RECOGNITION OF THE LATE AERONIAN, VALGU, AND IREVIKEN CARBON ISOTOPE EXCURSIONS IN THE ILLINOIS BASIN, MIDCONTINENT UNITED STATES

The Knapp Creek core from Johnson County, east-central Iowa, United States, contains some 228 feet of dolomitic Silurian strata which were deposited in the Illinois Basin. It includes (from oldest to youngest) the Blanding, Hopkinton, Scotch Grove, and Gower formations. The Silurian formations are unconformably bounded by both the Devonian Wapsipinicon Formation at the top, and the Ordovician Maquoketa Formation at the bottom. The Tete des Morts and Mosalem formations, historically considered the lowest part of the Silurian in Iowa, are not present in this core.

Carbon isotopes recovered from the core indicate the presence of the late Aeronian excursion, the Valgu Excursion (early Telychian), and the Ireviken Excursion (early Sheinwoodian). Conodonts recovered from just above the Valgu Excursion demonstrate the presence of the Pterospathodus amorphognathoides angulatus Zone. Additional conodonts recovered from the core include Aspelundia fluegeli fluegeli, Distomodus staurognathoides, Aulacognathus kuehni, Aulacognathus bullatus, Pterospathodus amorphognathoides amorphognathoides, and Kockelella ranuliformis. This core demonstrates that late Aeronian through early Sheinwoodian strata form a substantial portion of the Iowa stratigraphic succession, including the rarely preserved Aeronian–Telychian boundary interval. Additional Aeronian and Rhuddanian strata are potentially preserved within the Tete des Morts and part of the Mosalem formations. The presence of the Ireviken Excursion allows precise identification of the Llandovery–Wenlock boundary in sampled sections, but widespread facies diachroneity limits correlation into unsampled sections.