USING ORGANIC CARBON-U CORRELATIONS IN BLACK SHALES TO CONSTRAIN BASIN-SCALE PALEOCEANOGRAPHIC CONDITIONS

While it is recognized that Upper Pennsylvanian cyclothemic black shales contain high amounts of total organic carbon (TOC), there is lateral variation across the midcontinent in the concentrations found in individual outcrops, which reflects changes in depositional environment. That is, not all black shales form in deep, anoxic marine basins. Similarly, the total gamma ray intensity is known to be “high” in black shales, as compared to other sedimentary rock types, largely driven by high concentrations of U. Reducing conditions are associated with the preservation of both OC and U in sediments of Desmoinesian age, often leading to correlations in their concentrations. Previous work indicates that organic matter type, not just quantity, may be an important control on U preservation in black shales. If this observation holds for a wide variety of depositional settings, this would provide a way to use gamma-ray spectrometry to assess basin-wide variations in OC preservation.

To test this hypothesis, we have undertaken an outcrop study of the Excello Shale, which was deposited under generally reducing conditions over a large area from Oklahoma to Iowa during the Desmoinesian. We have extracted bitumen from samples collected in Oklahoma, Kansas and Missouri, fractionated it with Si-Al gel chromatography, and will use gas chromatography to assess basin-wide variations in OC type. We have also performed gamma-ray spectrometry on the outcrops, and developed a stratigraphic framework within which to assess the variations between OC and U concentrations. From this, we will develop a model linking U concentrations with OC type that can be applied to determine basin-scale changes in biogeochemical conditions during the Pennsylvanian, using gamma-ray logs from cores. Basin-scale paleoenvironmental models will provide additional insights in our understandings of the development of cyclothems and similar shales in economically important petroleum basins. These models will be used to improve our understanding of the links between carbon cycling and paleoceanographic conditions.