-D distribution of organic carbon in the WIS. We have applied these methods to the Niobrara Formation and the Sharon Springs Member of the Pierre Shale. in general, TOCs are higher in the southern part of the study region (roughly 38-42 degrees N and 100-105 degrees W). In addition, at some times, TOCs are highest in the eastern and western portions of the study region. in only a few intervals are TOCs highest in the deepest part of the basin. Organic-carbon accumulation rates (OCAR) follow roughly the same patterns.
Distributions of TOC and OCAR do not support stable stratification and anoxic bottom waters as the principal mechanism for organic accumulation because such waters would occupy the deepest part of the basin. Higher TOC values in the southern portion of the basin might provide support for intrusion of anoxic water from Tethys. However, higher values on the eastern and western margins of the basin are not consistent with this scenario. Rather, they indicate coastal upwelling or influx of terrestrial organic matter; available geochemical data suggest the latter is not the case. Coastal upwelling may have been wind-driven or a local manifestation of the estuarine circulation that dominated the general circulation of the seaway.