DEFINING A LLANDOVERY-WENLOCK (SILURIAN) TRANSGRESSION IN WISCONSIN AND ILLINOIS USING HIGH-RESOLUTION BIOSTRATIGRAPHIC, CHEMOSTRATIGRAPHIC AND LITHOSTRATIGRAPHIC DATA

Throughout most of the north- and south-central portions of North America, the Llandovery-Wenlock (Silurian) boundary is characterized by the presence of a major unconformity. This feature marks one of the most prominent sequence boundaries (Plaines-Brandon Bridge Sequence Boundary) within the Silurian of the midcontinent. Due to the magnitude of this unconformity, critical biostratigraphic, lithostratigraphic and chemostratigraphic data, representing most of the Telychian (late Llandovery), are missing over much of this region. As a result, the transition at the Llandovery-Wenlock boundary generally appears to be abrupt, suggesting a considerable time gap. In extreme examples, only part of the early Wenlock (Sheinwoodian) Ireviken Excursion is found overlying late Aeronian strata, as the earlier (lower) portion of the excursion may be missing along with most of the older Telychian rocks.

In contrast, the area centered on southeastern Wisconsin and northeastern Illinois generally preserves most of this missing Telychian section. Here, a relatively thick interval of these rocks is represented by the Brandon Bridge Formation, which overlies the unconformity. Importantly, this unit provides critical data on the depositional, biostratigraphic and chemostratigraphic nature of the Telychian and on the initiation of the following Ireviken Excursion. Depositionally, the Brandon Bridge represents a transgressive sequence that onlaps the underlying unconformable surface (Late Aeronian to possibly earliest Telychian in age). In addition, the Brandon Bridge has produced detailed biostratigraphic information, based primarily on conodonts from both the Lower Pterospathodus and Upper Pterospathodus Zonal Groups. Trends in the chemostratigraphic (∂¹³C_carb) data parallel biostratigraphic patterns, demonstrating that transgression was of long duration, ranging over several million years. This evidence also suggests that the development of the Ireviken Excursion might have begun in the early Telychian as opposed to early Wenlock.