INTEGRATED CONODONT BIOSTRATIGRAPHY AND CHEMOSTRATIGRAPHY (δ13CCARB, 87SR/86SR, MAJOR AND TRACE ELEMENTS) OF THE WAUBAKEE FORMATION (UPPER SILURIAN) IN EASTERN WISCONSIN: A NEW APPROACH TO AN UNRESOLVED CHRONOSTRATIGRAPHIC PROBLEM

The uppermost Silurian unit in Wisconsin is the Waubakee Formation, a finely-laminated dolostone deposited in restricted, hypersaline environments. Although its upper contact is clearly marked by a major unconformity below mid-Devonian strata, the lower boundary is lithologically cryptic, even with high-resolution elemental geochemistry generated using handheld XRF. This interval is sparsely fossiliferous and consequently, its age is poorly constrained. However, new data from the Waubakee Formation in several drill cores from eastern Wisconsin can be used to establish its age and stratigraphic correlations with similar Silurian (“Salina”) facies in the Michigan basin.

An integrated approach that combines conodont biostratigraphy with strontium and carbon isotope data can confidently determine the age of these problematic strata. The assemblage of conodonts, which were recovered from the lower 12 feet of the unit, contains specimens of Pseudooneotodus beckmanni, Pseudooneotodus bicornis, Panderodus unicostatus, Panderodus greenlandensis, Decoriconus fragillis, Wurmiella excavata, and a single fragmented Pa element of Ozarkodina sp. These taxa (the first diagnostic conodonts discovered in the Waubakee) are not indicative of a specific Zone, but the collection resembles depauperate faunas of mid to late Silurian biotic events. Strontium isotope analyses (⁸⁷Sr/⁸⁶Sr) of multi-element conodont samples from five stratigraphic horizons yield results ranging from 0.70844 to 0.70849. Carbon isotope stratigraphy shows a well-defined +2.5‰ excursion in the lower part of the formation. Moreover, the positive carbon isotope values are immediately overlain by a negative excursion of nearly -2‰, before return to a baseline of ~0.5‰ (a distinctive pattern observed throughout the Michigan Basin). Combined, this data precisely constrains the lower Waubakee to the Homerian, coincident with the Mulde Excursion.

These findings demonstrate the power of an integrated approach to resolving the most challenging chronostratigraphic problems. Application of these tools to problematic facies opens up new frontiers in determining the depositional history of restricted marine basins.