EVIDENCE FOR LATE DEVONIAN AND EARLY CARBONIFEROUS GLOBAL COOLING IN THE APPALACHIAN BASIN

The Late Devonian Spechty Kopf Formation in Pennsylvania (PA), and unnamed correlative units in Maryland (MD) and West Virginia (WV), comprise an enigmatic lithologic unit (ELU) that is known to occur in a 30 ± km-wide belt extending 400 km from northeastern PA, across western MD, into north central WV. The ELU commonly attains a thickness of nearly 400 m in northeastern PA, but thickness varies locally from 0 to 730 m. We interpret a glacial to periglacial control on the origin of the ELU, based on the following features (in ascending stratigraphic order): 1) a basal nonbedded polymictic diamictite containing matrix-supported clasts (of unknown provenance) up to 2 m in diameter (tillite), 2) bedded siltstone and sandstone containing matrix-supported polymictic clasts, up to 2 m in diameter (dropstones), and 3) laminites (glacial varves). Striated facets, which we interpret to be glacial in origin, are present on some clasts. The tillite, dropstones, varves, and faceted clasts in the ELU are indicative of deposition at or near the terminus of either continental ice centered in Gondwana (South America), or glaciers that debouched from undocumented highlands to the south. In either case, the study region was situated in the vicinity of 30 degrees south during the Devonian/Carboniferous transition (Scotese, 2002), which is approximately coeval with the onset of Late Devonian global "ice house" conditions (Frakes et al., 1992). The ELU unconformably overlies the Upper Devonian Catskill Formation, which is characterized by red beds with caliche, suggesting that deposition occurred under semiarid conditions (high pressure, consistent with paleolatitude). The ELU and the overlying Lower Mississippian Pocono Formation contain plant debris and coal, indicating that deposition occurred under a humid climate (low pressure, inconsistent with paleolatitude). This humid climate probably developed during the Late Devonian-Early Mississippian in response to a subpolar low-pressure system that was associated with a polar front along the northern edge of the ice. Aridosols and other features of aridity in Upper Mississippian red beds that overlie the Pocono Group indicate that high pressure and aridity returned to the study region once global warming and ice melting caused a poleward migration of the subpolar low-pressure system.