SILURIAN DEPOSITIONAL SEQUENCES OF THE CENTRAL UNITED STATES

The Silurian depositional history of the central United States (WI, IL, IA, IN, MI) has been the subject of numerous studies since the mid-1800’s but has persistently remained poorly understood. Generally characterized by small, and widely spaced natural outcrops of pervasively dolomitized carbonates, it has been difficult to develop a reliable lithostratigraphic framework for these rocks. Biostratigraphically precise fossils are not present in many stratigraphic intervals of the Silurian, making reginal correlations problematic. In addition, the oversimplification or misidentification of “reefs” in these rocks has also been a problem in establishing valid depositional trends. Beginning in the 1960s, the development of extensive rock coring programs and deep quarries, provided a unique opportunity to study these rocks. As a result, it was possible to establish a well-defined lithostratigraphic framework and related depositional sequences for major portions of the Silurian in this region. Additional biostratigraphic studies have also provided an increasingly important temporal control for the correlation of Silurian rock units. The depositional sequences were controlled by glacial-eustatic sea level changes and are represented by similar appearing, unconformity bound transgressive-regressive depositional packages, which are recognizable throughout and far beyond the study area. The transgression at the beginning of each sequence is related to a prominent extinction event and is commonly associated with examples of exceptional fossil preservation. More recent geochemical studies show that early stages of these sequences are also characterized by a ∂¹³C excursion which further defines the previously recognized depositional sequences. In turn the character of these depositional sequences has been an exceptionally reliable predictor for locating ∂¹³C excursions. Wenlock and early Ludlow rocks of the region exhibit the best documented examples of these sequences which resolve previous mistakes while establishing broad and accurate correlations of the regional Silurian. Unfortunately, the decreasing availability of useful quarry exposures and absence of new drilling programs has made it difficult to undertake additional comprehensive studies throughout the region.