THE GEOLOGY OF THE JAMES RIVER STATE PARK, EASTERN BLUE RIDGE/WESTERN PIEDMONT, CENTRAL VIRGINIA

The James River State Park (JRSP) lies astride the boundary between the Blue Ridge and Piedmont geologic provinces in western Buckingham County, Virginia. We produced a 1:10,000 scale geologic and total magnetic intensity map of the JRSP that details both bedrock and surficial units. Our research is aimed at understanding the tectonic significance of the enigmatic boundary between the Blue Ridge and Piedmont.

Bedrock in the park consists primarily of Evington Group metasedimentary rocks that includes phyllite and schist, graphitic meta-limestone and banded marble, medium-grained quartzite, and greenstone. The Evington Group forms part of the distal Laurentian (Blue Ridge) cover sequence. At JRSP these units crop out in narrow NE-SW striking outcrop belts that are repeated by faults. The main foliation strikes NE, and elongation lineations plunge gently to the NE and SW. Strain analysis in the quartzite indicates that this unit experienced apparent flattening strains and dextral shear. Microstructures are consistent with mid-greenschist facies conditions during deformation. The JRSP occurs across a broad zone of dextral transpression that forms a structural transitional between the eastern Blue Ridge and western Piedmont in central Virginia.

A suite of steeply dipping NNW- and N-striking diabase dikes cut the metamorphic rocks. These dikes are correlated with the widespread ~200 Ma CAMP dikes throughout the central and southern Appalachians. Surficial deposits at JRSP include alluvial deposits that border the James River, and two sets of strath terraces up to 40 m above the modern river.