ABSTRACT Rocks of the Upper Cretaceous Tuscaloosa Formation (Cenomanian) and Eutaw Formation (Santonian) in southwestern Georgia and southeastern Alabama record an interval of fluvial and nearshore marine deposition. In the vicinity of Columbus, Georgia, basal units of the Tuscaloosa Formation consist of a residual paleosol built on crystalline rocks of the Appalachian Piedmont covered by conglomeratic sandstones deposited in braided stream systems flowing across the mid-Cenomanian Coastal Plain unconformity. The unconformity, which separates Cretaceous detrital rocks from underlying metamorphic rocks and residual paleosols built on those metamorphic rocks, lies primarily within the Tuscaloosa Formation in this region and is marked at the modern surface by the geomorphic Fall Line. Mapping of the unconformity across the region reveals areas of significant paleorelief associated with a number of distinct paleovalleys incised into the mid-Cenomanian surface. The most distinct of these lie immediately east of the Alabama-Georgia state line, within 15 km of the modern Lower Chattahoochee River Valley. Spatially, these distinct paleovalleys lie immediately north of a Santonian estuarine environment recorded in the Eutaw Formation, disconformably above the Tuscaloosa Formation. Collectively, paleo-valleys in the mid-Cenomanian surface, the fluvial nature of the Tuscaloosa Formation in southwestern Georgia and southeastern Alabama, and the estuarine environment in the younger Eutaw Formation suggest a persistent (~10 m.y.) paleodrainage system that may be a forerunner to the modern Chattahoochee River.

ABSTRACT A local clay lens up to 60 cm thick in the Eocene Castle Hayne Limestone at the abandoned Fussell Quarry, Duplin County, North Carolina, is identified as a bentonite. It is composed of authigenic smectite with sparse euhedral biotite and apatite. Scanning electron microscope examination shows that the bentonite consists of relic bubble-wall shards altered to smectite. Smectitic columnules, rod-shaped casts of elongate pipe vesicles in pumice fragments derived from early dissolution of nearby small glass shards, also occur. This association is considered diagnostic of a silicic air-fall ash. K-Ar and Rb-Sr biotite dates from the bentonite are 46.2 ± 1.8 Ma and 45.7 ± 0.7 Ma, respectively, and a fission-track age of apatite is 51.0 ± 2.0 Ma; this later date is considered to be incorrect. Biotite compositions determined from electron microprobe analyses on 100 crystals suggest derivation from a single volcanic source no more than 4000 km from the bentonite. Possible sources of the ash include Bermuda; Highland County, Virginia; and the Caribbean; however, because of distance, prevailing wind direction, and similarity in age and composition, the volcanic swarm in Highland County, Virginia, is the suggested source.