CRYPTIC FAULTING AND BASIN INVERSION REVEALED THROUGH GEOCHRONOLOGY OF VOLCANIC CLASTS IN CONGLOMERATE, REFINED USING 3-D MODELS OBTAINED BY UAS

Detailed ‘boots on the ground’ field mapping is the backbone of many geologic studies, but modern techniques can refine interpretations made solely on field observations. In this study, we combine field exploration, geochronology of detrital clasts in conglomerate, and virtual models created with the use of unmanned aerial systems (UAS or drones), to better understand a structurally complex region in the southern Appalachian orogen.

in the Blue Ridge of SW Virginia, Neoproterozoic rift basins formed during initial breakup of the supercontinent Rodinia (~760-750 Ma) were later overprinted by Paleozoic contractional tectonism. Volcanic and clastic sedimentary rocks of the Mount Rogers Formation (MRF) formed during early stages of intracontinental rifting. Rankin (1993) described the stratigraphy of the MRF as a lower section of bimodal volcanics and sedimentary rocks, overlain by an upper section of rhyolite lavas and pyroclastics. Based on field relationships and guided by interpreted laws of superposition, the “lower” MRF has long been assumed as “older.” Recent geochronological data for rhyolites in the upper and lower MRF (e.g., Tollo et al. 2012) show that some of the ages (~760-749 Ma) largely overlap; thus the stratigraphic sequence is more complicated than the field evidence would suggest.

Our study is focused on conglomerates in the lower MRF, which contain boulder- to cobble-sized clasts of rhyolite. Zircon U-Pb geochronology of the detrital clasts yield ages ranging from ~780 to ~752 Ma, consistent with ages of both underlying and overlying rhyolites. Significantly, the youngest ages reveal that the conglomerates must be younger than at least some of the upper MRF rhyolites. The conflicting ages may be explained by basin inversion, where compression and shortening are superimposed onto formerly extensional basins. During this process, later shortening of syn-rift basins can produce complex structural and stratigraphic patterns that include cryptic extensional faults, some of which may be reactivated as compressional structures.

To further investigate cross-cutting relationships between the volcanic and sedimentary strata, we utilized UAS and structure-from-motion software to make 3D models of key outcrops, in order to extract quantitative measurements and create more precise cross sections.