INSIGHTS INTO THE GEOMETRY AND EVOLUTION OF THE CHATHAM FAULT AND DAN RIVER BASIN NEAR CHATHAM, VIRGINIA

In the southern Virginia Piedmont, Upper Triassic lacustrine rocks in the Dan River Basin (DRB) occupy the hanging wall and high-grade metamorphic rocks compose the footwall of the NE-striking, SE-dipping, rift-bounding Chatham normal fault. Rock cores obtained during uranium exploration near Chatham, Va., yield key data that bear on the geometry of the Chatham fault (CF). The lithologic characteristics and mineralogical composition of the fault rocks also provide insights into the fault’s evolution. This information, combined with new field data on the basin geometry and previously unpublished thermal maturity data, permits a reconstruction of the DRB and CF and provides a framework for understanding the Mesozoic evolution of the region.

Two lines of evidence indicate significant latest Triassic or younger regional uplift. The fault core consists of finely comminuted gouge, generally 10 cm to 2 m thick, within which the presence of dickite (a polymorph of kaolinite that forms at elevated temperatures and pressures) suggests paleo-burial depths of 4-7 km for rocks that are presently within 100 m of the surface. Vitrinite reflectance (Ro) data from black shales in the basin include a mean random Ro of 3.2%, determined by M.L. Malinconico at the Solite Quarry (Va.-N.C. state line), implying 6-8 km of exhumation, and Ro of 1.9% near Chatham, measured by Humble Geochemical for W.S. Henika (Va. Tech), suggesting 5-7 km of exhumation.

In a one-square-kilometer area overlying the Coles Hill uranium deposit, we mapped the shallow subsurface configuration of the CF using 19 inclined core holes that were collared in the Triassic hanging wall and penetrated the CF. The fault dip varies between 20° and 40° to the SE with an average dip of 35°, gentler than previously reported. This result, combined with the inference of 4-7 km of post-rifting regional uplift, allows an interpretation that the CF originated as a listric normal fault, with the present level of exposure representing the gentler dipping, deeper part of the fault. This interpretation is consistent with new attitude data for the Triassic strata in the DRB. Alternatively, the CF was entirely gently- to moderately-dipping, reactivating a Paleozoic thrust fault. In either case, the present hanging wall represents the preserved deep keel of an originally much larger DRB.