FRACTURE ORIENTATIONS IN MIOCENE SEDIMENTARY ROCKS, SOUTHEAST GEORGIA COASTAL PLAIN

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, locally conglomeratic, clayey sandstones with interbedded mudstone and siltstone. 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. Outcrops throughout the study area contain ubiquitous systematic joint sets. This study was undertaken to record fracture morphology, lithology, and the relative ages of the fracture sets found in the rocks of the Coosawhatchie Formation. A data set of 785 joint measurements was analyzed using stereonets and rose diagrams. Two joint set orientations were found to be dominant; 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°. Dips of joints range from 75° to 90°. Relative age relationships, determined from joint terminations, indicate that set II is older than set I. Age relationships among the minor sets showed that set V is younger than the other two minor sets (III and IV). Age relationships between minor sets III and IV have not been determined. A strong relationship between lithology and fracture propagation exists. Fractures are better developed in sandstone layers than in clay-rich units. With increasing percentage of clay in sandstones, fractures are poorly developed and are typically non-linear. In outcrops with interbedded sandstone and mudstone, fractures in sandstone layers die out or terminate abruptly at contacts with mudstone layers. The joint orientations in this study are consistent with orientations and deformation sequences noted by previous workers in the Coastal Plain of GA (Bartholomew et al. 2000; Davis and Rich, 2005). The presence of systematic joint sets throughout sedimentary rocks of the GA Coastal Plain suggests fracturing in response to regional tectonic stresses, rather than localized deformation associated with discrete structures.