A LATE PLEISTOCENE TO HOLOCENE ALLUVIAL SECTION AT COFFEE BLUFF ON THE OCMULGEE RIVER

In June of 2006, an organic mat rich in plant macrofossils was encountered in the concluding weeks of the initial field season in the Fernbank Museum's search for archaeological sites associated with the Spanish mission Santa Isabel de Utinahuca. The ancient wetland deposit, hypothesized to be Pleistocene in age, was at and immediately beneath the level of the Ocmulgee River at Coffee Bluff in Telfair County. Over three meters of alluvium were described and sampled overlying the organic mat, representing a nearly complete section through the Holocene back to the late Pleistocene. Well-preserved plant macrofossils in the organic mat, radiocarbon dated to 38,690 +/- 420 yr B.P., are dominated by species familiar in the area's contemporary floral communities. Laboratory analysis of a column of twelve samples from the overlying profile shows rapid changes in both mean grain size and sorting for the lower half of the profile, consistent with deposits of a braided stream. At approximately 1.2 m below the land surface there is a change to fining-upward depositional cycles in which both clay accumulation and sediment chemistry indicate a period of landform stability roughly coinciding with the Pleistocene-Holocene boundary at 10,000 yr B.P.