SEDIMENTOLOGY OF AN ILLINOIS EPISODE ICE-CONTACT LAKE, SOUTHWESTERN ILLINOIS

During the glacial maximum of the Illinois Episode, ice crossed the Mississippi River valley in southwestern Illinois. At that time it also dammed a small tributary valley to form an ice-contact lake. The remnants of this ice-contact lake are observed in a striking 15m high outcrop located in the Hickman Creek valley, ~ 10 km southeast of St. Louis, MO. The exposure is comprised of 0.1m to 1.0m tabular beds of alternating laminated and massive red and brown silt with distinctive thin gray clay interbeds. Some beds contain isolated pebble to cobble clasts or pre-fossil remains of spruce needles and gastropods. The sequence is capped by a thin deposit of pebbly loam diamicton (till) with the Sangamon Geosol developed in it. In addition to a detailed outcrop description of the lacustrine section, we are determining particle size distributions by laser diffraction methods (Mastersizer 2000), major and trace element contents through an energy-dispersive X-ray spectroscopy (EDX) analysis, and the anisotropy of magnetic susceptibility (AMS). Our preliminary interpretation is that the lake filled with episodes of suspension settling from meltwater plumes and possibly sediment gravity flows. Large clasts were deposited as ice-rafted debris. There may have been intervals of lake drainage. Our study should further elucidate sedimentary mechanisms including the relative importance of suspension settling and sediment gravity flow. Study of the pre-fossil assemblage may contribute to understanding the ice-proximal paleoclimate.