TIMING OF FAUNAL TURNOVER DURING THE MIDDLE DEVONIAN KACAK BIOEVENT IN THE EASTERN APPALACHIAN BASIN

Research in the realm of Devonian Bioevents has progressed to the point where the major global bioevents have been identified in a diverse array of sedimentary facies providing the ability to examine the effect of these events in the different geographic localities in order to better understand their underlying causes. One area of specific interest is the examination of the timing of the faunal change across the bioevent boundaries. Such studies require virtually complete stratigraphic sequences, preferably in areas with high rates of sedimentation, where the cloaking of the bioevents at diastemic facies boundaries is not an issue. In such cases, fine-scale stratigraphic sampling can nearly completely resolve the timing of faunal replacement across the bioevent boundaries in question.

One such example is the Eifelian-Givetian interval in the Hudson Valley of New York State where thick, stratigraphically complete sections have been examined spanning the Kačak Bioevent. In this region the Kačak Bioevent represents the extinction of a warm-water Stony Hollow Fauna that emanated out of equatorial regions into the Appalachian Basin during the late Eifelian and the subsequent replacement by the cooler-water, diverse Hamilton Fauna that migrated into the area from the south nearly coincident with the Eifelian-Givetian boundary. Fine-scale sampling of this interval indicates the turnover between the Stony Hollow Fauna and Hamilton Fauna occurred over a very short interval across a diverse array of biofacies ranging from deep-water, dysoxic to shallow-water, diverse environments. To date, only 2 taxa have been found to carry over between the two faunas: the brachiopod Longispina mucronatus, a generalist taxa found in nearly all facies ranging from the early-Eifelian Onondaga Fauna, through Stony Hollow Fauna, continuing upwards into the Hamilton Fauna above, and the goniatite Paradiceras discoideum that first appears in the upper portion of the Stony Hollow Fauna continuing well up into the overlying Hamilton Fauna. These findings to indicate that the Kačak Bioevent occurred as a result of a water-mass turnover associated with an influx warm, relatively dysoxic water that invaded the basin during the latest Eifelian that was very quickly replaced by cooler, more oxic waters that persisted through the Hamilton interval.