ABSTRACT
Iowa has one of the most complete terrestrial records of Quaternary Glaciation on earth: multiple Pre-Illinoian tills, a small area that was glaciated during the Illinoian, and multiple Wisconsinan advances. The pre-Last Glacial Maximum (LGM) Wisconsinan till, the Sheldon Creek Formation, lies under the LGM Des Moines Lobe (DML) till and extends beyond its terminal moraine. These glacial deposits have been a subject of debate since glacial stages were first assigned in the 19th century. Robert V. Ruhe drew a western boundary for the formation in the 1950s, at the time called the Tazewell, while the eastern boundary was thought to roughly coincide with the LGM DML boundary. East of the DML lies the Iowan Erosion Surface, a landform region dominated by intense erosion that occurred during the late Wisconsinan in eastern Iowa which beveled the landscape, removing loess, paleosols, and glacial landforms. However, a decade of detailed surficial mapping in north-central Iowa, through the STATEMAP program, has finally drawn an eastern boundary beyond the DML. The lithologic data indicates that the Sheldon Creek Formation shares a similar Keewatin ice source as the LGM DML. Lithostratographic data and radiocarbon dates from the area indicate that the Sheldon Creek was deposited in two phases, one from ~44-38 ka, and a later advance from 34-29 ka. These ages are seemingly asynchronous with the buildup of Labrador ice, and may indicate a more complex buildup to the LGM than previously thought.
As an added bounus: The Sheldon Creek dates, coupled with the timing of the James Lobe in the Dakotas could also suggest major implications in regards to the drainage of the northern Great Plains.
Biography
Mr. Kerr is a Geologist at the Iowa Geological Survey. He has a BS and a MS from the University of Iowa. His research interests include glacial stratigraphy, landscape development, and periglacial geomorphology.
