POSSIBLE LATE DEVONIAN GLACIOGENIC INFLUENCE ON COEVAL, FORELAND-BASIN, BLACK-SHALE DEPOSITION

The recent interpretation of Upper Devonian, Appalachian diamictites as tillites and the concurrent discovery of a glacial dropstone in Upper Devonian black shales (Cleveland Shale Mbr., Ohio Shale) from eastern Kentucky, which have been correlated across the basin through palynology, suggest that alpine glaciation characterized at least the latest Devonian parts of the Acadian/Neoacadian orogen. Moreover, the fact that dropstones are found in coeval black shales means that at times glacial products had direct access to adjacent, foreland-basin seas via tidewater glaciers, and that such tidewater and outwash influxes may have had major influence on the development of organic-rich, black shales in adjacent seas. The origin of these Late Devonian black shales has been a subject of major controversy for some time: That is, were they the result of basinwide anoxia and stratified water columns, or greatly enhanced organic productivity? The recently discovered contemporaneity and connections between Late Devonian glaciation and coeval black-shales suggest yet other likely causal relationships that impinge on both the alternatives above. It is well-known, for example, that in Antarctic regions today, glacial fertilization of adjacent seas with terrestrial nutrients via air-borne, meltwater and iceberg input has dramatically enhanced marine organic productivity, and there is no reason to expect that something similar did not occur in the semi-enclosed Acadian foreland basin. Similarly, the vast influx of fresh meltwater, especially during waning glaciation, would have generated a surficial layer of fresh water on adjacent seas, and the resulting halocline could have contributed to basin stratification, anoxia and widespread, organic-matter preservation. Although other factors may have influenced formation of the Cleveland black shales, we suspect that glacial contributions were significant. Hence, in considering the origin of foreland-basin black shales, the possibility of nearby glacial influence may be critical in determining origins.