LATE EIFELIAN (MIDDLE DEVONIAN) BIOTIC TURNOVER PATTERNS IN EASTERN NORTH AMERICA -- WITH IMPLICATIONS FOR GLOBAL BIO-EVENTS NEAR THE EIFELIAN-GIVETIAN STAGE BOUNDARY

The late Eifelian was a time of substantial geographic, environmental, and biotic change in the marine realm. Compilation of eastern North America faunal data, within a revised stratigraphic correlation framework, indicates that this interval comprises three discrete bioevents, with the last being the widely recognized Kaèák Event in the ensensis conodont Zone, closely preceding the Eifelian-Givetian stage boundary. The two earlier bioevents, defined herein, occur within the australis to eiflius zones, and are characterized by striking faunal changes in the Appalachian Basin and coeval formations in adjacent areas of eastern North America.

The first, Bakoven Event (australis Zone), is marked by a ~90% reduction in species diversity from the Onondaga Limestone into the overlying Bakoven black shale, as well as in up-ramp units in Ohio. This is associated with eustatic/tectonic deepening and widespread hypoxia. The second, Stony Hollow Event (kockelianus/eiflius zone), is marked by an incursion of tropical Old World Realm Cordilleran Province taxa, including distinctive atrypid brachiopods, corals, and bivalves, into the subtropical shelf region of eastern North America. The incursion is recorded in faunas of the Stony Hollow, Hurley, and Cherry Valley members of the Marcellus subgroup in the Appalachian basin and equivalent units in the Michigan and Iowa basins (Rogers City, Miami Bend, Lake Church, and Spillville formations). The Stony Hollow Event is less evident in the coeval Delaware and North Vernon formations in the Wabash Platform of central Ohio and southern Indiana. The incursion of this fauna appears to be associated with altered current patterns and/or climatic regime.

Approximately coeval formations in Europe show the probable signatures of these events, indicating that the Bakoven and/or Stony Hollow may be global-scale events. In particular, the late Eifelian successions in the Cantabrian Mountains of Spain, and the Eifel and Rhenish Schiefergebirge areas of Germany show evidence for faunal turnover during the australis-kockelianus zones. The Bakoven, Stony Hollow and Kaèák Events appear to be associated with changes in sea level, climatic gradients, and major fluctuations in the global carbon cycle. Collectively the three events make up a multi-phase bio-crisis in the late Eifelian.