BEDROCK AND SURFICIAL GEOLOGY OF THE JAMES RIVER VALLEY IN THE WESTERN PIEDMONT, VIRGINIA

In west-central Virginia, the James River flows from southwest to northeast along the enigmatic terrane boundary between Blue Ridge and Piedmont provinces. We produced a series of bedrock, surficial, and geophysical maps to 1) understand the regional stratigraphy, 2) decipher the kinematic history of deformation at this boundary, and 3) elucidate the Neogene fluvial history in the region.

The bedrock stratigraphy in the eastern Blue Ridge includes Neoproterozoic metasedimentary and metavolcanic rocks of the Lynchburg Group and Catoctin Formation. The Evington Group overlies the Catoctin Formation and forms part of the distal Laurentian cover sequence. Major rock types include phyllite, graphitic meta-limestone, quartzite, and greenstone. The western Piedmont is underlain by a monotonous sequence of phyllite and quartzose to plagioclase-rich meta-wacke. Steeply dipping NNW- to N-striking diabase dikes cut the metamorphic rocks, and are correlated with 200 Ma CAMP event.

Evington Group strata crop out in parallel NE-SW trending belts. Individual lithologic units are repeated by both folding and faulting. The main foliation typically strikes NE-SW, and, where present, elongation lineations plunge gently. At the outcrop and hand sample scale, multiple fabrics occur. Evington Group quartzites experienced flattening strain and dextral general shear during deformation under mid-greenschist facies conditions. The boundary between the Blue Ridge and western Piedmont is a broad zone (3-5 km wide) of dextral transpression. Late-stage oblique normal faults cut older metamorphic fabrics.

A flight of Pleistocene to Pliocene strath terraces, with treads up to 40 m above the modern river, occur on both sides of the James River. The James River is primarily a bedrock channel with a thin, and transient, veneer of sand and gravel. The James River’s modern floodplain is underlain by 2-5 m of silty sand and includes a subtle levee system. Alluvial islands in the James River include relict parts of the floodplain cut by marginal river channels, and larger mid-channel islands that are multi-generational landforms.