FINDING THE ANCESTRAL CHATTAHOOCHEE RIVER: EVIDENCE FOR A PERSISTENT MESOZOIC AND CENOZOIC PALEODRAINAGE SYSTEM IN THE SOUTHERN APPALACHIAN PIEDMONT AND COASTAL PLAIN PROVINCES OF EASTERN GEORGIA AND WESTERN ALABAMA

The surface expression of the Upper Cretaceous (middle Cenomanian) Coastal Plain unconformity in western Georgia and eastern Alabama is marked by the geomorphic Fall Line and separates crystalline basement terranes of the southern Appalachian Piedmont to the north (Dadeville complex, Opelika complex, Uchee belt, and Pine Mountain belt) from flat lying sedimentary rocks (Tuscaloosa and Eutaw Formations) of the Coastal Plain to the south. Over a distance >100 km - from Talbot County, Georgia to Elmore County, Alabama - the boundary between the two provinces is marked by a distinct Upper Cretaceous (Cenomanian) residual paleosol built on the underlying Paleozoic and older crystalline basement. Paleocontours constructed on the Coastal Plain unconformity in this region revealed the presence of 3 distinct paleovalleys ranging from 3 to 6 km in width. These paleovalleys, which were incised into the underlying crystalline basement and residual paleosol, were subsequently filled with fluvial sands and gravels of the lower Tuscaloosa Formation. Similar paleotopographic analysis of the Coastal Plain unconformity east and west of these middle Cenomanian paleovalleys indicate they are largely confined to the region of the modern Lower Chattahoochee River Valley. These isolated, middle Cenomanian paleovalleys, as well as a younger estuarine environment in the overlying Eutaw Formation (Santonian), indicate the existence of a persistent (>10 Myr) Upper Cretaceous paleodrainage system in this region. Because this paleodrainage system lies in the immediate vicinity of the modern Lower Chattahoochee River Valley, we speculate that it may represent an ancestral Chattahoochee River. If correct, the geologic record of Mesozoic sedimentary basins in the Gulf of Mexico, the late Cenozoic record of the Apalachicola River delta at the down-gradient mouth of the Chattahoochee River, and incision of the modern Chattahoochee River across prominent northeast-striking, stratigraphically-controlled ridges of the southern Appalachians, suggests this ancestral Chattahoochee River may be part of a persistent paleodrainage system that was active over an interval spanning >150 Myr.