REFINING THE TIMING OF FAUNAL TURNOVER DURING THE MIDDLE DEVONIAN IN THE APPALACHIAN BASIN: NEW FINDINGS ON THE EARLIEST OCCURRENCE OF THE HAMILTON FAUNA IN EASTERN NEW YORK STATE

A major faunal turnover in the Middle Devonian of eastern North America occurs between the Stony Hollow-Rogers City Fauna and the Hamilton-Traverse Fauna near the end of the Eifelian Stage. This large-scale turnover has been shown to occur across most of ENA in both the Appalachian and Michigan basins, during the latest Eifelian. The first appearance of the Hamilton Fauna has long been identified as occurring in the Halihan Hill Bed of the Oatka Creek Formation, which lies above the East Berne Member (EBM) shale interval.

Recent attention has focused on investigating the precise timing of this turnover in the stratigraphically expanded interval found in eastern New York State. Lying between the top of the Cherry Valley Mbr. and the Halihan Hill Bed in eastern New York State, the EBM is composed primarily of dark-gray to gray shale with thin siltstones and sandstones near the top, interpreted to represent the highstand and falling-stage systems tracts of the lowest 4th-order stratigraphic sequence of the Oatka Creek Formation. The fauna of this interval is very sparse, with a few beds dominated by non-diagnostic small, dysoxic-tolerant bivalves. Of specific interest in this interval is a thin (~30 cm) shell bed near the middle of the interval known as the Dave Elliot bed. This unique concentrated shell bed in the EBM provides critical insight into the interval between the Stony Hollow and Hamilton faunas in ENA. Preliminary investigations of this bed have yielded the earliest occurrence of a number of key Hamilton Fauna taxa including the brachiopods Tropidoleptus and Mediospirifer. This bed also displays a faunal gradient similar to that seen in the overlying Hamilton with biofacies ranging from deeper chonetid-dominated assemblages in gray siltstone to shallower spiriferid-coral bearing sandstones. Thus the biotic turnover is bracketed as occurring within the first small (5th order) cycle above the Cherry Valley Mbr.