GEOMORPHOLOGY OF AEOLIAN SAND STRINGERS IN WESTERN WISCONSIN AND SOUTHEASTERN MINNESOTA

Subtle (1-5 m high), elongate (<20 km long, 10-100 m wide), aeolian deposits termed “sand stringers” have been identified and described throughout the western Great Lakes region beyond the Late Wisconsin glacial margin. Although previous studies have attempted to map their distribution, and to a minor extent examine their composition and absolute age (e.g. Zanner 1999; Koch 2004; Millett et al. 2018), little is known of their geomorphic history and paleoenvironmental significance. This study aims to build on prior work to determine their distribution, orientation, morphology, composition, depositional chronology and processes of formation. The following methods were utilized to better understand these features: 1) GIS/remote sensing techniques to map and analyze their spatial distribution and morphology; 2) ground penetrating radar (GPR) analysis, sediment augering, and pedon descriptions to characterize their stratigraphy; and 3) optically stimulated luminescence dating (OSL) to constrain the timing of their formation and evolution.

Although prior mapping efforts noted >500 sand stringers in western Wisconsin and southeastern Minnesota, our more conservative mapping approach identified, with high confidence, 246 such features. Of these, 91% were orientated WNW-ESE. OSL ages on five stringers range from ~12.9 ka to ~8.8 ka. Comprehensive investigation of one stringer resulted in GPR imagery showing sub-parallel and slightly undulating stratigraphy in the stringers, but 3D analysis is still needed. Augering revealed three distinct units consisting of a lower coarse sand unit interpreted as glacial outwash, a middle unit of fine to medium sand interpreted as aeolian sand, and an upper unit composed of fine sand and silt interpreted as loess.

From this analysis we hypothesize that sand stringers formed in a post-permafrost landscape with sparse vegetation. Thawing allowed sand to be mobilized and the lack of vegetation allowed for aeolian transport and deposition. Initial deposition within the sand stringer consisted of aeolian sand, shifting to loess, possibly as the climate warmed and the landscape began to stabilize.