REVISED PENNSYLVANIAN LITHOSTRATIGRAPHY OF THE BROAD TOP COALFIELD, SOUTH-CENTRAL PENNSYLVANIA

The structurally complex, isolated Broad Top coalfield consists of a heterolithic, dominantly siliciclastic sequence of Pennsylvanian sedimentary rocks containing several commercial, low-volatile bituminous coals. It covers an area of 75 mi² and is situated about 25 mi east of the Main Bituminous coalfield in the northern part of the Broad Top synclinorium. Remarkably, despite a general understanding of the coalfield since the work of Gardner (1913), the detailed areal geology has not been well studied and quantified until now.

In response to a 2016 request from the Pa. Department of Environmental Protection, to help fulfill its regulatory responsibilities for mining, the Pa. Geological Survey (PAGS) agreed to undertake a comprehensive new study investigating and mapping the bedrock geology of the Broad Top field as part of a cooperative STATEMAP agreement. Although this ongoing project includes multiple goals and objectives, fundamental to the study was a thorough review and reevaluation of the lithostratigraphy of the Broad Top coal measures and its connection to the regional stratigraphic framework of the Main Bituminous field.

The present study relied on numerous diamond-drill records and underground mine maps from various sources and reinterpretation of the PAGS Broad Top reference core hole drilled in 1984, which provided the first evidence of Glenshaw Fm. (lower Conemaugh Gp.) marine zones in the coalfield—supplying a key to regional correlations. Based on analysis of local and regional datasets, identified Glenshaw marine zones in Broad Top include (all following series of units in ascending stratigraphic order) the Woods Run, Noble, and Ames. Principal Broad Top coals and their Main Bituminous field counterparts (in parentheses) representing key beds defining formational contacts are the Wood (Brookville), Speer (Upper Freeport), and Rogers (no equivalent). Additional important mineable coals include the Fulton (Lower Kittanning), Barnett (Middle Kittanning), Twin (Middle Kittanning rider), Barnettstown (Upper Kittanning), and Kelly (Lower Freeport). Thus, the newly recognized or redefined mappable bedrock geologic units in Broad Top consist of the Pottsville, Allegheny, Glenshaw, and Casselman Fms. and range in age from late Early to Late Pennsylvanian (middle Morrowan to Virgilian).