REDOX CONDITIONS, DEPOSITIONAL STYLES, AND CARBON PRESERVATION IN BLACK SHALE SEQUENCES: A DEVONIAN-AGE CASE FROM THE ILLINOIS BASIN

The widespread deposition of black shale facies during Middle-Late Devonian through Early Mississippian time is of keen interest to earth historians, tectonicists, and geochemists alike. This pivotal time period encompasses 1) the largest known marine mass extinction, 2) an episode of large-scale sequestration of organic matter in tectonically-controlled marginal marine basins, and 3) the burgeoning influence of the terrestrial biosphere in the global carbon cycle. Geochemical, sedimentological, and stratigraphic explorations of Devonian-Mississippian black shales, especially those of the Appalachian Basin, have yielded valuable but often conflicting insight into the dominant paleoenvironmental controls responsible for episodes of anoxia required to drive enhanced organic preservation in marginal marine basins. We attempt to further unravel the biogeochemical and sedimentological mechanisms responsible for black shale formation in the Illinois Basin by coupling interpretations gained from bulk geochemical/isotopic studies with detailed sedimentological findings, both derived from a USGS core that was recently recovered in Iroquois Park, Louisville, KY. Preliminary results from the Late Devonian black shale facies of the Camp Run and Clegg Creek members of the New Albany Shale indicate that 1) the abundance of organic carbon ranges from 4% in the lower Camp Run to over 20% in the uppermost present Clegg Creek, 2) the stable isotopic composition of bulk organic matter falls within a narrow range (d¹³C=-30.5 to -29‰; d¹⁵N=-1.0 to +1.3‰), and 3) the bulk C/N ratios range from 25 to 40. Grey shale facies within the New Albany Shale are characterized by lower C abundances and C/N ratios, and heavier C and N isotopic signatures. Preliminary trace metal results (Mo/Al, V/Al) from the upper 6 meters of the 30-meter black shale interval identify significant enrichment of redox-sensitive elements, suggesting that additional metal analyses will provide a more complete history of anoxia, euxinia, and will potentially deepen our understanding of the mechanisms controlling carbon preservation. These results will be sample-matched with core x-radiography in order to elucidate the linkages between redox conditions, styles of deposition, and their overall effect on carbon preservation.