Geochemical paleoredox indicators in Devonian–Mississippian black shales, Central Appalachian Basin (USA)

The degree of anoxia that existed during accumulation of Devonian–Mississippian black shales in central Kentucky has been debated widely. In this study, geochemical data were used to elucidate paleodepositional environments for these units. Using analyses from ten cores collected from the outcrop belt in central Kentucky, sulfur, carbon, and selected trace-element relationships were compared. The Sunbury Shale (Tournasian, Lower Mississippian) and the Cleveland and Huron members of the Ohio New Albany Shale (Fammenian, Upper Devonian) show different relationships between carbon, sulfur, and trace elements. C–S–Fe relationships for the Sunbury suggest anoxic, possibly even euxinic, conditions prevailed during sediment accumulation. Analysis of C–S–Fe data suggests that the Cleveland may have accumulated under anoxic conditions, with intermittent dysoxia, whereas the Huron Member may have accumulated under a relatively wide range of conditions, from anoxic, to dysoxic, to possibly oxic. These three units exhibit different degrees of trace-element enrichment, with the approximate order of enrichment relative to an average shale being Mo>Pb>Zn>V>Ni>Cu>Cr>Co. The Sunbury shows the highest levels of enrichment, followed by the Cleveland, with the Huron showing only slight enrichment for most of these trace elements. Differing degrees of enrichment may reflect differences in depositional environment during accumulation. High Mo concentrations in the Sunbury (generally 200–550 ppm) may infer euxinic conditions prevailed during sediment accumulation. The Cleveland and the Huron both show considerably lower Mo contents (averaging around 100 ppm). Geochemical ratios, including Ni/Co, V/Cr, and V/(V+Ni), also indicate variable paleoredox conditions for these shales. Based on previously established thresholds, V/Cr and Ni/Co ratios infer at least anoxic conditions during accumulation of the Sunbury, anoxic to dysoxic conditions during Cleveland accumulation, and dysoxic to oxic conditions for the Huron. V/(V+Ni) ratios tend to indicate consistently lower oxygen regimes than do other paleoredox indicators, and this discrepancy is greatest for the Huron Member. It is suggested that thresholds established for paleoredox indicators in previous studies should not be applied strictly, but that relative differences in these indicators collectively can infer variations in the degree of anoxia. In addition, relationships between Corg and redox elements may help elucidate the role of anoxia in OM accumulation. It may be concluded from this study that the Devonian–Mississippian black shales of central Kentucky accumulated under variable bottom-water conditions. At least anoxic conditions prevailed during accumulation of much of the Sunbury Shale and the upper part of the Cleveland Member, and possibly euxinic conditions for the Sunbury. Bottom-water conditions may have been intermittently anoxic and dysoxic during deposition of the lower Cleveland. During accumulation of the Huron Member, it is likely that conditions ranged from anoxic to dysoxic to marginally oxic, possibly being close to normal marine conditions at times during accumulation of the lowermost Huron sediments.