QUATERNARY LANDSCAPE DEVELOPMENT IN THE DRIFTLESS AREA OF SOUTHWEST WISCONSIN

While it has long been agreed that the Driftless Area of southwest Wisconsin was never overrun by Quaternary glaciers, the timing and rates of landscape development within this unglaciated region are less well understood. Early Quaternary incision of the Upper Mississippi River system is established by reversed magnetic polarity of lacustrine sediment associated with the Bridgeport moraine found near the mouth of the lower Wisconsin River; the relationship indicates approximately 200 m of incision occurred prior to about 790 ka. Recent coring on upland ridges along the eastern margin of the Mississippi River and south of the Wisconsin River confirm the occurrence of a pre-Illinoian glacial advance that crossed east of the modern Mississippi River against the western margin of the Driftless Area, although the age relationship between this ice advance and the pre-Illinoian Bridgeport sediment is unclear. Pre-Illinoian and Illinoian ice advances blocked tributary drainages of the upper Mississippi River. Slackwater lacustrine deposits related to these ice advances occur in the lower Wisconsin, Grant and Platte Rivers and elsewhere in similar settings. Multiple abandoned meanders on tributaries of the lower Wisconsin River and Mississippi River provide insight into the timing of late Quaternary bedrock incision. Elevation comparisons between a modern bedrock valley bottom and the bedrock valley bottom of an adjacent cut-off valley meander whose basal fill correlates with 500 ka glacial deposits in NE Iowa indicate valley bedrock incision averaged about 0.048 mm/yr over the last 500 ka. A similar comparison for another bedrock valley meander that was cutoff following Illinoian age (OIS 6-8) valley backfilling indicates an average bedrock incision rate of 0.046 mm/yr since about 130 ka.