RI-34 Subsurface Till Stratigraphy of the Todd County Area, Central Minnesota

Abstract

Drilling logs, cuttings samples, and a limited amount of split-tube samples--acquired as a by-product of a crystalline basement drilling project-- provide evidence for informally naming nine new till units in central Minnesota. The Elmdale till may represent the first Pleistocene ice advance into central Minnesota. Other tills of northwestern provenance are the Eagle Bend, Meyer Lake, Green, and Browerville. Although shale is generally rare, these tills typically contain appreciable amounts of Cretaceous rock fragments, chiefly from the Greenhorn Limestone. The Second Red, First Red, Sandy, and Red Sandy are pre-Wisconsinan tills of northeastern provenance. They compose only a minor portion of the Pleistocene deposits in the Todd County area, but indicate alternate northeastern and northwestern ice advances. The surf icial Wadena till had a northeastern source, as indicated by lithology, texture, and drumlin orientation. Its relatively high carbonate content, which led earlier workers to ascribe a northwestern provenance, was deri ved through incorporation of older drif t. The Elmdale, Second Red, and Eagle Bend tills are thought to be pre-Illinoian; the First Red, Meyer Lake, and Green tills, pre-Illinoian or Illinoian; the Sandy and Browerville, Illinoian; and the Red Sandy, Illinoian or early Wisconsinan in age. A new radiocarbon date of 36,970 B.P. from a wood sample above the Wadena till supports an early Wisconsinan age designation for the surficial till over much of Todd County.