DRAINAGE NETWORK EVOLUTION IN THE EASTERN GULF COASTAL PLAIN DETERMINED BY DETRITAL-ZIRCON GEOCHRONOLOGY

We present detrital-zircon geochronology from the modern Mobile and Apalachicola rivers, as well as previously reported Mesozoic and Cenozoic strata in the eastern Gulf Coastal Plain to advance understanding of drainage network evolution in the southeastern United States. The modern Mobile and Apalachicola rivers are defined by two prominent populations associated with Appalachian (490-270 Ma) and Grenville (1200-900 Ma) ages. Each river system can be distinguished based on proportions of Appalachian versus Grenville grains, in which the Mobile River exhibits a greater proportion of Grenville grains and the Apalachicola River exhibits a greater proportion of Appalachian grains. Both river systems contain an abundance (18% Mobile, 26% Apalachicola) of zircons with Th/U values < 0.1, demonstrating a metamorphic sediment source terrane. This relatively high abundance of metamorphic grains provides a unique approach to differentiate paleo-Tennessee River interpretations from paleo-river systems towards the southern Appalachian hinterland. A similar spatial trend in age spectra and Th/U values is present in Mesozoic and Cenozoic strata, indicating long-lived drainage divides demarcating a primarily foreland network (Mobile) and a primarily piedmont network (Apalachicola). Metamorphic detrital-zircon grains in the eastern Gulf Coastal Plain exhibit Taconic, Acadian, and Neoacadian age populations that temporally correlate to garnet growth ages in the eastern Blue Ridge province and detrital populations from recycled Paleozoic units in the Valley and Ridge province. Our results provide a framework for differentiating between paleo-river networks, help define Mesozoic-Cenozoic sediment provenance, and better understand landscape development in the southeastern United States eastern Gulf Coastal Plain.