TILLS IN THE SWIFT FALLS DELTA, GLACIAL LAKE BENSON

The Swift Falls delta is located south of Swift Falls, Swift County, west-central Minnesota. The Swift Falls Delta complex is made up of an upper and a lower deltaic component, both which were deposited by the ancestral East Branch Chippewa River into Glacial Lake Benson. The upper delta was deposited during the high water (earlier) phase of Lake Benson. The goal of this study was to determine the origin, manner of formation, and relative age of the sediments and sedimentary features found in a gravel pit in the upper delta, and to determine if the Swift Falls delta was ice proximal or distal.

The study site is located very close to the delta apex. Sedimentary features at the site were observed and described in the field. Then photographed and sampled. The samples were then subjected to sieving and point counts for composition analysis. The sediment facies of the site included bedded sediments which ranged in size from sandy silt to (in pockets) sandy gravel, possible topset beds, and channel cut and fill units. Also found were three different diamicts, one of which was traceable the length of the exposure. The other two diamicts were traceable for 30m and then pinch out. Other diamicts were found as discreet "clasts" (ice rafted?), flow lenses, and pockets. In the deltaic sediments an upward warping of the layers indicate an intrusion of a flowtill. This interpretation is supported by the sharp contacts between the diamicts and the sands and gravels. Lithologic analysis showed that the sands, gravels, and diamicts likely came from the same source.

All these features described provide argument for an ice proximal setting of the upper delta of the Swift Falls complex. Features indicate deltaic sedimentation with flow tills periodically intruding and altering bedding forms and ice rafting of diamict clasts.

Research for this study was funded by a grant from the N.S.F.-R.E.U. program (NSF-EAR 9820249).