ANALYSIS OF BEDROCK JOINTS IN UPPER CRETACEOUS-MIOCENE ROCKS OF THE GEORGIA COASTAL PLAIN

Sedimentary rocks of the Coastal Plain in Georgia consist of Upper Cretaceous - Pliocene sandstone, limestone, and mudstone. Georgia straddles the divide between the Atlantic Ocean and the Gulf of Mexico, as well as the northern terminus of the Peninsular Uplift which extends from Florida into south-central Georgia. Joints in sedimentary rocks deposited in Georgia may thus reflect regional tectonics of the southeastern margin of North America. Systematic bedrock joints were measured in an un-named Upper Cretaceous Eutaw sandstone equivalent near the Fall Line at Macon, the Eocene Ocala Limestone in the Dougherty Plain of southwestern Georgia, and sandstones of the Miocene Hawthorne Group (?) in the Tifton Upland east of the Dougherty Plain. Joints measured in the Upper Cretaceous sandstone show a primary trend of 80°-260°, approximately parallel to structural strike of nearby Piedmont metamorphic rocks and the Goat Rock Fault. Fractures measured in the Ocala limestone exposed in the stream bed of Ichawaynochaway Creek in southwestern Georgia, combined with 100-m stream segment orientations, have a strong trend of 0° -180° suggesting E-W extension, with two secondary sets at 324°-144°, and 18°-198°. Interestingly, approximately 60 km to the west, the secondary trend of 324°-144° is the dominant trend in the Ocala, suggesting NE-SW extension. At the same distance to the east, joints in Miocene sandstones exposed in the Tifton Upland have a preferred orientation centered around 35°-215°, which indicate NW-SE extension. To the north, a small data set of joints in exposures of the Hawthorne (?) in Crisp County indicate a weakly dominant set oriented at 330°-150°, approximately parallel to the axis of the Peninsular Uplift in Florida. Although joints can form in many different ways, these trends may suggest early association with Piedmont structures and continental margin subsidence in the Late Cretaceous, a possible north-trending Eocene-Miocene hinge line in southwestern Georgia that separated Gulf-ward subsidence from Atlantic subsidence, and weak indication that late Miocene Peninsular Arch uplift extended into south-central Georgia further than previously thought. Regardless of their causes, it is now becoming increasingly possible to establish joint domains in the Coastal Plain of Georgia.