MAGNETIC SUSCEPTIBILITY OF THE LATEST TELYCHIAN-EARLY SHEINWOODIAN (MIDDLE SILURIAN) SUCCESSION, SOUTHEASTERN INDIANA AND NORTHERN KENTUCKY: IMPLICATIONS FOR STRATIGRAPHIC INTERPRETATION OF DIAGENETICALLY ALTERED UNITS

Magnetic susceptibility (MS) values are technically independent of lithology; however, there are overarching controls on the distribution of clay minerals, coarser detrital particles, carbonate production, and diagenetic processes that genetically link MS patterns to facies shifts that can be predicted within a sequence stratigraphic framework. Silurian strata in the Cincinnati Arch region, comprising mixed carbonate-siliciclastic deposition in an epeiric ramp, provided an opportunity to test these relationships. Samples were collected from the latest Llandovery-early Wenlock Osgood, Lewisburg, and Massie Formations, as well as portions of the underlying Brassfield and overlying Laurel Formations. Bulk low-field MS measurements of irregular lithic fragments revealed consistently low values for the transgressive, carbonate-dominated Brassfield, Lewisburg, and Laurel sediments, as well as the silty to calcarenitic falling stage sediments of the upper Massie Formation. High MS values are characteristic of the clay-dominated highstand sediments of the lower Massie Formation. Most interesting, however, is the upward decrease in MS values observed within the Osgood Formation: although the entire formation is characterized by rhythmically alternating argillaceous dolostones and dolomitic mudstones, the highest MS values of the entire section occur in mudstones and tabular carbonates low in the Osgood, whereas values equal to or below those of the Lewisburg occur in mudstones and carbonates in the upper Osgood. This suggests that the dolomitized carbonates and mudstones of the lower Osgood represent highstand conditions and are highly condensed; high MS values reflect increased siliciclastic clay influx, possible oxidation of pyrite during re-working, and potentially even deposition of eolian ferromagnetic particles during sediment-starved intervals. In contrast, the upper Osgood represents falling stage conditions wherein increased influx of detrital carbonate and quartz silt resulted in low MS values. Hence, even in units where dolomitization has obscured primary sedimentary fabrics, fossil content, and other facies indicators, MS patterns can shed light on eustatic processes and differentiate between carbonates of dramatically differing origins.