Quaternary Geology of the Itasca - St. Croix Moraine Interlobate Area, North-Central Minnesota

Abstract

During the St. Croix Phase of the Late Wisconsinan Substage, two lobes of the Laurentide Ice Sheet terminated and formed an interlobate junction in Cass County, Minnesota. Coarse textured, yellowish- to reddish-brown supraglacial sediments (Brainerd Till) were deposited as the north-south trending St. Croix Moraine by a southwestward advance of the Brainerd Sublobe of the Rainy Lobe. Brainerd Till is characterized by a high percentage of reddish crystalline rocks derived from a source area to the northeast and also by a low percentage of carbonate, probably derived by incorporation of underlying carbonate·-rich till. The terminal position of the Brainerd .Sublobe is also marked by a continuous fosse and dump ridge at the head of the westerly grading Oshawa outwash plain. A second outwash plain was formed behind the St. Croix Moraine during retreat of the Rainy Lobe at the end of the Itasca-St. Croix Phase. A contemporaneous advance of the Wadena Lobe deposited the Lower Red Lake Falls Formation, a light olive brown to gray sandy loam till which contains moderate amounts of carbonate clasts and sparse northeast-source rock types. The bulk of the sediments were deposited as the east-west trending Itasca Moraine. Within that moraine are numerous sets of transverse compressional ridges with a broad curvature to the northeast, reflecting a south-southwesterly course for the Wadena Lobe at its terminus. An advance of that glacier to a position some 25 km south of the Itasca Moraine is indicated by extra-moranic ti11 and pitted outwash. Outwash sediments from the Wadena Lobe dominate the proglacial and interlobate areas. The surfaces of two pitted outwash plains grade south and southeastward from the Itasca Moraine and bury the northern part of the St. Croix Moraine. Sediment texture in both the Itasca and St. Croix moraines becomes coarser to the southeast across the study area. Carbonate content decreases and northeast-source rock types increase in the same direction. These variations are best explained as the result of incorporation of underlying till during several glaciations of the Wisconsinan Stage prior to and including the Itasca-St. Croix Phase. The Hewitt Till, deposited by a southwestward advance of the Wadena Lobe during the earlier Hewitt Phase of the Wisconsinan Glaciation, is a compact, dark brown, sandy lodgement till. It is completely buried by outwash sediments close to the Itasca and St. Croix moraines, but gradually emerges to become the dominant surficial deposit as the outwash sediments thin to the southwest. The Hewitt Till surface in the study area is drumlinized, forming the northernmost exposed portion of the Wadena Drumlin Field. Trends of Wadena drumlins show a gradual shift from S 22° W in the eastern part of the study area to S 64° W in the western part. Fabric measurements on drumlins show a strong preferred orientation of elongate stones dipping to the northeast, indicating a southwest advance of the Wadena Lobe during the Hewitt Phase. Eolian sands form a thin blanket over the Wadena drumlins and outwash sediments in the southeast part of the study area. Barchan dunes developed in a small area where an abundant supply of fine-grained sand and a long northwesterly fetch were present. Eolian activity probably occurred during the mid-Holocene Hypsithermal Interval, approximately 8,000 to 5,000 B. P., when the climate was more arid and prairie vegetation dominated the area.