GEOLOGIC MAPPING AND STRATIGRAPHIC FRAMEWORK OF THE MISSISSIPPIAN GREENBRIER GROUP AND BLUEFIELD FORMATION IN THE MONROE COUNTY AREA OF WEST VIRGINIA

The transition from the carbonate-dominated upper part of the Mississippian Greenbrier Group through the carbonate/siliciclastic lower part of the Bluefield Formation and the chiefly siliciclastic upper part of the Bluefield Formation is an import stratigraphic interval in the central Appalachian Basin. Used as drilling targets, an understanding of the framework of the “Big Lime” (Greenbrier Group) and the “Little Lime” (including the Glenray and Reynolds Members of the Bluefield Formation) is key to defining aquifer boundaries and a guide for mineral and energy exploration. These and other locally-named lithostratigraphic units were established in a multi-county geologic report by Reger and Price (1926). Because of locally applied lithostratigraphic names with vague type area descriptions and inconsistent unit identifications in the original study, thus geologic mapping in this region can be confusing. For example, the informal Raines Corner limestone, outcropping in the west-central part of Monroe County is apparently a facies equivalent of the Reynolds Limestone that can be traced as a regionally continuous horizon. For mapping and hydrogeologic reasons, we are documenting the outcrop occurrence of the upper part of the Greenbrier Group, particularly the Union and Alderson Limestones and intervening Greenville Shale (where present). The lower part of the Bluefield Formation is about 200 ft thick, comprising the basal shale of the Lillydale Member, limestone of the Glenray Member, an unnamed mixed shale and siltstone interval, and limestone of the Reynolds Member, which marks the top of the lower part of the Bluefield. The upper part of the Bluefield is about 600 ft thick composed chiefly of undifferentiated siltstone, shale, paleosols, and minor sandstone, except where the locally thick (up to 80 ft) sandstone of the Droop Member occurs. The upper part of the Bluefield Formation is overlain by the basal sandstone of the Stony Gap Member of the Hinton Formation. Geologic mapping on lidar-derived basemap imagery and conodont biostratigraphy are being applied to constrain the stratigraphic framework of these units in the Monroe County area.

Reger, D.B., and Price, P.H., 1926, Mercer, Monroe, and Summers Counties, County Geologic Report CGR-15, West Virginia Geological Survey, 1068 p., 6 plates, scale 1:62,500