Kentucky Dropstones and the Case for Late Devonian Alpine Glaciation in the Appalachian Basin (USA) with Implications for Appalachian Tectonics and Black-Shale Sedimentation

A nearly three-ton, 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, and most recently, an in-situ boulder was noted from the subsurface Bedford Shale in eastern Kentucky. Other, anomalous, igneous and metamorphic boulders have been found near Cleveland exposures in the area, but none were in situ, prompting previous workers to ascribe their origins to ice-rafting in proposed, Pleistocene proglacial lakes, accompanying glaciation to the north. The in-situ nature of the new find, dated age of the boulder lithology (470 Ma), as well as compositional aspects of this and other likely dropstone boulders, strongly suggest Appalachian origins. Although Gondwanan glaciation was present far to the south at the time,these finds suggest that ice rafting related to alpine glaciation in the newly elevated Acadian highlands 300 to 400 km to the east was more likely, and this scenario is supported by paleogeography, as well as probable tillites and other dropstone-bearing sequences in more proximal deposits of the same age in Pennsylvanian and Maryland.

The co-occurrence of black shales and nearby glaciation also suggests yet another set of factors conducive to black-shale deposition. Clearly, 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 the salinity-stratified water column and amplified euphotic-zone bioproductivity through increasing levels of meltwater-derived nutrients. In fact, episodes of nearby, waxing and waning glaciation in the Acadian highlands may have influenced the course of third- and fourth-order transgressive-regressive cycles observed in the Appalachian black-shale sequence, with the most organic-rich black shales occurring during periods of deglacial melting and transgression.