NEW FINDINGS FROM OLD ROCKS: GEOLOGY OF THE LAKESIDE VILLAGE QUADRANGLE, CENTRAL VIRGINIA PIEDMONT

The Lakeside Village 7.5-minute quadrangle is located along the James River in the central Virginia Piedmont. In this area, the Chopawamsic terrane, a middle to late Ordovician volcanic-plutonic arc, borders the Goochland terrane, a middle Proterozoic to middle Paleozoic continental fragment. Regional and reconnaissance studies over the past 100 years had delineated the major crustal elements of the area, but finer geologic details remained unresolved. In the Chopawamsic terrane, the Columbia pluton was interpreted to truncate structures in metavolcanic and metasedimentary rocks of the Chopawamsic formation. More recent quadrangle mapping shows that the southeastern margin of the pluton is interfingered with the surrounding layered rocks at the map scale, and both units are folded into recumbent, northeast-plunging, northwest-verging folds with generally northwest-trending axial surfaces. These structures are refolded by a later generation of upright to overturned northeast-trending folds. The southeastern boundary of the Chopawamsic terrane is defined by Paleozoic ductile transpressional faults that underwent brittle normal reactivation in the Mesozoic. A large, north-trending diabase dike displays apparent right-lateral offset along the brittle faults, suggesting continued post-Jurassic dextral motion. Other findings of detailed mapping include felsic volcanic rocks, ultramafic rocks, and coarse-grained diorite in fault-bounded blocks between the Goochland and Chopawamsic terranes. These findings highlight the importance of 1:24,000-scale mapping, even in areas that have been the subject of many years of study at smaller scales.