STRATIGRAPHY AND GEOMORPHIC EVOLUTION OF SAND STRINGERS IN THE UPPER MISSISSIPPI RIVER BASIN

The upper Midwest United States is host to a variety of aeolian landforms distributed beyond the Late Wisconsin glacial ice margin, including sand stringers – subtle (1-5 m high), elongate (<5 km long, 10-100 m wide) deposits. To better understand sand stringer formation and evolution, we investigate stratigraphy of two sand stringers: Good-1 (Goodhue County, Minnesota) and ECC (Eau Claire County, Wisconsin). The following methods were used: (1) collection and description of soil cores, (2) particle size analysis via laser diffractometry, and (3) volumetric and mass-dependent magnetic susceptibility (MS) measurements.

Good-1 is on a broad, flat valley between two low-order streams and is ~0.75 km long, 60 m wide, and 4 m high. It is composed of ~1.5-2.5 m of silt loam with A-B-C horizonation and enhanced MS overlying a structureless fine sand unit with iron staining, Fe-Mn nodules, and low MS. ECC is on a broad upland and is ~1.2 km long, 10 m wide, and 2 m high. ECC is composed of ~1 m of sandy loam with A-E-B-C horizonation and enhanced MS overlying a stratified unit with alternating 1-8 cm thick iron-cemented, red sand and light brown, structureless sand with fluctuating MS. Prior research reported optically stimulated luminescence ages of 8.9-11.3 ka from the stratified unit.

Stratigraphy at Good-1 is interpreted as aeolian sand overlain by thick accumulations of loess with a well-developed surface soil. We hypothesize aeolian sand was deposited during the late Pleistocene with loess accumulation following retreat of the Des Moines Lobe, which has been documented regionally. Good-1 was likely stable with pedogenesis dominating throughout much of the Holocene. At ECC, we interpret complex stratigraphy below ~1 m resulted from alternating periods of stabilization and aeolian erosion/deposition from the late Pleistocene to early Holocene. A well-developed soil and increasing silt in the upper ~1 m suggests waning aeolian activity and pedogenesis through the Holocene.