THE EMSIAN STAGE (UPPER LOWER DEVONIAN), APPALACHIAN BASIN: STRATIGRAPHY, SEQUENCES & T-R CYCLE IB, VOLCANICS & GEOCHRONOLOGY, & NEW DIRECTIONS

Emsian strata in the Appalachian Basin have been largely overlooked by geologists and paleontologists over the years. Recent basinwide analyses of Emsian and Eifelian strata (>350 outcrops) have vastly improved knowledge of this interval, making the Appalachian Basin one of the better known Emsian successions globally. These Emsian strata are now shown to comprise the Esopus and Schoharie fms. (NY, e PA); the Beaverdam and calcareous shale mbrs. of the Needmore Fm. (cent. PA, MD, VA, WV); and the Huntersville Fm. (except a thin capping sandstone; sw VA, WV). The latter was long mistakenly correlated with the overlying Eifelian Onondaga Fm.

Five Emsian “third order” depositional sequences are correlatable throughout the basin. Three of these occur in “lower Emsian” strata of the Esopus Fm. and equivalents (Beaverdam, lower Huntersville). The remaining two sequences occur in “upper Emsian” strata of the Schoharie Fm. and equivalents (calcareous shale, upper Huntersville). These two subdivisions appear equivalent to the international Subcommission on Devonian Stratigraphy's proposed Zlichovian and Dalejan substages of the Emsian Stage.

The Devonian T-R cycle model of Johnson et al. (1985) assigned one major T-R cycle to Emsian and lowest Eifelian strata, based on what now is seen as too little available data. Based on a greater knowledge, Emsian T-R Cycle Ib is now proposed to comprise five major T-R cycles. This reinterpretation is supported by geochronologic data from Appalachian Basin K-bentonites (lower Emsian Sprout Brook K-bentonites, 408.3 +/- 1.9 Ma; and lower Eifelian Tioga Middle Coarse Zone K-bentonites, 391.4 +/- 1.8 Ma; dates from Tucker et al., 1997), indicating a ca. 17.2 million year duration for the Emsian – making T-R Cycle 1b unjustifiably an order of magnitude greater than other Devonian T-R cycles.

One of the problems in utilizing the Appalachian Basin Emsian as a global standard is poor biostratigraphic resolution. New collaborative studies with global experts are presently underway, in a search for globally correlatable taxa (goniatites, conodonts, spores, dacryoconarids, brachiopods, ostracodes), along with an attempt to correlate the major Emsian cycles worldwide using magnetic susceptibility. Hopefully, results will allow correlation of the eastern U.S. record to the global Emsian.