ANALYSIS OF BRITTLE DEFORMATION FEATURES IN THE CHANNEL OF THE JAMES RIVER, WESTERN PIEDMONT VIRGINIA: TRADITIONAL AND REMOTE SENSING TECHNIQUES

This study analyzes brittle deformation features such as joints and shear fractures exposed in the bedrock channel of the James River in the western Piedmont of central Virginia using traditional field observations and measurements combined with analysis from Google Earth and drone imagery. In contrast to most of the Virginia Piedmont, bedrock is well-exposed in the James River (channel width 100 to 200 m) during low water conditions which affords the opportunity to image large expanses of bedrock. Our purpose is to characterize the bedrock fracture geometry and resolve the timing and orientation of regional paleostress patterns.

The western Piedmont is underlain by a suite of Neoproterozoic to Early Paleozoic meta-igneous and metasedimentary rocks. These well-foliated metamorphic units typically strike ENE to NE and are moderately to steeply dipping. NNW-striking diabase dikes crosscut older metamorphic rocks. Fractures in the metamorphic units are dominated by WNW- to NW-striking, steeply dipping extensional fractures that are post-metamorphic, and interpreted as formed by orogeny-normal compressive stresses in the late Alleghanian. Fractures in diabase dikes are complex and include two primary sets – an earlier formed NNW-striking set parallel to the overall dike trends, and younger set of oblique reverse faults that strike NE. These faults have east trending P-axes, and we interpret the younger fracture sets to be related to ridge push-related compressional stresses. Fracture density is greater within the diabase dikes than in the older metamorphic units.

Google Earth and UAV images provide us additional ways to analyze regional fracture sets. Eleven 10,000 m² study areas in the James River were analyzed for lineament density and orientation. A number of drone photographs were analyzed using the program FracPaq. A total of 723 lineaments were digitized from 10 images taken at 30 m above water level with 398 striking NE, and 325 striking NNW. The dominant lineament set in both Google Earth and drone image analysis strikes to the ENE to NE, parallel to both the regional foliation and layering in the metamorphic units. UAV imagery provided spectacular detail of subaqueously exposed geologic contacts, fluvial sediment, and riparian vegetation.