CHARACTERIZATION OF THE BEARPEN RHYOLITE, A PREVIOUSLY UNDESCRIBED UNIT IN THE MOUNT ROGERS FORMATION
Our recent mapping on and to the northeast of Bearpen Ridge in Grayson County VA, reveals a distinctive and previously undescribed phenocryst-rich rhyolite that rests stratigraphically between the Fees Rhyolite and the upper MRF rhyolites. For its prominent exposures on Bearpen Ridge, we informally refer to this unit as the Bearpen rhyolite. The Bearpen is distinguished by 3-5 mm bright pink K-feldspar phenocrysts that tend to accumulate in clusters. Feldspar size and amount varies between outcrops. Other phenocryst minerals found in the Bearpen include quartz +/- plagioclase, although the amount of each is subordinate to the K-feldspar. The matrix is dark gray and flows around the phenocryst grains. Dark gray clasts are also found in the matrix, which appear to be phenocryst-poor Bearpen, possibly caused by autobrecciation. Portions of the Bearpen have flow banding present at the outcrop scale. Due to the position of the Bearpen between the lower MRF and upper MRF rhyolites, a more detailed study of its characteristics may help to better define the contact between the lower and upper MRF.