DECIPHERING THE GEOLOGY AT A PRESUMED TERRANE TRIPLE POINT: GLADSTONE 7.5' QUADRANGLE, CENTRAL VIRGINIA

The Gladstone 7.5’ quadrangle is located in central Virginia at a presumed terrane triple point between the Blue Ridge (BR) and Western Piedmont (WP) terranes and the northernmost extent of the Smith River Allochthon (SRA). The goals of our study are: 1) provide a 1:24,000 scale bedrock and surficial geologic map of the quadrangle; 2) decipher the regional stratigraphy, and 3) determine the structural geometry and kinematic history of these enigmatic terranes.

The bedrock geology consists of the eastern BR cover sequence composed of Neoproterozoic Lynchburg Group arkosic meta-sandstone, Ediacaran Catoctin Formation greenstone, and Evington Group meta-pelite interlayered with quartzite, carbonate, and greenstone. The WP is characterized by a monotonous sequence of low-grade quartzose phyllite and metagraywacke with higher grade mica schists and amphibolites to the southeast. Seven NNW-striking CAMP-related diabase dikes cut the older metamorphic rocks. Bedrock units are concealed by 1) multiple strath terraces up to 40 m above the modern channel of the James River and 2) floodplain deposits adjacent to the James River and other streams.

We’ve discovered a small inlier (<2 km²) of Grenvillian basement at the eastern edge of the Evington Group in the hanging wall of a previously unrecognized thrust fault at the Blue Ridge-Piedmont boundary. The Evington Group is characterized by a set of parallel NE-SW striking lithologic belts repeated by folding and faulting. In contrast to previous regional studies, we recognize a broad NE-trending transition zone between the BR and WP rather than a narrow mylonite zone. Structural evidence in the Gladstone quadrangle is incompatible with a low-angle and rootless SRA in central Virginia.