LONG-LIVED UPPER CRETACEOUS PALEODRAINAGE SYSTEM IN THE U.S. SOUTHWESTERN GEORGIA-SOUTHEASTERN ALABAMA REGION

In southwestern Georgia-southeastern Alabama, the Gulf Coastal Plain unconformity, or “Fall Line”, separates deformed and metamorphosed crystalline basement of the Appalachian metamorphic core from flat lying sedimentary units of the Gulf Coastal Plain. In the Columbus, Georgia-Phenix City, Alabama region of the Lower Chattahoochee River Valley (LCRV), basement rocks are part of the Uchee terrane, a Neoproterozoic (620 – 640 Ma) belt of presumed peri-Gondwanan origin metamorphosed to amphibolite facies during Alleghenian accretion to Laurentia. In the immediate vicinity of the unconformity, amphibolites of the Phenix City Gneiss are typically overlain by kaolinitic sands and gravels of the Tuscaloosa Formation, an Upper Cretaceous (Cenomanian to Santonian) predominantly fluvial clastic unit which formed atop rocks of the Uchee belt following the breakup of Pangea. Quartzite clasts and pebbles of rare gneiss in the Tuscaloosa Formation suggest derivation of sediment from local quartzofeldspathic gneiss of the Columbus Metamorphic Complex, the nearby Pine Mountain belt and potentially Inner Piedmont-eastern Blue Ridge terranes farther north. Preliminary mapping along the Coastal Plain unconformity in the vicinity of the LCRV suggests the presence of one or more large (>2km) paleovalleys in the Upper Cretaceous (Cenomanian) surface. Interestingly, these occupy a similar position to a paleodrainage system identified in the overlying (Santonian to Campanian) Eutaw Formation, with heavy mineral analysis of the Eutaw suggesting a paleodivide immediately west of the modern LCRV. Physical and chemical characteristics of sediment within Eutaw exposures along the Coastal Plain unconformity in the vicinity of the LCRV suggest distribution of Eutaw sediment occurred within disparate drainage basins. The characteristics of the sediment argue against simple autochthonous recycling of Tuscaloosa sediments, but towards transport processes sourcing rocks preexisting the Santonian stage and which included a variety of metamorphic minerals. Collectively, these data suggest the possibility of a long-lived (>25 m.y.) drainage system in the vicinity of the modern LCRV.