IDENTIFICATION OF AN EARLY, HIGHER (343m) PHASE OF GLACIAL LAKE BENSON, SWIFT CO., MINNESOTA

Sediments of Glacial Lake Benson, a proglacial lake that formed in the present-day Minnesota River, were initially mapped by Diedrick and Rust (1975), and Geomorphologic features (deltas and strandlines) were first mapped by Rittenour et al. (1998). Subsequent studies of Glacial Lake Benson have focused on the nature, origin and stratigraphy of Lake Benson sediments. Previous reconnaissance mapping identified deltaic sediments 8 km east of Benson, Minnesota. This deposit was initially not attributed to Glacial Lake Benson because it occurs at an elevation of 343m and Glacial Lake Benson has a well-documented elevation of 328m. The goal of this study was to determine if a possible early, higher phase of Glacial Lake Benson existed.

Fieldwork attempted to identify sedimentologic and geomorphologic evidence for a higher lake level of Glacial Lake Benson. Deltaic sediments are present and topset/foreset contacts occur at 343m. Provenance analyses of the fine sand fraction of the delta sediments indicate a Des Moines lobe source. Importantly, these sediments appear to be overlain by Des Moines lobe till, indicating that an early, higher phase of Lake Benson may have ended with an advance of the Des Moines lobe. No 343m strandlines or tributary terraces were identified. Hummocky surfaces and marshes from later events dominate the topography and may be obscuring features of the early, higher (343m) phase of Glacial Lake Benson

Research for this study was funded by a grant from the N.S.F.-R.E.U Program (NSF-EAR 0640575).