ORGANIC MATTER ACCUMULATION IN DEVONIAN-MISSISSIPPIAN BLACK SHALES, EAST-CENTRAL KENTUCKY

Geochemical and petrographic differences between the Sunbury, Cleveland, and Huron Shales of east-central Kentucky suggest multiple controls on the accumulation of organic matter. Stratigraphic trends were determined following detailed analysis of a single core (DN6, Montgomery County, KY). Specifically, the role of inorganic input was evaluated using Ti/Al, K/Al and Si/Al as proxies for clastic input; C-S-Fe relationships, pyrite framboid size, Mo, V/(V+Ni), Ni/Co, and V/Cr were used to assess paleo-redox conditions during sediment accumulation; and C/P, Ba/Al, and P/Al were used as proxies for paleoproductivity. A terrestrial to marine maceral ratio -- (vitrinite plus inertinite)/(alginite plus bituminite) -- was used to evaluate changes in the source of organic matter.

C-S-Fe relationships, trace element ratios and pyrite framboid size distributions suggest anoxic conditions prevailed during deposition of a significant proportion of these units. However, bottom-water conditions were intermittently dysoxic, and geochemical parameters suggest that these periods of dysoxia may have been more likely to occur during accumulation of the Huron than during accumulation of either the Sunbury or the Cleveland. Productivity appears to have been an important factor in the accumulation of organic carbon: high C/P ratios are consistent with a productivity-anoxia feedback mechanism as originally proposed by Ingall et al. (1993) and recently applied to the Devonian of western New York (Murphy et al., 2000). In addition, nutrients from increased terrestrial weathering may have contributed to enhanced productivity. The triggering event that led to the establishment of anoxic bottom-water conditions and high productivity remains somewhat enigmatic. It also appears that productivity was just one factor in the accumulation of the organics in these units, as variations in sediment influx appear to be a factor controlling organic carbon content in the Sunbury, Cleveland and the lowermost part of the Huron. The increase in terrestrial organic components observed up-section may reflect vegetative changes occurring on land. Most of this change is attributable to increases in inertinite, much of which appears to be fusinitic in origin.