PRE-ILLINOIAN ADVANCES OF THE LAURENTIDE ICE SHEET

Pre-Illinoian glacial deposits are best preserved across Iowa, Nebraska, and Missouri. In Missouri the Pre-Illinoian LIS advanced at least 6 times to the same approximate terminus along today’s Missouri River. This sequence has been dated using cosmogenic-nuclide ratios within paleosols directly beneath respective tills. The LIS first reached this area at ~2.4 Ma (possibly 2 separate advances), followed by two more early-Pleistocene glaciations in rapid succession at ~1.3 Ma. The LIS reached this position again during the Middle Pleistocene at ~0.76 Ma, and twice between 0.45-0.25 Ma. The matrix lithology of the tills changes systematically with younger age, becoming progressively coarser in texture and enriched in igneous rock fragments. This trend indicates a progressive loss of highly weathered regolith across the Canadian Shield and entrainment of less-weathered materials over time.

Several additional generalizations of the early LIS are possible based on observation. First, multiple tills (up to 5) are commonly preserved in direct superposition, and B horizons with undeformed pedogenic features are usually preserved beneath younger tills. Given that periglacial erosion likely removed part of the upper solum, preservation of B horizons means that there was only minor subglacial erosion. Any model of the early LIS that predicts extensive subglacial deformation and erosion of older subjacent deposits in these areas does not reflect actual conditions. Secondly, outwash and other glacial-fluvial features are much less common within pre-Illinoian deposits compared to younger (Illinoian and Wisconsinan) sequences, particularly near the terminus. This could reflect preservation bias, due to long periods of erosion removing recessional outwash prior to burial by the next glaciation. However, these features are also not associated with tills of similar age that were probably deposited during the same or successive marine isotope stages. It would seem that meltwater was nearly absent as the ice front initially receded, and this interpretation is re-enforced by a corresponding lack of loess associated with these advances. As a drastic hypothesis, might sublimation have contributed significantly to the ablation of the early LIS near its terminal positions?