GEOLOGIC MAPPING IN NORTHWEST GEORGIA HELPS DEFINE THE EXTENT OF DECOLLEMENT IN THE CHATTANOOGA SHALE

The Sulphur Springs, Cedar Grove, Trenton and Durham 7.5 minute quadrangles (Georgia-Alabama) expose gently folded Paleozoic strata on the northwest side of the Valley & Ridge province. Geologic mapping indicates that throughout the area stratigraphy is detached at the horizon of the Chattanooga Shale (Devonian). Although no more than 6 m thick when undeformed, the Chattanooga marks a pronounced contrast in ductility compared with overlying Mississippian carbonates; especially with cherty dolostones of the Fort Payne Formation. Thrust faults rise from a concealed decollement in Conasauga shales (Cambrian), ramp-up through the Knox Group (Cambro-Ordovician) and overlying Ordovician-Silurian strata and flatten out in the Chattanooga. The resulting detachment probably originated as a splay from the Kingston fault but was reactivated repeatedly as fault ramps advanced from hinterland to foreland. The Lookout Valley, Wills Valley and McClemore Cove anticlines appear to be horses in the resulting duplex. They fold the roof thrust, which is consequently out of sequence. The Lookout Mountain fault is formed by imbrication of the detachment and probably equates with the Cranmore Cove fault in Tennessee.