THE RELATIONSHIP OF DEPOSITIONAL SEQUENCES, AND ∂13C EXCURSIONS TO ECHINODERM DIVERSITY IN THE SILURIAN REEFS OF WISCONSIN AND ILLINOIS

A high diversity echinoderm fauna has been described from the Silurian “Niagaran” Racine Dolomite of Wisconsin and Illinois. Since 1861, nearly seventy crinoid taxa along with eleven cystoids and one blastoid have been reported from the Racine. New research on the sequence stratigraphy, chemostratigraphy and depositional environments of the Racine and related rock units of the Midwestern U. S. clarifies various aspects of the occurrence, age and character of this fauna. Widespread geographically, many Racine taxa occur only in reef related environments. A single locality at Racine, Wisconsin has the highest reported diversity (fifty-four taxa) of any Silurian reef worldwide. Reefs of this region occur in different environmental settings and have different echinoderm characteristics. Most reefs exhibit common echinoderms but can have dramatic differences in diversity. Adjacent non-reef beds exhibit a lower echinoderm diversity, and abundance. The Racine echinoderm fauna is not a single entity but represents two distinct and successive Wenlock associations. The localities of southeastern Wisconsin (Racine) and northwestern Illinois are early Sheinwoodian in age while those of northeastern Illinois (Bridgeport, Hawthorne) are late Sheinwoodian. Each association is found within a separate depositional sequence representing climatically driven (glacio-eustatic) periods of sea level change. In the early stage of each sequence, transgressive flooding culminating with the peak of a ∂¹³C excursion (early Sheinwoodian Ireviken excursion, late Sheinwoodian Mulde excursion) coincides with pervasive deposition of clean carbonates, and extensive reef growth with a distinctive echinoderm fauna. Extensive study of the trilobite taxa found in these Silurian sequences shows pervasive extinction and replacement of all taxa in biomere-like patterns. The lack of a modern and comprehensive study of Racine echinoderms precludes a similar conclusion about extinction patterns of this group. With the exception of the unique Holocystitidae most published studies of Racine Dolomite echinoderms date from the late nineteenth and early twentieth centuries. Dolomitization is commonly cited as a hindrance in the study of Racine echinoderms however, there are large existing collections that have never been studied.