SAND WEDGES POINT TO PERMAFROST AND AEOLIAN ACTIVITY IN THE CHIPPEWA VALLEY CA. 18-13 KA

A variety of geomorphic evidence suggests that permafrost was once widespread in the Driftless Area and nearby landscapes such as the Chippewa Valley of western Wisconsin. Permafrost may have covered this landscape from ca. 25-24 ka, when Late Wisconsin ice was at or near its LGM position, until 14-13 ka, or perhaps even until 12.5 ka in preferred locations, as the ice front was variously receding from the region. Our work adds to this chronology for permafrost in the Chippewa valley.

Previous and ongoing work in the Chippewa Valley, outside of the Late Wisconsin margin, has indicated that strong winds and aeolian processes also dominated the region during and immediately after the LGM. Dates on sandy aeolian landforms - sand ramps, sand sheets, parabolic dunes and linear dunes (“stringers”) - on the valley floor range between ca. 9 and 13 ka, pointing to the end of what may have been a prolonged period of sand mobilization. Loess deposition may have begun at ca. 24 ka, and continued until at least 13 ka. The general lack of loess on the sandy valley floor, coupled with thick loess deposits on the immediate eastern and southeastern sides of large bedrock ridges in the valley, seems to indicate that much of this loess was later deflated and removed, likely on very strong winds, as assisted by saltating sand. An OSL date of ca. 13.2 ka from interbedded loess and aeolian sand from a large ridge appears to constrain the end of this major aeolian interval.

We present new data from five sand wedges at two different sites in and near the valley that not only constrain the later part of the permafrost interval, but also link it to aeolian activity in the basin. Wedge morphologies indicate formation within a dry, sandy, windswept, permafrost environment. The dates on these sand wedges, which range from ca. 18.6 to 13.7 ka, provide the first numerical control on the permafrost interval in this region. Together, ages from sand wedges and aeolian landforms indicate a dry, sandy, windswept Chippewa Valley landscape, lasting from at least 18 ka until ca. 13 ka.