STRATIGRAPHY AND DEPOSITIONAL FRAMEWORK OF THE TWOMILE LIMESTONE (LATE PENNSYLVANIAN) OF WEST VIRGINIA

The Twomile limestone (of I.C. White, 1885) is a mapable unit up to 2 m thick that occurs within the Conemaugh Formation of south-central West Virginia. The type area occurs about 5 km north of Charleston and the unit can be traced throughout much of Kanawha and Lincoln Counties. The texture is dominated by micrite with subordinate amounts of intraclasts, pelloids, and skeletal material including mainly octracodes and Spirorbis. Bioturbation is common and cross-lamination is locally developed. Recent workers have interpreted the Twomile limestone as a lacustrine (i.e. freshwater) deposit. Detailed facies analysis of the Twomile limestone at 12 outcrop locations indicates that it is a basal “transgressive” unit that overlies a widespread, well-developed calcic vertisol. The presence of Spirorbis indicates the lacustrine setting was located close enough to the coast to experience intermittent connection to the sea. The overlying strata consist of a coarsening-upward, lake-fill sequence of shale to sandstone which is either truncated by fluvial channel sandstones or capped by another thick, calcic vertisol. The paleosol-bounded, ‘transgressive-regressive' cycle containing the Twomile limestone has many similarities with fifth-order sequences reported about 70 km west of the Twomile type area. This suggests that paleosol-bounded allocycles in the coastal plain were base-level controlled and tied to glacioeustatic sea level cycles. Lithostratigraphic correlations indicate that the Twomile limestone is probably equivalent to one of the 3 closely spaced marine units (Lower Brush Creek , Upper Brush Creek , or Cambridge Limestone) in the middle of the Glenshaw Formation (Lower Conemaugh Group). Preliminary palynologic studies imply a slightly older age.