PALEORELIEF ON THE UPPER CRETACEOUS COASTAL PLAIN UNCONFORMITY IN EASTERN ALABAMA AND WESTERN GEORGIA

Within the southeastern U.S., the Upper Cretaceous Coastal Plain unconformity (CPu) lies within the Tuscaloosa Formation, between a residual paleosol and overlying quartzofeldspathic detrital units. Detrital rocks of the upper Tuscaloosa Formation are predominantly poorly indurated, variably-arkosic and kaolin-rich rounded to sub-rounded sands and gravels. Some exposures exhibit cross bedding and mottling. The lowermost unit of the Tuscaloosa Formation consists of a reddish-orange, sandy to silty clay exhibiting yellow-white mottling, and is interpreted as a paleosol built on the underlying metamorphic rocks prior to deposition of the overlying detrital units. In western Georgia and eastern central Alabama, underlying metamorphic “basement” units consist of schist, quartzite, gneiss, and amphibolite belonging to the Uchee belt, Pine Mountain belt, Opelika complex, and Dadeville complex. Previous work indicates the existence of paleovalleys on the CPu in the vicinity of the Lower Chattahoochee River Valley (LCRV) in western Georgia. This study aims to map the CPu west of the modern LCRV in order to determine the westward extent of these Upper Cretaceous paleovalleys. The spatial positions of sedimentary units above the CPu, as well as the paleosol at the base of the Tuscaloosa Formation and underlying metamorphic rocks, were used to constrain the location of the CPu west of the LCRV. In areas where contact between Tuscaloosa Formation detrital units and underlying paleosol or metamorphic basement could not be directly observed, the contact was interpolated. Interpolated points of the CPu were derived from over 300 exposures, compiled into a database, and then plotted in Google Earth. Elevations on the directly observed and interpolated locations of the CPu were then contoured to create a paleotopographic map of the Upper Cretaceous CPu. Preliminary data suggests the presence of gentle, north-south oriented paleovalleys immediately west of those identified in the LCRV, and may suggest the presence of multiple paleodrainage systems originating in the headwaters of the Upper Cretaceous Appalachian highlands in this region. In addition to paleotopographic analysis, we assess the origin of these newly identified paleovalleys and their likely depositional environment in a fluvial or shallow marine setting.