SEDIMENTARY FACIES OF THE SWIFT FALLS DELTA : STRATIGRAPHY AND PALEOENVIRONMENT

The East Branch of the Chippewa River, Pope and Swift Counties, west central Minnesota flows from the Blue Mounds through Swift Falls to Benson, Minnesota. During the late glacial retreat of the Des Moines lobe, outwash from an ice margin at or near the Blue Mounds flowed down the Glacial East Branch of the Chippewa into Glacial Lake Benson creating the Swift Falls Delta. The purpose of this study was to reconstruct the depositional environments of the Swift Falls Delta in an effort to determine: a) the source of Glacial East Branch outwash; b) the proximity of the ice margin and c) the origin of the Blue Mounds - an enigmatic glacial landform.

A stratigraphic study was performed on a 50m north-south tending excavation through the delta. Individual units were identified, described and analyzed in the laboratory. Sedimentary features of the Swift Falls Delta include: locustrine silts, debris flow diamicts, partial turbidite sequences and scour and fill channels that represent a dynamic proglacial environment. Individual units may have formed as a result of: seasonal freeze-thaw outflow, ice margin readvances, and proglacial debris flows. These deposits may be of specific enough character to be traced up the East Branch of the Chippewa terraces to the Blue Mounds.

The origin of the Blue Mounds has long eluded researchers. With continued study of ice proximal features and glacial deposits in the East Branch of the Chippewa drainage, it may be possible to ascertain if there was an ice margin at the Blue Mounds at the time of Glacial Lake Benson. Research for this study was funded by a grant from the N.S.F.-R.E.U. Program (NSF-EAR 0353621).