80-13 New Constraints on the Post-MIS 3 Advance of the Green Bay Lobe of the Laurentide Ice Sheet From Near the Baraboo Hills, South-Central Wisconsin, USA

Session: Recent Advances in Glacial Geology, Geomorphology, and Chronology

Presenting Author:

Authors:

Abstract:

Considerable uncertainty exists regarding the early phase of advance of the Green Bay Lobe (GBL) towards its MIS 2 maximum position. This is due to the inherent difficulty in dating the advance of ice sheets, the relative paucity of datable organics from the last glaciation due to permafrost conditions in the region, and uncertainty regarding the pre-glacial topography of subsequently glaciated central Wisconsin. To identify unambiguous stratigraphic and chronologic evidence of the early advance of the glacial lobe, we collected a rotosonic core from a valley immediately south of the Baraboo Hills and west of the maximum position of the GBL in a setting conducive to archiving the earliest glaciogenic sediment from the advancing GBL.

The valley, today drained by Honey Creek, is located just beyond the margin of the GBL. The basin is in a bedrock valley opening toward the advancing ice margin and along the major meltwater pathway for the GBL (the lower Wisconsin River). Clayton and Attig (1990) recognized that outwash from the GBL dammed the valley to form a lake; they named it glacial Lake Black Hawk. The core, collected 4.25 km west of the outwash face, penetrated 34.7 m to the bedrock surface. It contains, from base upward, a gravel lag rich in exotic lithologies (outwash); an interval of laminated silt with layers of terrestrial plant macrofossils (lacustrine); an interval of sand with layers of peat (nearshore); a second interval of laminated silt with macrofossils (lacustrine); and a final interval of sand (nearshore and aeolian).

Six organic samples were analyzed by AMS radiocarbon. The basal age, ~1.5 m above the outwash, of 31.5 – 32.2 ka cal yr BP provides constraint on when ice advanced into the headwaters of the Wisconsin River and outwash had aggraded to the level of the bedrock floor of Honey Creek valley.  The youngest age, from ~1 m below the top of lake sediment, of 22.9 – 23.2 ka cal yr BP indicates that glacial Lake Black Hawk drained prior to the final retreat of the GBL.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8540

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