SUBTLE VARIATION IN PALEO-OXYGENATION AND ITS EFFECTS ON BIOTIC TURNOVER IN THE MIDDLE DEVONIAN APPALACHIAN BASIN OF NEW YORK

The Middle Devonian succession of the Appalachian Basin of New York preserves a series of rapid biotic turnovers, influenced by depth and lithology. In this study, multivariate statistical analyses of faunal data collected from multiple horizons indicate an asynchronous, depth mediated replacement of various biofacies of the Windom Shale of the Hamilton Group by faunas of the overlying Tully Limestone. This complex pattern runs counter to previous views that the Hamilton and Tully units contain the same repetitive biofacies, and should thus be grouped as one ecological evolutionary subunit. Biotic elements typical of the most dysoxic and deepest shelf settings of Tully first appear earlier in the deepest regions of the latest Windom. In slightly shallower dysoxic settings of the latest Windom these faunas interfinger with biofacies common to more oxygenated settings seen slightly earlier in the Windom. Once onset of Tully Limestone deposition occurs in this facies, a fauna distinctive from that of the deepest Tully is found. Thus, the dysoxic biofacies of the deepest Tully settings is coeval with more typical Windom biofacies, but both are replaced in slightly shallower settings of the Tully by yet a third dysoxic biofacies.

A detailed analysis of these dysoxic biofacies was performed to better understand the paleoenvironmental constraints on this regional biotic transition. Although almost all samples of the latest Windom Shale and the lower Tully Limestone contain some percentage of silt, a correlation of other lithologic components with biofacies is observed. The most dysoxic fauna is nearly always found in siltstones or sandstones that contain at least one mineral from a suite of pyrite, phosphate, or chamosite. The moderately dysoxic biota occurs in rocks with a higher calcareous component. Finally, the more oxygenated Windom faunas are found in shalier units that are often bioturbated.

Thus, a multifaceted turnover event is recognized between the Windom and Tully, in which three distinct faunas interacted with one another. Each fauna can be assigned separate, but related, depth and lithologic parameters. These results suggest that detailed dissections of turnover boundaries may reveal depth influenced changes and previously unrecognized paleoenvironmental gradients.