PRO-GLACIAL BRAIDED STREAM DEPOSITS, POPE COUNTY, MINNESOTA

The Alexandria Moraine, west-central Minnesota, was deposited during several advances of the Des Moines, Superior, and Wadena lobes of the Laurentide ice sheet. The Blue Mounds, a large stagnation feature (Reid, 1974), dominates the southwesterly end of the Alexandria Moraine. The Mounds extend northwest-southeast for approximately ten kilometers and are up to one kilometer wide. Near the southeastern end of the Mounds a water gap allowed melt water to drain southward into Glacial Lake Benson. The study area is between the water gap and Upper Swift Falls Delta, a feature of Glacial Lake Benson. Presently the East Branch Chippewa River occupies the water gap. Evidence of ice blocks was found in the delta indicating close proximity to an ice sheet (Reiss and Maus, 1999).

The study area is a small gravel pit located approximately two kilometers south of the gap. Basal fine-grained material is horizontally bedded with small current and climbing ripples, both consistent with a fluvial environment. This sequence was scoured and is overlain by coarse flood deposits. Grain counts of the lower fine-grained and upper coarse-grained material are all low in carbonates and chert, but moderate to high in quartz and shale. This composition is characteristic of the Des Moines Lobe (Patterson and Southwick, 1998). Lenses of coarse material located within the sand are probably remains of ice blocks that were buried during deposition and later melted.

The observed depositional patterns are similar to Miall's (1981) Bijou Creek type. This suggests an ephemeral, braided stream system subject to infrequent, violet flash floods at the time of deposition.

Research for this study was funded by a grant from the National Science Foundation -- Research Experiences for Undergraduates (REU) Program (NSF-EAR 9820249).