PYRITE FRAMBOID SIZE DISTRIBUTION: ENVIRONMENTAL IMPLICATIONS FOR THE ACCUMULATION OF DEVONIAN - MISSISSIPPIAN SHALES, CENTRAL KENTUCKY

The depositional environment that existed during the accumulation of Devonian - Mississippian black shales of the eastern US has been a topic of debate in recent decades. In this study, pyrite framboid size distribution was evaluated for four cores from the Devonian outcrop belt of central Kentucky to determine the role that anoxic-euxinic conditions may have played in the accumulation of these black shales. For samples from the Appalachian Basin, pyrite framboid size distributions suggest that the Sunbury Shale and the upper part of the Huron Shale Member of the Ohio Shale were deposited under predominantly euxinic conditions, whereas the lowermost part of the Huron Member and the Cleveland Member of the Ohio Shale were deposited under predominantly dysoxic to oxic conditions. For samples from the Illinois Basin, pyrite framboid size distributions suggest that the Clegg Creek and the Morgan Trail members of the New Albany Shale indicate dysoxic to oxic conditions were present, whereas data for the Camp Run Member suggest the sediments were deposited during repeated intervals of euxinic deposition followed by predominant dysoxic - oxic conditions which allowed development of the bioturbated fabric seen in these gray shales. These results are not completely in agreement with previous research that utilized C-S-Fe and trace element relationships to establish depositional conditions during organic-rich sediment accumulation. For example, previous research has indicated that the Cleveland Member of the Ohio Shale and the Clegg Creek Member of the New Albany Shale were deposited under euxinic conditions. Reasons for these differences in interpretation may include syngenetic and diagenetic processes including winnowing of framboids in the depositional environment and atypical growth of the framboids