North-Central Section - 49th Annual Meeting (19-20 May 2015)

Paper No. 3
Presentation Time: 2:15 PM


WOIDA, Kathleen, U.S. Dept of Agriculture, Natural Resources Conservation Service, 210 Walnut Street, 693 Federal Bldg, Des Moines, IA 50309 and LENSCH, Richard, retired from U.S. Dept of Agriculture, Natural Resources Conservation Service, 503 West 7th Street, Suite 3, Atlantic, IA 50022,

It is generally accepted that southwest Iowa underwent 6-7 major episodes of glaciation during the Pleistocene Epoch, which are represented by till layers separated by paleosols. Because Illinoian and Wisconsinan ice did not extend into southern Iowa, these Pre-Illinoian tills lie very close to the land surface. Prior to deposition of Wisconsinan loess, dissection of the landscape exposed several of the buried layers. Much later, erosion of the loess blanket again uncovered the upper paleosols on many slopes. The Clarinda soil series is mapped where these gray paleosols, now overlain by 0-45 cm of loess, are the dominant parent material for the modern soil. Clarinda soils occur over an elevation range of 70 m, reflecting the thick till package.

During the 2006 Adams County soil survey update, 78 locations were logged where gray paleosols lay exposed along roadcuts and hillslopes. The top elevations of the exposures fell into four groups, separated by 5-9 m. We propose that these paleosols represent four Pre-Illinoian interglacial stages. Projecting these elevations onto the Clarinda series map may give an initial approximation of the distribution of Pre-Illinoian tills in the county.

Drilling in 2006 and 2014 allowed for textural and morphological study of the upper three paleosols in a buried state. Solum thickness varies from >7 m in the uppermost paleosol (Yarmouth-Sangamon) to <2 m in the lower Pre-Illinoian paleosol (PIP-2). All three have clay or silty clay textures with >45% clay. The two younger paleosols are welded soils and include an upper solum in a non-till parent material, most likely loess. PIP-2 is sandier and very dark with an organic carbon content more typical of a Late Wisconsinan paleosol. Solum depth to secondary carbonates decreases markedly with paleosol age due to re-saturation by overlying tills. Thin-sections show frequent channels and fused granular structure at the top of PIP-1 and PIP-2, probable evidence for preservation of an A-horizon. Strongly developed vertic features (wedge structure, slickensides, and stress-oriented clay) on both macroscopic and microscopic scales attest to intense or prolonged conditions of wetting and drying. Future collaborative work will attempt to correlate the tills to a regional stratigraphic framework and characterize organic carbon stabilization in PIP-2.

  • Woida_Pleistocene paleosols_Slides.pptx (25.7 MB)
  • Woida_Pleistocene paleosols_Narrative.docx (28.1 kB)