GEOLOGY OF THE COLUMBIA 7.5' QUADRANGLE, VIRGINIA PIEDMONT

Rocks exposed throughout the Columbia 7.5’quadrangle in the central Virginia Piedmont provide evidence for multiple Paleozoic orogenic events. A sequence of metavolcanic and interlayered metasedimentary units, correlative with the Chopawamsic Formation and the Ta River Metamorphic Suite, are the oldest rocks in the quadrangle. Felsic to intermediate metavolcanic units are structurally overlain by mafic metavolcanics to the east. Granodioritic to granitic rocks of the Columbia pluton intrude the metavolcanic rocks. Mapping indicates that granitic rocks formerly mapped as the Carysbrook pluton are part of the texturally heterogeneous Columbia pluton. Porphyroblastic schists with discontinuous lenses of quartzite crop out in the eastern part of the quadrangle and are correlated with the late Ordovician Quantico Formation. The contact between the Quantico schists and the underlying Chopawamsic metavolcanic rocks is interpreted to be conformable. Porphyroblastic phyllites and quartzites of the late Ordovician Arvonia Formation are exposed to the west. A metadiorite stock (1 km²) intrudes metagraywackes and metarhyolites in the northwestern part of the quadrangle. The metamorphic grade increases from greenschist facies in the northwest to amphibolite facies in the east. En-echelon Mesozoic diabase dikes, striking NNW to NNE, cut all Paleozoic units. Three major synformal structures are exposed in the quadrangle: the Long Island synform, the northern termination of the Arvonia synform, and the southern termination of the Columbia/Quantico synform. The Long Island synform is a complex series of folds that plunge moderately to the southwest and are kinematically compatible with dextral transpression. The Arvonia synform is a relatively simple, asymmetric, gently southwest-plunging structure. Foliation in the Columbia/Quantico synform defines a complex moderately northeast-plunging structure. A northwest-striking joint set is prominent throughout the quadrangle.