CHASING THE HANGENBERG AND KINDERHOOK-OSAGE BOUNDARY EXCURSIONS IN THE U.S. MIDCONTINENT: “DUMBBELL STRATIGRAPHY” AND THE SUB-BURLINGTON UNCONFORMITY

The latest Devonian to Early Mississippian (Famennian to Osagean) was marked by significant changes in the ocean-atmosphere-biosphere system that include two major biogeochemical events. This interval records two of the largest positive δ¹³C excursions of the Phanerozoic; the Hangenberg excursion (which occurs at the Devonian-Mississippian boundary) and the Kinderhook-Osage Boundary Excursion (KOBE) which spans the K-O boundary. Some of the best exposed strata of this age are located in the U.S. midcontinent, notably the Iowa, Illinois, and Missouri tri-state region, which includes the type area for the Kinderhookian Stage.

Deposition occurred across a shallow carbonate shelf with a northeast-southwest depositional strike; Inner Shelf facies in central Iowa transition to Proximal/Central Middle Shelf facies in southeast Iowa and northern Missouri, Distal Middle Shelf facies in western Illinois and central Missouri, and eventually transitioning to Outer Shelf facies farther to the south/southeast. Forming a dumbbell-like pattern, strata are thinnest in the Central Middle Shelf and thicken both northwestward and southeastward into the Inner Shelf and Distal Middle Shelf deposits. This condensed interval of the Central Middle Shelf greatly complicates stratigraphic correlations across the state of Iowa and into Missouri/Illinois.

Through integration of sequence stratigraphy, δ¹³C chemostratigraphy, and conodont biostratigraphy, results from the tri-state area indicate an expanded record of the Devonian-Mississippian boundary and the Hangenberg excursion, whereas the KOBE is absent and the K-O boundary is marked by a significant hiatal surface. Conversely, up-ramp in central Iowa an expanded record of the K-O boundary and the KOBE is present, while the Devonian-Mississippian boundary is more condensed and the Hangenberg excursion is absent. This shelf geometry, resembling a dumbbell shape, suggests the lack of stratigraphy across the K-O boundary (sub-Burlington unconformity) was unlikely to have been the result of subaerial exposure.