LATE PLEISTOCENE FLUVIO-LACUSTRINE DEPOSITS AT FULTON, TENNESSEE: SLACK-WATER DEPOSITS OR TRIBUTARY AGGRADATION ALONG THE ANCESTRAL OHIO-MISSISSIPPI RIVER VALLEY?

Late Pleistocene glacial floods along the Ohio-Mississippi river valley have been postulated by many workers; however, the extent and landscape effects of such floods are less certain. We have investigated exposures of a late Pleistocene fluvio-lacustrine succession near Fulton, Tennessee, to assess evidence for glacial floodwater and sediments. The fluvio-lacustrine succession lies between late(?) Pleistocene terrace gravels and the Peoria loess near the terminus of the Hatchie River valley. The succession comprises an older (>43 ka, C-14 yrs) overbank-paleosol interval unconformably overlain by a younger (23.8 to 18.0 ka, C-14 yrs) lacustrine-overbank-point bar interval. Aside from the basal quartzite and chert gravel and sand, the sediment is principally composed of silt and minor clay. The lacustrine, overbank, and point-bar deposits are generally olive brown to greenish-gray and contain common to abundant, fine to coarse organic debris. Root traces, varying from fine to medium size, and vertical burrows are rare to abundant within the lacustrine deposits, whereas, fine to coarse root traces are common to abundant in the overbank deposits. The lacustrine deposits contain common to abundant gastropod shells, chara remains, and ostracod valves, and less common bivalve shells. The gastropod shells are of subclass pulmonata and include terrestrial and subaqueous varieties. The ostracode species include varieties common in modern aquatic environments of the northern Midwest and Great Plains.

Our initial results show little or no evidence of slack-water deposition. The sediment characteristics as well as the sedimentary structures all suggest deposition of forest organic debris and loess-derived silt in overbank, marsh, pond, and channel environments of an ancestral Hatchie River course. The aquatic remains are consistent with other late Pleistocene faunas in the Midwest and show no systematic variations related to glacial-water inflow. Our preliminary interpretation is that the fluvio-lacustrine sediments aggraded in the lower Hatchie River valley because of base-level rise due to glacial outwash aggradation in the Ohio-Mississippi River valley. We are currently awaiting carbon and oxygen isotopic results of gastropods and bivalves to further evaluate this interpretation.