FROM OUTCROP TO REGION: REVEALING THE STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF THE UPPERMOST MOUNT ROGERS FORMATION, SW VA, THROUGH DETAILED OUTCROP ANALYSIS

Detailed analysis of key outcrops in geological field areas can provide significant insight into the overall stratigraphy of a region. This is especially helpful in regions that lack continuous outcrop due to significant weathering and vegetative cover, as in the Blue Ridge of SW Virginia. In a study of the Mount Rogers Formation (MRF), outcrop RHR-905 is a heavily weathered, 200m long road cut on Rocky Hollow Rd., Grayson County, VA. Composed of non-welded and welded tuffs, arkose, and mafic and felsic dikes, this outcrop provides information about the relative timing of events in the upper MRF.

Volcanic and sedimentary rocks of the MRF were emplaced during the initial rifting of Rodinia along the eastern margin of Laurentia, which began ~780 mya. During intracontinental rifting, a series of volcanic eruptions created rhyolitic lavas, pyroclastic deposits, and basalts. As rifting continued, coarse alluvial sediments were deposited within rift valleys. As volcanic activity waned, these rocks were overlain by glacial deposits of the Konnarock Formation (KF).

The strata in outcrop RHR-905 dip to the southeast, with older tuffs overlain by arkosic sandstones. Non-welded lithic tuff dominates the volcanic portion of the outcrop, but a small high-angle reverse fault exposes the contact with underlying welded tuff. The apparent reduction in intensity of welding from the stratigraphically lower welded tuff is consistent with textures commonly seen in the upper phases of recent ignimbrite sequences, in which the most densely welded tuff is found in the center of the sequence.

The arkose overlies the tuff sequence, although a thin (~2.5 cm) zone of green clay-like material at the contact may be evidence for high strain along the contact. Therefore, the stratigraphic relationships are uncertain. The arkose contains abundant volcanic lithic clasts, which we are currently analyzing with thin-section study. If our analysis confirms the lithic clasts are sourced from the tuff, then the arkose must be stratigraphically younger. The mafic and felsic dikes are the youngest components of the outcrop and cross-cut both the tuffs and the arkose. We cannot determine the relative age of the dikes, however, as they do not intersect each other. Mafic dikes of similar orientation occur in the KF, so they must be younger than the glacial deposits.