Organic Geochemistry of the Cretaceous Western Interior Seaway: A Trans-Basinal Evaluation
Richard D. Pancost, Katherine H. Freeman, Michael A. Arthur, 1998. "Organic Geochemistry of the Cretaceous Western Interior Seaway: A Trans-Basinal Evaluation", Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, Walter E. Dean, Michael A. Arthur
Download citation file:
We used hydrocarbon distributions to establish the dominant sources and controls on organic matter deposition and preservation in rocks from the latest Cenomanian-early Turonian interval in cores representing a three-site transect across the Cretaceous Western Interior Seaway. Here we present the results of free n-alkane and isoprenoid hydrocarbon analyses. Maturity varies slightly between the three cores: organic matter in the central basin core is most mature (equivalent vitrinite reflectane ~0.55), while in the western and east-central basin cores organic matter is less mature (equivalent vitrinite reflectance -0.4). Geographic controls on compound abundances were dominant over lithologic controls, with terrestrial sources dominating on the western margin proximal to the Sevier highlands and a mixture of marine and terrestrial sources present in the central basin. Pristane and phytane in samples from the east-central basin exhibit 13C-enrichment relative to the other sites; it is proposed that this difference results from a greater Tethyan contribution to surface waters at that site than at the others. Interpretation of pristane/phytane ratios is complicated by maturity differences but indicates that depositional conditions were most reducing at the basin margins.
At a given location, variations in compound abundances through lithologic cycles primarily record increased degradation of organic matter during deposition of bioturbated limestones. In those same beds, selective degradation of marine organic matter and/or lower fluxes of marine organic matter also resulted in preferential preservation of terrestrial organic matter. These processes affected the compound-specific record of the Cenomanian-Turonian positive carbon isotope excursion, which is expressed in the δ13C values of pristane and phytane but is less clear in δ13C values of other compounds. This is likely a function of multiple sources for individual compounds, whose relative contributions are influenced by both surface-ocean and early diagenetic processes.
Figures & Tables
This volume presents the results of a coordinated, multidisciplinary study of Cretaceous carbonate and clastic rocks in cores collected along a transect across the old Cretaceous seaway that extended from the Gulf Coast to the Arctic by a team of academic, industry and U.S. Geological Survey scientists. The overall goal was to construct a subsurface transect of mid-Cretaceous strata that were deposited in the U.S. Western Interior Seaway. In particular, the papers in this volume focus on the Graneros Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation and equivalents in cores from six drillholes from western Kansas, southeastern Colorado and eastern Utah.