UPPER DEVONIAN GLACIATION IN THE OHIO SHALE OF EAST-CENTRAL KENTUCKY

This is a report on the recent discovery of a large granite boulder found imbedded within the Upper Devonian (Famennian) Ohio Shale, Cleveland Member in Rowan County, Kentucky. This boulder measures 1.7 x 1.3 m and is at least 0.75 m thick. Compositionally it was originally a biotite granite, with quartz, K-feldspar (microcline) and biotite as the main mineral constituents. Thin-section examination of samples from this granite boulder shows that it has been subjected to low-grade (greenschist) metamorphism and deformation. This is evidenced by the presence of highly strained quartz, along with bent or kinked biotite that has been altered to chlorite. The boulder is imbedded within the Cleveland Member of the Ohio Shale (a black, organic-rich shale) as evidenced by the presence of upturned shale layers or a “mud drape” along its sides. Though relatively unweathered, the boulder appears quite worn with beveled or flattened surfaces and rounded corners and edges. Factoring in its size and density (2.70 g/cu cm), we estimate the weight to be at least 3 tons.

Taking into account the size, weight, shape and exotic lithology of this boulder, we suggest that it is an ice-rafted dropstone that was transported to and then released from a melting iceberg at this site. Two possible scenarios can be invoked to explain its presence in the Ohio Shale. One possibility suggests the occurrence of alpine glaciation in the Acadian highlands some 200-250 miles east of this locality. Most paleogeographic reconstructions for this time period place Kentucky and the Acadian mountains at around 30 deg. south latitude. These glaciers would have had to extend from the Acadian highlands west to sea level, with icebergs calving off along their western edge. The other possibility suggests that this dropstone was derived from along the west coast of Gondwana to the south. There exists substantial evidence that this part of Gondwana (specifically north-central South America) was covered by a continental glacier at this time. This dropstone would have had to be rafted into Kentucky by an iceberg derived from this ice sheet. Support for this depends upon the size and extent of this ice sheet, as well as the presence of an unobstructed, paleo-ocean current that flowed north along the west coast of Gondwana and eventually into the interior of North America.