CHARACTERIZATION OF THE BEARPEN RHYOLITE, A PREVIOUSLY UNDESCRIBED UNIT IN THE MOUNT ROGERS FORMATION

The Neoproterozoic Mount Rogers Formation (MRF), located in the Blue Ridge of SW Virginia and NW North Carolina, represents an intracontinental rifting event that occurred ~760-750 Ma during the first of two pulses that led to the formation of the Iapetus Ocean. The MRF, which rests on Mesoproterozoic basement (~1.3-1.0 Ga) along a sheared unconformity, is divided into two parts. The upper MRF consists of the Buzzard Rock, Whitetop, and Wilburn Rhyolite Members. The lower MRF comprises bimodal volcanic rocks, including the Fees Rhyolite Member, interlayered with coarse siliciclastic deposits. Each of the rhyolite bodies in the upper and lower MRF can be distinguished by: 1) its distinct phenocryst assemblage; and 2) its textures that indicate emplacement by either flows or pyroclastic processes.

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.