TILL STRATIGRAPHY WITHIN THE FREMONT CHANNEL, WESTERN IOWA

The Fremont channel is a deep (>100 m) buried valley in western Iowa, subparallel to today’s Missouri River. This study is based on cuttings from 10 water wells within the Fremont channel, which were provided by the Iowa Geological Survey. The valley is filled mostly with till, and the surficial deposits at the well locations fall within 3 age groups: (1) Late Wisconsin (~14 ka) Dows Fm. deposited by the Des Moines Lobe, (2) Middle Wisconsin Sheldon Creek Fm. (~40-30 ka), and (3) various Middle Pleistocene tills (pre-Illinoian, > ~190 ka). Three Middle Pleistocene tills are recognized just south of the study area, and these commonly are given the informal names “A1”, A2”, and “A3”, youngest to oldest.

Laboratory analysis identified or confirmed multiple tills within the Dows Fm., two tills within the Sheldon Creek Fm. and multiple pre-Illinoian. Most stratigraphic contacts, however, were first recognized upon initial inspection of the cuttings; contacts generally are marked by stratified sediment and weathering zones. Wisconsin-age tills are easily identified by high shale contents within the coarse sand. Two tills of the Middle Wisconsin Sheldon Creek Fm. are distinguished from both younger (Dows Fm.) and older (pre-Illinoian) deposits by higher carbonate contents. The younger Sheldon Creek till is distinctly sandier than the older, which suggests a means of distinguishing these two on a regional basis. At least 4 Middle Pleistocene tills are present in northern Iowa. The youngest of these is much sandier than the A1 at locations just to the south. In two wells, however, this youngest Middle Pleistocene till overlies a typical A1, separated by a thin oxidized zone and stratified sediment, respectively. Further study might focus on whether these two tills represent separate glaciations, or just a minor ice-sheet fluctuation.