THE FREMONT CHANNEL IN WESTERN IOWA: A POSSIBLE ARCHIVE OF EARLY AND MIDDLE PLEISTOCENE GLACIATIONS IN THE MIDWESTERN U.S

The Fremont channel is a deep (>100 m) buried pre-glacial valley in western Iowa. The Iowa Geological Survey is studying this feature to assess its potential as a future groundwater source. The available data, primarily from well cuttings, indicate that the valley is filled mostly with till, but sand & gravel are present at various depths. If the sands occur consistently between the same tills, the chances of a laterally continuous aquifer are much greater, hence there is an emphasis on identifying specific tills.

Stratigraphic contacts within the glacial sequence are recognizable in cuttings based on both weathering profiles and interbedded outwash. For example, in one set of cuttings four such visual contacts separate 5 tills. Three of these tills are distinctly sandier than any previously known from this region. Based on other parameters; however, all five resemble both the Early Pleistocene “B-tills” (~1.3 Ma in age based on paleomagnetism and interbedded ash deposits) and the youngest Middle Pleistocene till, the “A1” (< 0.63 Ma). However, no more than two “B-tills” and a single “A1” have been recognized previously, suggesting that the valley could preserve a more complete record of glaciation than any other location known in Iowa.