Eastern Section Meeting

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Deposition and Structural Features of the Basal Morgantown Sandstone of the Casselman Formation (Pennsylvanian) of the Greater Pittsburgh Region


In southwestern Pennsylvania, exposures of the Morgantown Sandstone (Pennsylvanian) of the Casselman Formation are comprised of generally massive, cross-bedded sandstone that varies between 3 and 20 meters. The base of the unit is commonly marked by pebble or cobble conglomerate containing clasts of diverse composition, including carbonate, and rounding in a sandy matrix. Underlying units include gray to black shale, and unfossiliferous limestone; each of which may be deformed by folding, loading structures, and/or truncations. Pebble-filled fractures resembling sedimentary dikes rarely cut the carbonate rocks. A thin horizon of coal a few to several centimeters thick commonly crops out among the lowest sandstone. The coal is generally strongly disrupted by faults that may transect and offset the layer or be coincident, in which case the sheared coal may accommodate sliding and related detachment. Locally conglomeratic sandstone, which resembles a debris flow, overlies cobbly debris containing platy shale clasts, some of which may be folded. The floating clasts may rarely consist of rootless, isoclinal folds. Occasionally, folding and other soft sediment deformation is recorded by underlying beds, which may be gently truncated at the contact. In places absent of conglomerate and coal, the base of the sandstone rests directly upon undeformed shale. Wherever exposure is sufficient, the base of the unit is clearly undulating. The unusual character of the basal Morgantown is known within an area of at least 400 square miles. Throughout the dozens of outcrops visited, nearly all display structural and sedimentary features that record disruption at the basal contact with the underlying strata. The structural and stratigraphic features at the base of the Morgantown are interpreted as having formed contemporaneously with the deposition of the sandstone as a regional, abruptly emplaced sand flow. Although commonly attributed to accumulation as a channel sandstone, no channel banks or well-sorted traction deposits are known. Furthermore, the thickness of the section between the base of the Morgantown Sandstone and the persistent Ames Limestone of the Glenshaw Formation varies between 15 and 35 meters throughout the Pittsburgh region. This further leads to the conclusion that the base of the Morgantown records scour.