POSSIBLE RELATIONSHIPS BETWEEN LATE DEVONIAN ALPINE GLACIATION AND BLACK SHALES

Upper Devonian diamictites from the central Appalachian Basin, interpreted to be glacial in origin, may now support connections with likely glacial dropstones from Upper Devonian black shales (Cleveland Sh.) in eastern Kentucky, based on stratigraphic, biostratigraphic and paleogeographic evidence. Together, the dropstones and diamictites, separated by about 500 km, provide evidence for alpine glaciation in the Neoacadian orogen and for tidewater glaciers in the Appalachian foreland basin. Both occurrences are related to a period of Late Devonian global cooling and to a Neoacadian, transpressional, tectonic regime that created coastal mountains high enough to generate alpine glaciation, while producing sufficient foreland subsidence to force transgressing seas eastward to the hinterland.

This occurrence also points out an important association between glaciation and black shales. Glacial meltwater is a major source of terrestrial nutrients and dissolved organic matter that can stimulate organic productivity in nearby seas, and iceberg-prone seas are typically areas of enhanced organic productivity and sequestration of organic carbon in underlying sediments. In addition, glacial meltwater draining into adjacent seas commonly generates a fresher, lighter, surface layer that initiates a salinity stratification, or halocline, and this kind of density stratification may prevent circulation of oxygen-rich surface water to greater depths, thereby promoting anoxia below the halocline. Hence, Late Devonian tidewater glaciation may have contributed to both anoxia and enhanced organic productivity, and at least for upper parts of the Cleveland Shale, coeval glaciation must now be considered as another control on organic-rich sedimentation.