ENVIRONMENTAL GRADIENT ANALYSIS IN THE UPPER ORDOVICIAN TACONIC FORELAND BASIN, CENTRAL APPALACHIANS, USA

Bretsky’s (1970) classic study of paleocommunities in the Upper Ordovician Taconic foreland basin suggested three dominant marine assemblages ranging from Pennsylvania to Virginia. Multivariate analyses of his data reveal a primary environmental gradient related to depth, and a secondary gradient related to carbonate content. We compare gradient analyses between central Pennsylvania and southwestern Virginia to quantify how these Ordovician assemblages responded to these two primary gradients. Preliminary analyses in central Pennsylvania show abundance peaks of Lingula, Orthorhynchula, Rafinesquina, Onniella, Sowerbyella, Triarthrus, and graptolites moving from high-energy, shoreface environment to an offshore environment below storm wavebase. This faunal gradient is similar to the findings of Springer and Bambach (1985) in southwest Virginia, although some major differences occur between the shallow portions of the gradients from the two areas. In central Pennsylvania the brachiopod Orthorhynchula is abundant in siliciclastic shoreface environments, whereas in southwestern Virginia the brachiopods Hebertella and Zygospira and the bivalve Ambonychia dominate in shallow subtidal carbonate environments. These faunal differences between regions suggest that the carbonate-siliciclastic gradient along the axis of the Taconic foreland basin provided an additional environmental gradient controlling the composition of the marine benthic assemblages.