ABSTRACT The Tonica Depression (TD), a late Quaternary sediment‐filled basin in northeastern Illinois, formed as a thermokarst lake as evidenced by a basal trash layer, its depositional history, and its occurrence on a late‐Pleistocene permafrost landscape. Stratigraphy and radiocarbon ages reveal that a multiproxy paleoenvironmental record extends throughout the late Wisconsin. Burning of organically enriched sediments has abbreviated the Holocene sequence. The lowest stratum is a glacial diamicton that forms the basin's floor, and at a slightly higher elevation, the rim. Lithofacies 1, supraglacial and debris flow deposits, are draped by undated Peoria Silt (PS). PS yielded molluscs and Coleoptera indicative of spruce forest ( Picea ) in a cold and harsh climate. Lithofacies 2 (ca. 17 500–18 500 cal yr BP) is a “trash zone” with white spruce ( Picea glauca ) wood, cones and needles with pollen, Coleoptera, molluscs, and vertebrates, including an elk‐moose. A “marl,” Lithofacies 3 (ca. 14 600–17 400 cal yr BP), attests to the deepening of the lake (>2 m) as also reflected by the molluscs, coleoptera, and pollen. The environment was an open spruce forest, but the climate was warmer than the previous ones. The top of this zone is composed of a brick‐red deposit with burned organic matter. Lithofacies 4 (ca. 14 600 cal yr BP) is composed of organic silt and lacustrine organic silt: a peaty silt loam. Finally, Middle Archaic artifacts on the rim of TD document human settlement around the depression during the dry and warm Middle Holocene (ca. 6747 cal BP). Tonica is the first well‐studied thermokarst lake from temperate latitudes in the United States, and it provides an excellent paleoenvironmental record of the late Pleistocene of north central Illinois.
Graham et al. (Mon,) studied this question.