LATEST DEVONIAN ALPINE GLACIATION FROM THE CENTRAL APPALACHIAN BASIN AND ITS POSSIBLE INFLUENCE ON COEVAL BLACK-SHALE DEPOSITION

Recently, a nearly three-ton, in situ granitic boulder was found embedded within uppermost parts of the Upper Devonian (Famennian; praesulcata Zone) Cleveland Shale Member of the Ohio Shale in Rowan County, northeastern Kentucky. Other, anomalous, igneous and metamorphic boulders have been found near Cleveland exposures in the area, but none were in situ. The in-situ nature of the new find, as well as compositional aspects of this and other boulders, strongly suggest an eastern Appalachian origin. Although Gondwanan glaciation was present far to the south at the time, the likely Appalachian sources suggest that ice rafting related to alpine glaciation in the newly elevated Neoacadian highlands ~500 km to the east was more likely. This is supported, moreover, by diamictites associated with lacustrine-like sequences containing dropstones reported by several workers from uppermost Devonian parts of the Hampshire, Rockwell and Spechty Kopf formations in northeastern West Virginia, western Maryland and central Pennsylvania. These deposits have been interpreted to represent tillites deposited during alpine glacial maxima, succeeded by glaciolacustrine sedimentation during glacial retreat, and we concur. Palynology indicates that the central Appalachian sequences are coeval with the Cleveland black shales containing the boulder.

The co-occurrence of black shales, likely dropstones and nearby glaciation also suggests yet another set of factors conducive to black-shale deposition. There is no doubt that the unique coincidence of temporal, tectonic, and paleoclimatic-paleogeographic factors at the time strongly favored black-shale deposition, but the presence of nearby glaciation indicated by the dropstones would have enhanced these factors through an influx of meltwater that intensified a salinity-stratified water column and amplified euphotic-zone bioproductivity by increasing levels of meltwater-derived nutrients. In fact, episodes of nearby, waxing and waning glaciation in the Neoacadian highlands may have influenced the course of third- and fourth-order transgressions and regressions seen in the Appalachian black-shale sequence, with the most organic-rich black shales occurring during periods of deglacial melting and transgression.