Late Eifelian (Middle Devonian) biocrises: Timing and signature of the pre-Kaák Bakoven and Stony Hollow Events in eastern North America

摘要

The late Eifelian (Middle Devonian) was a time of substantial physical and biotic change in the marine realm, herein termed the late Eifelian Biocrisis. This interval can be subdivided into three discrete bioevents, with the last being the widely recognized Kaák Event in the ensensis conodont Zone near the Eifelian-Givetian stage boundary. The earlier bioevents, within the australis to eiflius Zones, are defined by striking faunal changes in the Appalachian Basin and coeval formations in adjacent areas of eastern North America. The first, the Bakoven Event (australis Zone), is marked by a significant reduction in faunal diversity in the uppermost Onondaga Limestone and overlying Bakoven Black Shale that is associated with eustatic/tectonic deepening and widespread hypoxia. The second, the 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 to temperate shelf region of eastern North America. The incursion is recorded in the faunas of in the Stony Hollow, Hurley, and Cherry Valley Members of the Marcellus subgroup in the Appalachian and in equivalent units in the Michigan and Iowa Basins (Rogers City, Lake Church, and Spillville Formations). The Stony Hollow Event is much less evident in the coeval Delaware and North Vernon Formations in the Wabash Platform of Ohio and southern Indiana. The Stony Hollow bioevent appears to be associated with altered current patterns and/or climatic regime during the kockelianus-eiflius Zone interval. Approximately coeval formations in Europe and Morocco 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 Eifel and Rhenish Schiefergebirge areas of Germany, and in the Cantabrian Mountains of Spain show evidence for faunal turnover during the interval of the australis-. kockelianus Zones. Both the Bakoven and Stony Hollow and the terminal Kák Events appear to be associated with changes in climatic gradients and major disturbances in the global carbon cycle.