EVIDENCE FOR LATE DEVONIAN (FAMMENIAN) ALPINE GLACIATION IN THE APPALACHIAN BASIN: A GRANITIC LONESTONE FROM UPPER DEVONIAN BLACK SHALES IN NORTHEASTERN KENTUCKY

Recently, a nearly 3-ton granitic lonestone was found embedded in uppermost parts of the Upper Devonian (Famennian, praesulcata Zone) Cleveland Shale Member of the Ohio Shale in Rowan County, northeastern Kentucky. Dated zircons from the boulder provide an Early Ordovician (early Taconian) concordia age of 474±5 Ma with several cores exhibiting Late Mesoproterozoic (Grenvillian) ages of 1156±230 Ma; so the boulder clearly has an origin in the central Appalachians. After eliminating other possible origins, it seems most likely that the boulder is an ice-rafted, glacial dropstone, transported to and released at the site of deposition.

Interestingly, diamictites from Pennsylvania and Maryland, recently interpreted to be tillites, occur in the Upper Devonian-Lower Mississippian Rockwell and Spechty Kopf formations about 300 mi to the east of the boulder. In the Rockwell Formation, these diamictites overlie marginal-marine sediments thought to represent the Oswayo transgression. Palynological correlations between the Kentucky Cleveland Shale and Rockwell diamictites indicate that black shales around the boulder and at least the uppermost diamictite occur in the latest Devonian LN Miospore Biozone; two underlying tillites most likely occur in the LE Miospore Biozone. These relationships are further supported by lithostratigraphic and gamma-ray correlations.

Paleogeographic reconstructions place the probable tillites at about 30° S. latitude and allow for surface currents that could have transported boulder-carrying icebergs to northeastern Kentucky. Hence, we suggest that, at least briefly, alpine glaciers developed and extended from Acadian highlands westward to sea level during the Oswayo transgression such that icebergs were able to calve off into adjacent seas. More exotic boulder candidates for dropstones, one more than 16 tons in weight, are present in eastern Kentucky, suggesting that alpine glaciation may have been an important phenomenon during black-shale deposition.