STRATIGRAPHIC VARIABILITY IN TRACE-ELEMENT PALEOREDOX INDICATORS IN DEVONIAN-MISSISSIPPIAN MARINE BLACK SHALES, CENTRAL KENTUCKY

The degree of anoxia that existed during accumulation of Devonian-Mississippian black shales in central Kentucky has been debated. Trace-element concentrations and ratios were used to further elucidate paleodepositional environments for these units. The Sunbury, Cleveland and Huron Shales show significant differences in relationships between carbon, sulfur and trace elements.

Trace element data for a series of cores indicate significant enrichment factors in all three units. The magnitude of enrichment varies; the approximate order of enrichment relative to an average shale is Mo>Pb>Zn>V>Ni>Cu>Cr>Co. The Sunbury generally shows the highest levels of enrichment, followed by the Cleveland, with the Huron showing the lowest enrichment factors. Varying degrees of enrichment may suggest differences in depositional environment during accumulation. Mo levels infer anoxic conditions were more prevalent during accumulation of the Sunbury than during accumulation of either the Cleveland or the Huron. Geochemical ratios, including V(V+Ni), V/Cr, and Ni/C, also indicate variable paleoredox conditions for these shales. V/(V+Ni) ratios suggest inhospitable, low oxygen levels during accumulation of both the Sunbury and the Cleveland units (at least anoxic, and possibly even euxinic at times). V/Cr and Ni/Co ratios infer at least anoxic conditions during accumulation of the Sunbury, but suggest anoxic to dysoxic conditions during Cleveland accumulation. The possibility of anoxia during Huron accumulation is suggested by V/(V+Ni), but other indices (V/Cr and Ni/Co) suggest somewhat higher oxygen levels may have prevailed.

These data will be discussed within the framework of complementary studies that have used multiple geochemical and petrographic proxies to evaluate paleodepositional environments for these shales.