TIMING AND IMPLICATIONS OF LATE QUATERNARY SEDIMENT DEPOSITION AND LANDSCAPE EVOLUTION IN SOUTH-CENTRAL WISCONSIN

The modern landscape of south-central Wisconsin is an amalgam of glacial and glacial-lacustrine sediments associated with the Green Bay Lobe, and aeolian and alluvial sediments that mantle the far older bedrock-controlled landforms of the Driftless Area. On-going research west of the maximum extent of the Green Bay Lobe is documenting an increasing number of sites where large volumes of sediment were deposited during and since the last glaciation: a radiocarbon date from a piece of black spruce from Adams Co., WI, indicates at least 85 m of sediment have been deposited in the glacial Lake Wisconsin basin just west of the glacial margin since ~30.0 ka; radiocarbon dates from plant macrofossils suggest that at least 25 m of lacustrine sediment was deposited in the West Baraboo basin between ~21.6 and ~17.4 ka; and radiocarbon dates from plant macrofossils from slackwater lakes dammed in tributaries to the lower Wisconsin River document as much as 40 m of lacustrine sediment was deposited in those basins between ~21.0 and ~19.9 ka.

Notably absent throughout south-central Wisconsin are any deposits that are clearly older than Marine Isotope Stage 2 (MIS 2). This raises the question of what the ice-margin positions, drainage patterns, and overall landscape of south-central Wisconsin were like prior to the most recent glaciation. The current course of the Wisconsin River across the glacial Lake Wisconsin basin and around the east end of the Baraboo Hills is necessarily a post-glacial construct. Our recent research strongly indicates that the entirety of south-central Wisconsin drained to the northeast in pre-Quaternary time, and that the routing of the Wisconsin and upper Mississippi Rivers to the south is a result of likely middle Quaternary glaciations. In what direction, then, did water runoff in the interim between the apparent pre-Quaternary pattern of drainage toward the northeast and the modern drainage toward the south? The growing inventory of sedimentary basins with thick sequences of latest Pleistocene deposition in south-central Wisconsin suggest that MIS 2 materials deposited by the Green Bay Lobe may have exerted a primary control on routing the Wisconsin River to the south and southwest.