FRACTURE ORIENTATIONS IN MIOCENE SEDIMENTARY ROCKS OF THE SOUTHEAST GEORGIA COASTAL PLAIN AND THIER INFLUENCE ON STREAM SEGMENT ORIENTATION

Neogene sedimentary rocks in the vicinity of Statesboro, GA consist of clastic rocks of the Miocene Coosawhatchie formation. The rocks of the study area are weakly consolidated, fine to coarse grained, locally conglomeratic, clayey sandstones with interbedded mudstone and siltstone, interpreted to have been deposited near sea level. The presence of ghost shrimp burrows (Ophiomorpha nodosa) indicates that at least some of the Coosawhatchie formation was deposited in an intertidal or shallow marine environment. A study was undertaken to record morphology, lithology, and the relative ages of the ubiquitous systematic joint sets throughout the study area. A data set of 854 joint measurements was analyzed using rose diagrams displaying two dominant joint set orientations; I) 000°-180° +/-20° and II) 070°-250° +/-20°. Three minor joint set orientations are III) 035°-215° +/-15°, IV) 105°-285° +/-15°, and V) 140°-320° +/- 15°. The joint orientations in this study are consistent with those noted by previous workers. The presence of systematic joint sets throughout sedimentary rocks of the Georgia Coastal Plain suggests fracturing in response to regional tectonic stresses, rather than localized deformation associated with discrete structures. The stream net in the Southeast Georgia area includes reaches with preferred alignments in a few directions. Reaches displaying a rectangular drainage pattern are apparent at a variety of scales and are reminiscent of well documented fracture orientations. A rose diagram of stream segment orientation frequencies was created utilizing digital elevation models processed into 30 meter segments using GIS software. The rose diagrams showed a nonrandom distribution with identifiable trends NE/SW, NW/SE, and E/W. The strike frequencies of stream segment orientation correspond well with the dominant trends in fracture orientation. This suggests that the drainage patterns reflect both structure and the influence of a southeasterly regional drainage gradient. The orientations of stream segments can therefore serve as a proxy for fracture orientation.