TIMING OF ONSET OF MIDDLE DEVONIAN TAGHANIC BIOEVENTS RELATIVE TO SEA LEVEL CHANGES, EAST-CENTRAL NEW YORK STATE AND CENTRAL PENNSYLVANIA

Although recognized as a significant episode of faunal incursions and extinctions, the Late Middle Devonian (Givetian) "Taghanic Bioevent" is poorly understood in the type Taghanic area of western New York owing to unconformity development in that area; the timing of this incursion event, thus, has remained problemmatic. Discovery of new sections of the Cooperstown Member, New Lisbon Member, and Tully Formation clastic correlatives in the Butternut Valley-Otego Valley region in east-central New York, shows that a nearly complete stratigraphic succession exists for the Late Givetian in this area. A key temporal event corresponding to this succession is the onset of a complex series of biotic changes, during which the diverse, long-standing, endemic Hamilton Fauna was partly, to largely replaced by distinctive Old World taxa comprising the Tully Fauna. The new sections in eastern New York, as well as others, newly reported from central Pennsylvania, show that the faunal invasion occured during episodes of sea level highstand and that it occurred in, at least, two distinct pulses. In New York, Tullypothyridina (formerly Hypothyridina) first appears near the base of the New Lisbon Member in dark, dysoxic "Leiorhynchus" (Camarotoechia) facies. In Pennsylvania, the key Tully Fauna taxon Rhyssochonetes appears in correlative "Leiorhynchus" facies at the top of the Mahantango Formation. Both in New York and Pennsylvania, typical Hamilton taxa briefly reappear above this Tully Fauna vanguard in higher, more oxic facies. The base of the Tully Formation, above the New Lisbon and its Pennsylvania equivalent, marks a second transgression and a more pervasive and long-lasting incursion of Tully taxa. This pattern, coupled with evidence for earlier transgressive incursions of Emanuella and eventual destruction of endemic Hamilton Fauna shelf communities, was linked, in part, to a series of increasingly severe dysoxic, deepening events during the Late Middle Devonian.