Reports
The Mississippian System in Maryland
1989, Brezinski, D.K.
Report of Investigations 52
Abstract
The Mississippian System in Maryland comprises the Rockwell, Purslane, Greenbrier, and Mauch Chunk Formations in ascending order. The Rockwell Formation consists of interbedded siltstones, shales, and thin sandstones. The lower part of the formation spans the Devonian-Mississippian boundary and conformably overlies the Hampshire Formation. Intertonguing of marine and nonmarine facies characterizes the Rockwell of Garrett County. The lower marine unit correlates with the Oswayo Formation of northwestern Pennsylvania and the upper unit with the Sunbury Shale of Ohio. At Sideling and Town Hills only one marine unit is present. This marine episode can be correlated with the Riddlesburg Shale of Pennsylvania. Associated with and underlying the Riddlesburg Shale are shoreface sandstones and locally polymictic diamictites. Aggradation into alluvial plain deposition marks the end of Rockwell deposition.
The Purslane Formation consists of thick, cross-bedded, light-colored sandstones and conglomerates with thin intervals of red mudstone, carbonaceous shale and siltstone. An overall coarsening-upward in the Purslane indicates the change from low-gradient stream deposition in the lower part of the formation to high-gradient braided streams in the upper.
Erosion following Purslane deposition produced an unconformity separating it from the overlying Greenbrier Formation. The Greenbrier Formation in Maryland consists of four members. In ascending order they are: Loyalhanna, Deer Valley, Savage Dam, and Wymps Gap. The Loyalhanna consists of cross-bedded, green-gray to red, sandy limestone directly overlain by medium-bedded, purer carbonates of the Deer Valley Member. Interbedded terrigenous red clastics and marine sandstones, shales, and limestones characterize the Savage Dam Member, a tongue of clastic rocks which thicken dramatically to the east. The Savage Dam Member is overlain by a carbonate tongue known as the Wymps Gap Member, which thins both north and east in Garrett and Allegany Counties.
The uppermost Mississippian unit, the Mauch Chunk Formation, is characterized by red claystones, mudstones, and siltstones, and relatively thick, cross-bedded , green-gray and red-brown sandstones. A thin marine unit, correlated with the Reynolds Limestone of West Virginia, is present in southwestern Garrett County. Erosion near the end of Mauch Chunk deposition led to the development of an unconformity between it and overlying Pennylvanian units.
