THE WASHINGTON COAL COMPLEX OF THE DUNKARD GROUP - THE LAST OF THE CARBONIFEROUS THICK COALS IN THE APPALACHIANS

The Washington coal is the youngest major Paleozoic coal of the Dunkard Basin. Though thick, up to 11 ft, it contains many clay partings and has never been economically significant. In the north it is a coal complex, typically split into two to three coals, separated by siliciclastic beds, and ranges in thickness from 7 ft to 36 feet. It is underlain by a well-developed, widespread paleosol (Washington Fire Clay) and is overlain by black shale and the non-marine Lower Washington Limestone. Coals in the complex are not members of separate cycles. Sediments separating the coals are undisturbed and show very little evidence of bioturbation or pedogenesis, suggesting only temporary disruption of peat accumulation. The coal complex is thickest to the east, in Greene and Fayette Counties of PA, where the coals are split by sandstone. The most dramatic change occurs to the south, where the coal becomes very thin and often comprises only a concentration of thin coal lenses in gray shale. Nonetheless, this thin coal horizon is a rare occurrence in the southern half of the Dunkard Basin and, in combination with its distinctive yellow underclay, is considered to be one of the best stratigraphic markers of the region.

The character of the siliciclastic rocks between the coals changes laterally. In eastern Belmont and Washington Counties, OH, and western Marshall County of the northern panhandle of WV, the Washington coal complex contains two coals. The intervening shale includes a thin zone containing Lingula shells. These mark the last Paleozoic marine transgression into the Appalachians and are the only evidence of sea water incursion since a previous series of transgressions that occurred in the lower half of the Conemaugh Gp of the Missourian (Kasimovian), approximately 7 my earlier. This appears to match a similar gap in major global marine transgressions, with the Washington coal transgression equivalent to one of a pair of major transgression in the latest Pennsylvanian. The Elm Grove Ls, at the base of the Dunkard, also has been reported to contain marine fossils. In our study, however, it has yielded only non-marine fossils so far, and preliminary examination for conodonts and other microfossils has yielded only fish teeth and scales.