FIRST RECOGNITION OF HIRNANTIAN (UPPERMOST ORDOVICIAN) STRATA IN NORTHEASTERN IOWA AND NORTHEASTERN ILLINOIS BY MEANS OF d¹³C CHEMOSTRATIGRAPHY

The precise age of the strata overlying the Richmondian Maquoketa Shale in Iowa and coeval strata in northern Illinois, and the position of the Ordovician/Silurian boundary, have long been uncertain. In the absence of diagnostic fossils, these rocks have been referred to either the Upper Ordovician or to the Lower Silurian, and the presence of the latest Ordovician (Hirnantian) Stage has never been reliably documented. Our chemostratigraphic studies have resulted in the discovery of the globally distributed Hirnantian d13C excursion (HICE) in the Schweizer Member of the Wilhelmi Formation at Joliet, northeastern Illinois, and in the lower part of the Mosalem Formation at Bellevue, northeastern Iowa. In the former area, the apparently complete HICE has maximum d13C values of approx. +2.5‰ and occurs in an 8 m thick interval. Lack of samples from the uppermost HICE interval at Bellevue prevents establishment of its thickness but it exceeds 7 m, and the d13C values are similar to those from Joliet. In both areas, the d13C baseline values are +1‰ or less. This first positive identification of Hirnantian strata in northern Illinois and eastern Iowa has important implications for the interpretation of the Late Ordovician-Early Silurian geologic history of the Upper Mississippi Valley.