CONTEMPORANEOUS AND ACCELERATED SLACKWATER LAKE AND LOESS SEDIMENTATION IN SOUTHERN ILLINOIS DURING THE LAST GLACIAL MAXIMUM (SHELBY PHASE)

Although the Laurentide Ice Sheet did not reach southern Illinois during the last glaciation (Wisconsin Episode), active glacial lobes in the upper Midwest seasonally fed sediment to large meltwater streams (e.g., Mississippi and Wabash River Valleys). Sediment dams led to the impoundment of tributary valleys and formation of extensive slackwater lakes (e.g., glacial Lake Muddy and glacial Lake Saline). Fine-grained, stratified deposits (Equality Formation) in these former lake basins are up to 25 m thick and occur in at least two terrace levels. At depths > 3 m, the Equality Formation is typically calcareous and may contain plant debris, wood fragments, or mollusk shells.

A cutbank along the Big Muddy River (near Hurst, Illinois), into the upper terrace of glacial Lake Muddy, exposes a zone of fossiliferous Equality Formation. Mollusk shells identified include Pomatiopsis lapidaria (amphibious gastropod), Fossaria sp. (aquatic gastropod), Pisidium sp. and Sphaerium sp. (small bivalves), and Succineidae. Six radiocarbon ages on shells at depths of 3.5–6.0 m range between 21,400–19,800 ¹⁴C years before present [B.P.] (25,800–23,800 calibrated years B.P.). About 5 km northeast, three sets of aquatic gastropod shells in archival core samples of Equality Formation yielded similar ages (21,000–20,500 ¹⁴C years B.P.). Collectively, these new ages are similar to prior reported ages on peat, organic matter or mollusk shells in glacial Lake Saline (21,800–20,500 ¹⁴C years B.P.; Frye et al. [1972], Heinrich [1982]) and glacial Lake Kaskaskia (21,500–19,900 ¹⁴C years B.P.; [Grimley and Phillips, 2015]). Thus, peak sediment aggradation among glacial Lakes Muddy, Saline, and Kaskaskia was contemporaneous and coincident with the Shelby Phase, when the Lake Michigan Lobe was advancing to or at its maximum extent. Additionally, enhanced Shelby Phase sedimentation was chrono-correlative with accelerated accumulation of Peoria Silt (loess) just prior to and after the Mississippi River’s diversion at 24,400 cal yr B.P. In sum, sedimentation in southern Illinois’ slackwater lakes and in loess proximal to major river valleys was likely similarly affected by greatly increased glacial sediment loads in meltwater valleys at a time when the southern Laurentide Ice Sheet was at its maximal extent in Illinois and Indiana.