NEOPROTEROZOIC TO PALEOZOIC TECTONICS IN THE VIRGINIA BLUE RIDGE: COMPLEX CONTACTS IN AN INVERTED EXTENSIONAL TO COMPRESSIONAL SEQUENCE

Inverted stratigraphic and structural relationships occur where compression and shortening are superimposed onto formerly extensional regions. As a result, ordered stratigraphic sequences can become jumbled and age relationships of strata blurred. An example of such inversion occurs in the Blue Ridge of SW Virginia, where Neoproterozoic rift-related bimodal volcanic rocks of the Mount Rogers Formation (MRF; ~760-750 Ma) and overlying glaciogenic sedimentary deposits of the Konnarock Formation (KF) were overprinted by Paleozoic contraction (~340 Ma). The glaciogenic deposits of the KF have previously been suggested to be related to the 717-635 Ma Sturtian “Snowball Earth" glaciation, therefore determining the age of the KF and its relationship to the underlying MRF is of relevance to the geological community. The maximum age of the KF is bracketed by the youngest rhyolites of the MRF. The MRF-KF contact has been variously interpreted as faulted or unconformable or, more recently, conformable. Interpretations are hindered, however, by deep weathering and poor exposure between outcrops, and further complicated by structural inversion.

Our study is focused on the MRF-KF contact in the Rocky Hollow area of Grayson County, VA, where the contact is a complex zone of lower KF laminated mudstone (‘rhythmite’) intermixed with arkose and conglomerate, rhyolite, and rhyolitic tuff, all cut by mafic dikes. Late-stage rhyolitic eruptions appear to have disrupted the KF mudstones, resulting in convolutely deformed bedding and “baking" of the mudstones. Detailed mapping reveals that this sequence occurs in an extensional graben. To the west, the footwall of the basin-bounding fault exposes the older MRF Whitetop Rhyolite overlain by KF mudstone, sandstone, and diamictite. We find no evidence of rhyolite or mafic dikes in the upper KF deposits in the footwall; thus, the igneous activity apparently ceased as glacial deposition continued. In addition to field mapping at the 1:12,000 scale, we have employed uncrewed aerial systems (UAS, i.e. drones) and structure-from-motion software to make 3D models of key outcrops to further inform our interpretations of structural and stratigraphic relationships.