INTEGRATED PETROGRAPHIC AND PALYNOLOGICAL STUDY OF A MIDDLE SILURIAN SECTION AT BLUEGRASS, HIGHLAND COUNTY, VIRGINIA

Isolated spore masses and plant fragments containing spores were recovered from middle Silurian shaley and silty horizons within the top four meters of the McKenzie Fm, and lower half of the Williamsport Fm at Bluegrass, Virginia. A detailed analysis of the two units reveals that the former contains a mixed assemblage of marine and nonmarine palynomorphs, while the latter contains only nonmarine palynomorphs. These data for the Williamsport Fm conflict with existing lithostratigraphic models which posit a nearshore marine setting for this unit. To reconcile this discrepancy, the two units were re-sampled and are now the focus of this petrographic study. The upper part of the McKenzie is composed of very fine grained clay minerals and poorly sorted subangular quartz, while the lower part of the Williamsport contains arenite with little to no clay matrix and larger, more rounded quartz grains. Very few heavy accessory minerals are present, primarily zircon and tourmaline. Sandstone layers in the lower Williamsport are 20 to 75 cm thick, and bedding geometry changes upward from being laterally continuous to overlapping lenticular, suggestive of a lateral accretionary deposit. Thick beds at the top of the Williamsport consist of poorly sorted wacke with up to 25 percent clay matrix. These minerologic transitions suggest a transgressive-regressive shoreline environment of deposition. Combined with the palynologic signal, our study indicates that the roughly 10 m of section was deposited as an emergent deltaic plain containing both nearshore marine (tidalites) and nonmarine (fluvial) deposits. Thus, nonmarine sediments were deposited further west than previously reported, and coeval deposits should be investigated for more clues to early terrestrial life on the eastern margin of the Appalachian Basin.