Abstract

Outcrop and subsurface data are utilized in a regional lithostratigraphic study of the Devonian shales in parts of Pennsylvania, Maryland, West Virginia, and Virginia, a study area of some 40,000 km² (15,000 mi²).

The main mass of dark Devonian shale (Millboro) is split by a wedge of silty shale and siltstone (Mahantango Fm) carried into the basin as a distributary lobe centered on Fulton County, Pennsylvania. Where this silty wedge is present, the Millboro is divisible into the Marcellus below and Harrell Shale (with a lower greyish black Burket Member) above. Where silty strata are absent, shales in the Tioga Ash-base of Brallier interval are considered entirely Millboro.

The distributary (Fulton Lobe) is bounded on the north by Perry Bay and on the south by Grant Bay. These embayments result from an eastward shift of black shale (Harrell-Burket) during the sea level rise of the Taghanic onlap and produced the lobe shape of the distributary. In extreme eastern outcrops black shale is absent and Brallier rests on Mahantango. The contact is at the approximate Tully position. Along strike in Virginia, the Mahantango thins by stratigraphic convergence; the silty strata change over to black, fissile shale (Marcellus) and are incorporated into the type Millboro (Marcellus-Harrell). The formation thins by convergence across strike also. The convergence results from the transition from fairly shallow prodelta to deeper water distal black muds.

Several horizons in the interval provide good marker beds: Tully Limestone, Pokejoy Member, siltstones within the Mahantango, Landes Limestone and concretion zone, Purcell Limestone and Tioga.