T249 Advances in Understanding Pleistocene Landscape Evolution through the Window of Bedrock Topography (Posters)

Wednesday, 7 November 2018
9:00 AM - 6:30 PM
Halls J-K

EVIDENCE OF EOLIAN EROSION OF THE BEDROCK LANDSCAPES OF WESTERN WISCONSIN DURING THE LATE PLEISTOCENE

Paper number: T249-2
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

We report on a variety of geomorphic evidence suggesting that strong winds facilitated a widespread and long-lasting event which eroded and redistributed sediments in the Chippewa River valley of western Wisconsin. Although some key dates have yet to come in, we believe that this event coincided generally with the retreat of the MIS 2 ice from the region. As the Chippewa lobe of the Laurentide ice sheet reached its maximum extent at the northern margins of the Chippewa valley, it developed large valley trains of outwash. This outwash, as well as residuum from the sandstone ridges that dominate the region, provided the sand necessary to facilitate eolian sand erosion and transport across the region. We suggest that the proximity of the ice margin led to strong temperature and pressure gradients, and thus, strong storms that facilitated this event.

In many areas, parabolic dunes and long, low, linear "stringers" (ridges) of sand, silty-sand, and even loess are evidence of a period of remobilization of preexisting sediment on west-northwest winds. Well-formed sand wedges and ice-wedge casts on low uplands filled with clean sand suggest that much of this erosion and sand transport occurred in conjunction with a period of extensive permafrost. Ventifacts abound on the landscape, and are especially common in "wind-tunnel" areas between and beside isolated bedrock ridges, around which winds may have been funneled. On the windward, i.e., west and northwest, sides of these bedrock ridges, bare bedrock is common, residuum is universally thin, and loess is often absent. Ridges that occur downwind of areas with abundant sand supply are often blanketed with eolian sand on their western and northwestern flanks, driven partially up the windward slopes as sand ramps and climbing dunes. Thick loess commonly occurs on the east and southeast sides of the largest sandstone ridges, suggesting that winds from the west-northwest were often unidirectional and strong enough to form prominent wind shadows. Together, these features point to a permafrost/thawing permafrost landscape that was largely impacted, and in many places eroded, by saltating sand. In this poster, we provide examples, many of them photographs but some also as digital terrain maps, of the features described above.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318304

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