AN UPPER CRETACEOUS BONE BED AT THE SOUTH ATLANTIC-EASTERN GULF JUNCTION

A 2.0 km length of Hannahatchee Creek, Stewart County, western Georgia, exposes a 1.0 m thick concentrated bone-and-shell lag deposit, which has yielded Late Cretaceous fossils of 19 species of chondrichthyans, 10 species of osteichthyans, plus mosasaurs, plesiosaurs, marine and freshwater turtles, crocodylians, pterosaurs, and four dinosaur taxa. This lag deposit comprises the stratigraphic boundary between the Blufftown and Cusseta Formations, and the fossil bed also contains abundant mollusks and foraminifera which help constrain the age to the mid-Campanian (Zone of Exogyra erraticostata). The concentrated vertebrate material is largely transported and apparently reworked from the underlying Blufftown unit: however, some vertebrate remains, and many ammonites, are also found in the immediately superjacent lower Cusseta Formation. The uppermost 2.0 m of the Blufftown Fm. locally is further notable for sequences of storm beds containing abundant serpulid worm tubes (Hamulus sp.) and Lingula sp.

The admixture of fossils at Hannahatchee Creek indicates accumulation in a perimarine, primarily estuarine environment. Non-marine fauna include superabundant fossils of the gigantic estuarine crocodylian Deinosuchus rugosus, as well as other crocodylians, freshwater turtles and fish, and the obviously terrestrial dinosaurs. Additional indicators of proximity to the paleo-shoreline include abundant Teredo-bored, coalified wood, and the storm-runoff beds with abundant Lingula. Nevertheless, the typical mollusks and sharks in the assemblage are marine species, indicating strong influence of normal marine environments. The fossil bone concentrations, with almost universal disarticulation of skeletons and evident reworking of teeth and bones, indicate accumulation by marine transportation and deposition on a transgressive surface. Some components of the assemblage, however, are obviously autochthonous, including a bioherm of the ostreid Crassostrea cusseta. In addition to yielding the bulk of Late Cretaceous vertebrate fossils known from Georgia, this site is notably located at the Late Cretaceous paleogeographic boundary between the South Atlantic and Gulf Coastal Plains, and shares characteristics of vertebrate assemblages with both realms.