ICHNOLOGICAL SIGNATURES OF STORM-WASHOVER FAN DEPOSITS: YELLOW BANKS BLUFF, ST. CATHERINES ISLAND, GEORGIA

As the most extensive outcrop of Pleistocene(?) sand on the eastern shore of St. Catherines Island, Yellow Banks Bluff has been interpreted at different times in various ways. Previous research pointed toward its facies as marine, with two thin, continuous and burrowed dark-brown beds within the outcrop interpreted as marine hardgrounds linked to pre-Silver Bluff Pleistocene highstands. However, our examination of the outcrop, and particularly its trace fossils, yields a new and contrary hypothesis. We propose that the burrowed beds are storm-washover fans deposited between dunes and maritime forests. Trace fossils within the dark brown beds are identified as fiddler crab (Uca spp.) burrows formed on the tops of washover fans as post-storm colonization surfaces, an interpretation based on the burrows' distinctive J-shaped forms and on analogous colonization by intertidal fiddler crabs observed in modern washover fans on the island. Vertical tree-root traces crosscut the Yellow Banks Bluff deposits in places; the tree-root traces in turn were penetrated intensively by infaunal insects, which are evidenced by pervasive and multiple generations of backfilled burrows (Taenidium). In one exposure below and adjacent to Yellow Banks Bluff, a black peat to peaty mud with pine cones and much terrestrial wood is preserved in a freshwater swamp channel deposit. These facies are interpreted collectively as eolian (back dune), succeeded ecologically by maritime forests. The biogenically reworked tree-root traces and associated facies are nearly identical to those interpreted in the Pleistocene Raccoon Bluff Formation  on Sapelo Island, hinting at possible paleoenvironmental and stratigraphic equivalence. In short, these new ichnological observations require reinterpretation of the sea-level history of Yellow Banks Bluff facies; specifically, its paleoenvironmental setting was a relative lowstand rather than a highstand. Future research should therefore focus on better age determinations and possible correlation of Yellow Banks Bluff with similar outcrops in the Georgia barrier islands, while paying close attention to their distinctive suites of trace fossils.