NEW STRATIGRAPHIC AND CHRONOLOGIC CONTROLS ON THE FORMATION AND DRAINAGE OF GLACIAL LAKE WISCONSIN

Glacial Lake Wisconsin formed when ice from the Green Bay Lobe blocked the southward drainage of the Wisconsin River at the east end of the Baraboo Hills. The lake filled numerous small basins in the vicinity of the Baraboo Hills and a large, main basin farther north. At its maximum, the lake spilled over through the Black River in the northwest. The lake drained catastrophically when ice had retreated far enough to open the east end of the Baraboo Hills (e.g., Bretz, 1950; Clayton and Attig, 1989). Despite this long-standing recognition of the general history of glacial Lake Wisconsin, there is a scarcity of reliable geochronologic control on the lake’s formation and drainage. This reflects the general paucity of datable carbon found in sediments in the upper Midwest from Marine Isotope Stage 2 (MIS 2), as well as the difficulty preserving chronologic data in the sandy main lake basin.

Recent coring in the small basins near the Baraboo Hills is providing new insight into the history of glacial Lake Wisconsin. Geoprobe cores collected in the Reedsburg, Loganville, and West Baraboo basins contain between 15 and 45 m of fine-grained laminated lacustrine sediment with abundant plant macrofossils. In the Reedsburg and Loganville basins, the fine-grained sediment filled the basin down to a coarse lag of material immediately on the bedrock floors of the basins; coring did not reach bedrock in the West Baraboo basin. AMS radiocarbon dates from the Reedsburg basin indicate the lake had formed by 23.7 ka and persisted later than 20.3 ka. Dates from the West Baraboo basin indicate the lake persisted until at least 17.4 ka; the stratigraphic setting of this age suggests it likely represents an accurate proxy for the catastrophic drainage of the lake. These dates corroborate similar radiocarbon and OSL chronologies developed in the nearby Devils Lake gorge (Attig et al., 2011; Carson et al., 2012).

The sedimentary sequences from these three basins show no evidence of unconformities, soil horizons, or multiple lake sequences, suggesting that the basins were filled solely during MIS 2. Furthermore, the geometry of the basins is not conducive to easy or complete erosion of older sediment; this indicates that glacial Lake Wisconsin may have existed only one time rather than multiple times as has been previously inferred from the regional glacial geology.