FROM MACRO TO MICRO: ADVANCES IN EARLY TWENTIETH CENTURY CORRELATION OF MIDWESTERN SILURIAN ROCKS

During the 19^th and early 20^th centuries, lithology and macro invertebrate biostratigraphy were the primary tools used in the correlation of American Paleozoic sedimentary rocks. However, using this approach, the scattered small outcrops of American Midwestern Silurian dolomites were difficult to place in a stratigraphic framework. In 1930, Paul Dunn, a University of Chicago Ph.D. student, discovered arenaceous foraminifera in Silurian rocks from Missouri and began processing additional Silurian rock samples collected around Joliet, Illinois by a former University of Chicago student, Jerome Fisher. While many of the Fisher’s samples also contained foraminifera, the basal “Niagaran” strata in his sections exhibited a conspicuous horizon of exceptional abundance and diversity of these fossils. Expanding his sampling from Illinois to Nashville, Tennessee, Dunn was able to trace this horizon throughout this region in exposures of the Osgood Limestone and related units. A short time later Lew Workman, of the Illinois State Geological Survey, began a study of insoluble residues also using the Fisher samples. He determined that Dunn’s Osgood horizon cooccurred with a conspicuous increase of in the volume of insoluble residues characterized by green and red clay. Combined, these fossils and residues clearly defined the base of the “Niagaran “strata, sharply contrasting with underlying Silurian rocks. While both approaches provided a significant advancement in Silurian correlation, they also had important limitations. Dunn’s foraminifera were very long ranging with some taxa extending into the Devonian. They did not represent a true biostratigraphic zone but only a conspicuous “acme” interval of abundant and diverse fossils. Nevertheless, this was first example of long-distance correlation of Midwestern Silurian rock using microfossils. Workman’s insoluble residues were also useful in identifying Osgood and related strata over a broad region. However, residues from other parts of the Silurian were usually only of local importance because of changing facies and diagenetic processes. Both approaches were innovative in the use of a single set of samples allowing for precise comparisons of data. Their resulting long-distance correlation of Osgood related strata continues to be verified by more recent chemostratigraphic and conodont based biostratigraphic studies.