DETERMINING A GLACIAL BOUNDARY WITHOUT TRADITIONAL STRATIGRAPHIC AND GEOMORPHIC EVIDENCE: MAPPING THE MIDDLE WISCONSINAN SHELDON CREEK FORMATION IN NORTH-CENTRAL IOWA

The Iowa Geological Survey has recently redefined the Middle Wisconsin Sheldon Creek boundary in north-central Iowa. Glacial extents from relatively young advances are usually apparent from geomorphic features (i.e. moraines). However, exposures of the Sheldon Creek are extremely limited as this formation is overlain by the younger Late Wisconsinan Des Moines Lobe (DML) glacial advance. The original unit description is from a quarry exposure, and drill cores have been utilized to constrain the southern extent below the overriding DML. However, few outcrops exist to the west of the DML landform, and the influence of the Iowan (Erosion) Surface processes on the eastern side of the DML during the Last Glacial Maximum removed features typical of a glacial boundary. This erosion, combined with shallow bedrock in many areas, creates a spotty and incomplete stratigraphic record.

Without traditional geomorphic evidence for a boundary, the eastern margin of the Sheldon Creek was historically thought to coincide with the younger DML glacial advance. Stratigraphic evidence from core holes suggested otherwise, but few drill holes have full penetrations of the Sheldon Creek Formation; therefore, multiple lines of evidence were combined to determine the boundary. Over 200 cores were drilled over a 10 year period, and descriptive and analytical results were combined in piecemeal fashion with varying degrees of confidence in different data sets. The established stratigraphic framework was correlated based on a limited number of radiocarbon dates. Subtleties in geomorphic evidence also proved essential to unraveling the story. By combining all available stratigraphic, geomorphic, geochronologic, and lithologic data, an interpretive record was assembled for the Sheldon Creek glacial advance in Iowa. This evidence places glacial ice farther south than is traditionally recognized by regional models for the Middle Wisconsinan and may play a large role in understanding glacial dynamics for this time interval.