FAUNAL TURNOVER EVENTS WITHIN BLACK SHALE INTERVALS IN THE MIDDLE DEVONIAN APPALACHIAN BASIN

It has long been known that the rocks of lower Marcellus age (Early Middle Devonian-Late Eifelian) in eastern North America contain a fauna distinct from the lower Eifelian (Onondaga Fauna) below and the lower Givetian (Hamilton Fauna) above. Workers as early as the 1930’s noted that the fossils seen in the Stony Hollow Member of the Union Springs Formation in eastern New York State (NYS) could be found all across the eastern United States in a very restricted stratigraphic interval (Delaware Fm. of Ohio, Rogers City Fm. of Michigan, Spillville Fm. of Iowa, Lake Church Fm. of Wisconsin, Miami Bend Fm. of Indiana). It was later determined that this anomalous fauna represents an incursion of warm-water adapted taxa into the east-central U.S. out of central Canada, recording an interval of global warming during this time.

In the type area of New York State, the Stony Hollow fauna is bounded above and below by intervals of near black shale deposition (Union Springs shale below, East Berne shale above) and recent investigation of the boundaries between Evolutionary Ecological sub-Units within the Middle Devonian Appalachian Basin has shown that interval of dysoxia may play a key role in initiating large-scale biotic turnover. Lower Middle Devonian exposures in the Hudson Valley region of NYS preserve the most nearly complete stratigraphic sections through these boundary intervals anywhere in eastern North America, allowing for a highly detailed investigation of these turnovers. New information from a section of the at the top of the Stony Hollow interval shows the continuation of the Stony Hollow fauna upwards into the lower portion of the overlying East Berne shale, demonstrating that the fauna had not been eradicated from the basin at this point as had been earlier believed, indicating that the fauna remained in the basin through the later phases of the next sea level flooding event. Furthermore, this indicates that the final extinction of the Stony Hollow Fauna from the basin did not occur until the full onset of dysoxic conditions in the middle portion of the East Berne shale and that the timing of the biotic turnover event between the Stony Hollow fauna below and the Hamilton Fauna above was very rapid indeed.