ORGANIC PETROGRAPHY AND GEOCHEMISTRY OF THE SUNBURY SHALE: FURTHER INSIGHTS INTO THE DEPOSITIONAL SYSTEM

The Sunbury Shale represents the uppermost unit of a series of economically important Devonian-Mississippian black shales in Kentucky and surrounding states. The Sunbury and underlying Ohio Shale are of interest not only as trace element and rare-earth sources, but also as source rocks in the deep subsurface, and as indicators of the onset of the Acadian and Neoacadian orogenies. The Ohio Shales and their equivalents have received comparatively more attention than the thinner Sunbury Shale in Eastern Kentucky. Organic petrography and geochemistry of samples acquired from a new road-cut exposure was studied to expand our understanding of terrestrial vs. marine input and preservation during deposition of the Sunbury Shale. Previous works that include organic petrography of the Sunbury Shale suggest high terrestrial matter, primarily inertinite, input to the system and deposition under persistently anoxic conditions. Detailed geochemical analyses suggest that the dominant source of organic matter is marine phytoplankton, and that anoxia was not persistent, but that suboxic conditions prevailed, excepting in the middle of the unit. New organic petrographic results obtained from splits of samples studied for detailed geochemistry are presented. Terrestrial organic matter input increases, then decreases up section, with the greatest period of terrestrial matter input corresponding to the greatest period of bacterial decay of marine macerals. Anoxic to Euxinic conditions were most prevalent at the beginning and the end of Sunbury deposition, with suboxic conditions prevailing through the majority of the section. P, V, and Zn concentrations are correlated with marine maceral occurrences. Marine maceral facies indicate that relatively shallow (25-100 m), open-ocean conditions prevailed during deposition of the Sunbury Shale.