AN IMPORTANT EXPOSURE OF FOSSILIFEROUS WISCONSIN AND ILLINOIS EPISODE SLACKWATER LAKE SEDIMENTS IN WESTERN TENNESSEE

An important sequence of Quaternary deposits, inset within the Eocene Cockfield Formation, is currently exposed along the east bank of the Mississippi River where it is actively eroding into loess covered uplands and terraces in western Tennessee. The studied site (Fulton Section) consists of 1 km of continuous exposures (~ 20 m in height) that clearly display several cut and fill sequences ranging from pre-Illinoian to Holocene age. These sequences likely record aggradation and degradation of an ancestral Hatchie River system (river is now 3 km to south) that responded to changing sediment loads in the Mississippi River (or paleo-Ohio River) during Quaternary glacial-interglacial cycles. The stratigraphic record here can be correlated regionally to southern Illinois terraces.

High levels of aggradation are recorded at the Fulton Section by fossiliferous lacustrine sediment deposits of the Illinoian (Teneriffe Silt) and Wisconsinan (Equality Formation). Both units occur at maximum elevations of about 240 – 250 feet asl (73 – 76 m) (modern Mississippi River is at 210 ft. (64 m)), but, due to thick loess cover, occur within terraces at 270 – 300 feet asl (82 – 91 m). Where the older terrace was not eroded by stream incision during the last interglacial, the Teneriffe Silt contains a poorly drained Sangamon Geosol in its upper portion, overlain by ~1 m Roxana Silt and > 10 m of Peoria Silt (loess). The younger terrace contains Equality Fm. overlain by 9 m of Peoria Silt and has a surface elevation of ~ 270 feet asl. Radiocarbon ages on detrital wood are 24 to 18 ka in the Equality Fm. and > 45 ka in the Teneriffe Silt. The Equality Formation contains abundant wood and macrofossils of Picea glauca and some Fraxinus nigra in channel fills, suggesting a peak last glacial climate similar to today's southern boreal zone in the northern Great Lakes region. The Equality Fm. also contains abundant gastropods that vary from aquatic (Helisoma trivolvis, Physa sp., Fossaria parva, Stagnicola elodes, Stagnicola cf. S. emarginata, and fingernail clams) to amphibious (Pomatiopsis lapidaria) to terrestrial (Succinea ovalis, Stenotrema sp., Mesodon sp.).

The gastropod fauna indicate an overall change in environment from open-water lacustrine to near-shore lacustrine to wetland to footslope (redeposited loess), prior to loess deposition.