Age and correlation of Silurian rocks in Sheboygan, Wisconsin, using integrated stable carbon isotope stratigraphy and facies analysis

The exact age of Silurian rocks in eastern Wisconsin is uncertain; biostratigraphic precision here approximates +/- 2 million years. Much of this ambiguity stems from limited surface exposure, poor biostratigraphic control, and prominent facies changes north of the well-studied sections in southeastern Wisconsin. The primary objective of this study is to evaluate the efficacy of high-resolution carbon isotope stratigraphy in resolving this chronostratigraphic uncertainty. Development of a carbon isotope global standard has revolutionized chronostratigraphy in the Silurian and is applicable to Wisconsin rocks. Recent bedrock mapping in Sheboygan County led to the acquisition of 15 new bedrock cores, four of which were analyzed for carbon and oxygen isotopes. A core drilled in Quarry Park in the city of Sheboygan’s north side was sampled at less than 2-foot (0.6 m) intervals through more than 600 feet (180 m) of Silurian rock. Three additional cores, drilled nearby along the Lake Michigan shoreline (about 1,500 feet, 460 m, northeast of North Shore Park), were sampled at a similar interval. Correlation of these cores around the city of Sheboygan allows for construction of a composite section. Carbon isotope results from this composite section exhibit many of the patterns present in the global composite. As a result, the uncertainty on age assignment of strata within this interval may be reduced to approximately a 100,000-year scale. The Sheboygan-area chemostratigraphy provides a preliminary revision to the age of these Silurian rocks and provides a starting point for the future revision of Silurian chronostratigraphy throughout eastern Wisconsin and the Great Lakes region for improved prediction and assessment of natural resources.