The Earliest Occurrence of the Middle Devonian Hamilton Fauna In Eastern North America: Further Refining of the Timing of the Kacak Bioevent

A major faunal turnover representing the Kacak Bioevent in Eastern North America (ENA) occurs between the Stony Hollow-Rogers City Fauna and the Hamilton-Traverse Fauna. 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 of the EBM 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/ Mt. Marion formation. The fauna of this interval is very sparse, with a few beds dominated by non-diagnostic small, dysoxic-tolerant bivalves. Of specific interest is a thin (~30 cm) shell bed in the upper third 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. 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 displays a faunal gradient similar to that seen in the overlying Hamilton with biofacies ranging from deeper-water chonetid-dominated assemblages in gray siltstone to shallower-water spiriferid-coral bearing sandstones. Thus the Kacak Bioevent is bracketed as occurring within the first 4th order cycle above the Cherry Valley Mbr.