An Expanded Stratigraphic Record of the Devonian-Carboniferous Boundary Hangenberg Biogeochemical Event from Southeast Iowa (U.S.A.)

The Devonian-Carboniferous boundary in the type area of the Mississippian subsystem (tri-state area of Iowa, Illinois, and Missouri) has been historically difficult to identify. Many of the localities contain similar lithologies and stratigraphic successions, but chronostratigraphic correlation of seemingly identical lithologies can vary greatly in this interval and frequently this has led to miscorrelation. In particular, the similar lithofacies that comprise the McCraney Formation and Louisiana Formation have been a source of stratigraphic confusion for over 100 years. To investigate the Devonian-Carboniferous boundary interval in the Mississippian type area we selected two localities in southeastern Iowa, the H-28 core from Lee County outside of Keokuk, Iowa, and the Starr’s Cave outcrop located near Burlington, Iowa. In total, 62 conodont samples and 299 carbonate carbon isotope samples were processed for this study and recorded the Hangenberg positive carbon isotope excursion and 25 conodont species, including a diverse assemblage of siphonodellids. The Hangenberg excursion is recorded in over 20 m of strata in southeast Iowa, making this one of the thickest stratigraphic records of this important biogeochemical event yet recovered, and helps to define more clearly the position of the base of the Carboniferous System in the region. These results show that the “McCraney” Fm. at the Starr’s Cave outcrop and the coeval carbonate unit in the H-28 core are both the Louisiana Formation, and calls into question the use of the name McCraney throughout the State of Iowa