Clearville Siltstone of Middle Devonian Mahantango Formation in Parts of Pennsylvania, Maryland, West Virginia, and Virginia: ABSTRACT

Abstract

The Clearville siltstone of the Middle Devonian Mahantango Formation is an eastward-thickening and coarsening detrital clastic wedge which crops out in the Valley and Ridge province of south-central Pennsylvania, western Maryland, eastern West Virginia, and northwestern Virginia. Its west limit defines the west limit of the Mahantango Formation and can be traced in the subsurface of Cambria and Somerset Counties in Pennsylvania and in outcrop in Hardy and Mineral Counties in West Virginia. The Clearville siltstone in the western half of its surface occurrence consists predominantly of a sequence of interbedded siltstone and mudstone which is usually directly overlain by the Pokejoy Member of the Mahantango Formation, which exists as either a calcareous, fossiliferous siltstone or an argillaceous, fossiliferous limestone. To the east, the Clearville siltstone changes to a more complex sequence of several upward-coarsening cycles, the first of which is directly overlain by a Spirifer tullius zone that correlates with the Pokejoy Member to the west. The upward-coarsening cycles are interpreted as delta cycles, grading from prodelta mudstones and claystones near the base to medium to very thick-bedded delta front sandstones near the top of the cycles. The Pokejoy Member probably originated as an indirect result of either delta lobe abandonment or a eustatic sea-level rise following the first major phase of sedimentation of the Clearville siltstone. The sedimentation pattern of the Clearville siltstone is marked by two lobate regions of thickening: a more northern region long referred to as the Fulton lobe, and a more southern region referred to as the Frederick lobe. In both lobes, primary sedimentation appears to have been the result of westwardly flowing turbidity currents on a gently sloping shelf with later resedimentation and physical reworking from a combination of sublittoral processes such as tidal and storm surge currents. This physical reworking, as well as biogenic reworking, is particularly evident in the Fulton lobe, where two distinct sheet sandstone facies are recognized. This and other facies comparisons suggest lower rates of sedimentation in perhaps shallower water for the Fulton lobe than the Frederick lobe du ing Clearville deposition. Although very low porosity values are characteristic of the Clearville End_Page 1169------------------------------ clastics, fracture porosity and thick delta front sandstone development combine in the more eastern outcrop belts to increase the potential for a hydrocarbon reservoir in the Clearville siltstone. End_of_Article - Last_Page 1170------------