Search for: All records

ABSTRACT A pre‐Late Wisconsinan, post‐Illinoian, till sheet has long been recognized in north‐central Iowa, but has not been formally recognized or defined until now. Early researchers referred to these deposits as the ‘Tazewell’, and the term ‘Sheldon Creek’ was more recently used informally by the Iowa Geological Survey in guidebooks and reports. Recent mapping has extended the eastern margin significantly past previous interpretations. The Sheldon Creek Formation has similar lithologic characteristics to the overlying Alden Member of the Dows Formation, and the two units are distinguished mainly by stratigraphic position. Differentiation from underlying Pre‐Illinoian till units is accomplished using lithology, primarily matrix grain‐size and sand fraction lithology. A suite of 22 radiocarbon ages indicate two distinct, separate groupings within the Sheldon Creek data. These data strongly suggest ice advanced south to 42° N twice, once during Marine Isotope Stage (MIS) 3 and again during late MIS 3 or possibly early MIS 2. The presence of the Laurentide Ice Sheet in northern Iowa during MIS 3 has significant implications for ice sheet reconstructions during this interval. 

The tristate area of Iowa, Illinois, and Missouri contains some of the best-exposed Mississippian strata in the world, including the type area for the Mississippian subsystem, across a broad carbonate platform known as the Burlington shelf. Strata have been mapped as thinnest along the central middle shelf and thickening both up-ramp and down-ramp, forming a complex dumbbell-like stratigraphic pattern rather than a simple clinoform geometry thinning into the basin. Additionally, two significant hiatuses at the Devonian-Carboniferous boundary and Kinderhookian-Osagean boundary greatly complicate stratigraphic correlations across the region. As a result, the precise temporal relationships between strata deposited across the region remain uncertain. Two large biogeochemical events occurred during this interval that provide facies-independent chronostratigraphic tools: the Hangenberg event, which marks the Devonian-Carboniferous boundary, and the Kinderhookian-Osagean boundary event. To target these events, we collected 66 conodont samples and 1005 carbonate carbon isotope samples from three cores and three outcrops and integrated the results with existing data from key facies/depth transitions across the Burlington shelf. Our new data demonstrate a complex relationship among complementary stratigraphic thicknesses, where the Devonian-Carboniferous boundary interval is thin or absent in the up-ramp inner-shelf setting and preserved in a significantly expanded interval in the central to distal middle-shelf deposits of southeast Iowa and northeast Missouri. However, the overlying Kinderhookian-Osagean boundary interval is not preserved in this down-ramp setting but is preserved in significantly expanded strata in the up-ramp inner-shelf setting of central Iowa.