GLACIAL LANDFORMS IN CENTRAL MINNESOTA: A PERSPECTIVE ON THE GENESIS AND AGE OF DRUMLINS, MORAINES, AND GLACIO-TECTONIC THRUST RIDGES

Recent mapping in the Todd county region of central Minnesota provides insight into the genesis of drumlins, moraines, and thrust ridges associated with the Late Wisconsinan glaciation. Interpretations are based on descriptions and samples collected over the last 10 years from Rotary-sonic drilling, auger drilling, and outcrops. Textural and lithologic analyses were used to characterize units.

In central Minnesota during the Late Wisconsinan glaciation, Wadena-lobe ice deposited varying thicknesses of bedded sediments and stony, sandy, carbonate-bearing till. These sediments covered a partially eroded, dissected, and weathered landscape composed of multiple older glacial deposits, suspected to be pre-Wisconsinan in age. Wadena-lobe ice drumlinized large areas of the region chiefly by erosion and deformation of the pre-existing land surface. Older deposits were observed commonly buried by thin Wadena-lobe sediments on drumlin flanks, and in places, they formed part of the drumlin core. Wadena-lobe deposits are considered to be Late Wisconsinan in age. The underlying units, in places, have thick oxidation zones and truncated soil horizons, but a definitive age for any of these deposits has yet to be established. Two sites—one cored, the other auger-sampled—about 2 miles apart near Clotho, each encountered Wadena-lobe till over lake sediment, over a pre-Wadena-lobe till (a different older till at each site). Pollen counts of the lake sediment from the cored site indicate an interglacial period. Two marl samples from each site are presently being tested in an attempt to obtain ages by high resolution U/Th dating.

The southern portion of the St. Croix moraine in central Minnesota is composed of segments of Superior-lobe stagnation deposits, glacio-tectonic thrust deposits, and segments where thin, draped, Superior-lobe sediment covers a ridge of older materials. The Osakis moraine, a recessional moraine of the Wadena lobe, also has a segment formed by glacio-tectonics—indicated by inverted stratigraphy and a hill/hole pair.