THE LATE MIDDLE DEVONIAN GLOBAL TAGHANIC BIOCRISIS IN ITS TYPE-AREA (NORTHERN APPALACHIAN BASIN): GEOLOGICALLY RAPID FAUNAL TRANSITIONS DRIVEN BY GLOBAL AND LOCAL ENVIRONMENTAL CHANGES

The late Middle Devonian (middle-upper Givetian) “Taghanic (Pharciceras) Event” was originally named for goniatite turnovers in the northern Appalachian Basin during the deposition of the Tully Limestone. Subsequently, this biocrisis has been associated with the extinction of most of the long-persisting ‘Hamilton Fauna’ over a period of approximately 0.5 million years. In order to fully understand the cause(s) of this turnover, paleoecological reconstruction of faunas through this biocrisis using a high-resolution stratigraphic framework was undertaken along a complete onshore through offshore gradient in the type-area. This has revealed at least three main pulses of faunal transition within the northern Appalachian Basin, each comprised of the step-wise incursion and resulting replacement by an extra-basinal fauna.

The Taghanic Biocrisis is now recognized globally as a series of pulsed faunal transitions and extinctions, resulting in an end to faunal provinciality and, ultimately, in the appearance of a global cosmopolitan fauna. Global-scale environmental changes occurring at this time, namely temperature increases, changes between arid and humid intervals, rapid sea level fluctuations, and widespread black shale deposition, only account for some features of the step-wise faunal transitions recognized in the type-area. However, when global environmental changes are considered in the context of regional basin dynamics, and in particular the degree to which estuarine-type watermass circulation patterns were operating, all aspects of the faunal transitions correspond well with the hypothesized environmental changes. Application of this watermass circulation model to the faunal changes observed suggests that regional salinity was an important control on faunal distribution in the northern Appalachian Basin during the Global Taghanic Biocrisis.