CONSTRAINING THE TIMING OF LATE GLACIAL EVENTS IN THE LOWER WISCONSIN RIVER VALLEY USING OPTICAL (OSL) DATING OF EOLIAN AND FLUVIAL SEDIMENT

Near Arena, WI, in the Lower Wisconsin River Valley (LWRV), distinct tundra polygons are clearly visible on aerial photographs. The timing of formation of these permafrost features limits the age of the Elderon Terrace on which they formed and also constrains the timing of recession of the Green Bay Lobe (GBL) and the drainage of Glacial Lake Wisconsin (GLW) to around 16.3 ka (~14,000 ¹⁴C yr BP). Ice-wedge casts also record the timing of the degradation of permafrost adjacent to the retreating GBL around 15.1 ka (~13,000 ¹⁴C yr BP) (Clayton and Attig, 1989, 1990). We used ground-penetrating radar (GPR) to evaluate stratigraphy and to identify areas for detailed study. A backhoe trench measuring 15x3x2 meters exposed ice-wedge casts in cross-section. Excavation revealed indistinct but clearly identifiable wedge forms within an eolian sand sheet overlying glacial outwash. Ice-wedge casts seen in profile are preserved as cumulic A horizons typically thickened ~30cm as compared to surrounding sediment. Samples were taken from two ice-wedge casts for soil organic-matter analysis, particle-size analysis, and optical dating. Preliminary optically stimulated luminescence (OSL) ages of 15.2 and 16.1 ka from glaciofluvial sediment underlying the eolian sand provide minimum limiting ages for Elderon Terrace surface abandonment in agreement with previous work. OSL ages of 12.3, 12.6, and 13.1 ka (average 12.7 ka) from eolian sediment contained within ice-wedge casts and surrounding host sediment suggest that permafrost lasted until at least that time in southern Wisconsin and degradation occurred after that. These results indicate that permafrost adjacent to the GBL in the LWRV existed for a period of ~2,500 years after the abandonment of the Elderon Terrace.