FORENSIC RECONSTRUCTION OF SEDIMENTS AND ENVIRONMENTS AT THE TERMINI OF PRAIRIE ICE STREAMS

In the south-central area once covered by the Laurentide Ice Sheet, the locations of prairie ice stream margins are clearly demarcated by long, broadly arcuate terminal and interlobate moraines. Deposits attributed to single advances (advance plus retreat deposits) range in thickness from more than 35 to less than 3 m. In Illinois, the hummocky terrain that typifies morainic areas shows little or no evidence of sediment stacking, faulting, or other features suggestive of compressive flow. Ice-walled lake plains (IWLPs) are locally common in the chaotic topography, especially in areas of sublobe interference where flow paths were short, and sediment relatively thick. IWLPs are formed largely of fossiliferous, 1 – 8 m thick successions of rhythmically bedded silts and very-fine sand. Fossils include ostracode valves, pillclams shells, chirominid head capsules, and the leaves, stems, buds, etc., of terrestrial and aquatic plants. The full IWLP record from Illinois indicate that from ca. 21,800 to 20,800 and 18,000 to 16,800 cal yr BP, newly-formed dead-ice permafrost supported shrubs with prostate growth habits, indicating cold, windy conditions. The intervening hiatus (ca. 20,800 to 18,000 cal yr BP) is indicated by the distribution of ages from IWLPs throughout Illinois, as well as by a paraconformity sandwiched by well-dated fossiliferous, laminated sediment. The paraconformity probably represents summer temperatures that were too cold to support a lake, as well as temporary inactivity of the active layer. Notably absent from the fossil record are trees and vertebrates, although these elements are locally abundant in kettle-fills near or abutting against ice-walled lake plains. The difference in age of the IWLP shrub fossils and kettle needles approximates the span of time when topographic inversion occurred from the onset of local deglaciation to full melting of the dead-ice permafrost.