INTEGRATION OF MULTIPLE GEOPHYSICAL AND SOIL DATASETS IN THE CONSTRUCTION OF A GEOLOGIC MAP IN THE CENTRAL VIRGINIA SEISMIC ZONE

Geophysics, LiDAR and soil survey data augment geologic mapping in the Buckner 7.5ʹ quadrangle and studies of the active Fredericks Hall fault, which was delineated by aftershocks of the 2011 Mineral, VA M5.8 earthquake. The Buckner quadrangle contains rocks of the Ord. to Sil. Quantico Fm. (garnet-mica schist), the Ord. Ta River Metamorphic Suite (mostly layered amphibolite interleaved with Alleghanian granitoids), the Elk Hill Complex (strongly deformed biotite granitoid with amphibolite boudins) and the Paleozoic(?) Po River Metamorphic Suite (mica gneisses, schists, and intrusive granitoids).

Airborne gamma spectrometry for K, Th and U has proven very useful for geologic mapping. The Lakeside and Spotsylvania faults, which bound the Elk Hill Complex and separate it from the Ta River and Po River metamorphic suites, show distinct K+Th highs. A Th+U high corresponds to kyanite-muscovite-quartz schist interlayered locally with amphibolite. Radiometric K highs also mark belts of muscovite+K-feldspar pegmatites within granitoids. The Louisa County soil series map shows that amphibolites of the Ta River Metamorphic Suite correlate with the Fluvanna Series fine sandy loam, and the Quantico Fm. correlates with the Nason and Tatum silt loams. Distribution of these and other soil units assists in delineation of bedrock and surficial lithologies. Preliminary ⁴⁰Ar/³⁹Ar plateau ages of muscovite from foliated pegmatite in the Lakeside fault zone (294.5 ± 1.4 Ma) and from the kyanite-muscovite-quartz schist (286.8 ± 1.3 Ma) reflect cooling from Alleghanian high-grade metamorphism and suggest that major ductile deformation on these intervening structures ceased by the Permian.

Aftershocks that define the northern tip of the Fredericks Hall fault occur where a NE-trending LiDAR lineament, magnetic contrast, and linear radiometric K high are crosscut by a NNW-trending linear magnetic high. Prior to detailed mapping, the geophysical data were interpreted to represent the intersection of a micaceous fault zone with a Jurassic dike. Mapping demonstrates that the lineament occurs at a contact between biotite granitoid and layered amphibolite, and that the K high is the result of pegmatite within granitoid. Sparse outcrops suggest that the diabase dike may not reach the surface in the vicinity of the aftershock cluster.