INFLUENCE OF REDOX RHYTHMS ON MAGNETIC SUSCEPTIBILITY TRENDS IN PALEOZOIC MARINE STRATA: PILOT STUDIES FROM THE SILURIAN AND MISSISSIPPIAN OF THE EASTERN MIDCONTINENT

Magnetic susceptibility (MS) trends are detectable across a range of lithologies, but some geochemical fluctuations may exert a certain degree of control over local MS signatures. Rhythms controlling the redox state of iron—manifest as alternations between red- and green-colored lithologic units—are expressed in MS profiles of lithologically and temporally diverse successions in the Paleozoic of eastern midcontinental USA. Irregular lithic fragments, collected in order to minimize anisotropy of MS, were used to generate profiles from three intervals: the basal interval of the middle Silurian Osgood Formation of northern Kentucky, the lower half of the middle Silurian Maddox Member of the Wayne Formation of central Tennessee, and the Lower Mississippian Henley Member of the Borden Formation of southern Ohio. Collectively, these units comprise pure siliciclastic mudrocks from different Paleozoic intervals (Osgood Formation and Henley Member) as well as argillaceous carbonates (Maddox Member) that are correlative to a sampled, non-carbonate succession (Osgood Formation). In all of these sections, regardless of age and lithology, red-green intervals were strongly separated with respect to MS values. Specifically, there was no overlap between red- and green-colored samples, with 100% of samples from all three sections being characterized by red samples that are entirely above, and green samples that are entirely below, the the average MS value for the section. There is no overlap between red and green MS samples in siliciclastic nor carbonate successions, with the highest green sample having a lower MS value than the lowest red sample universally. Thus, the pattern of elevated MS in red samples relative to green samples is supported across time (Silurian and Mississippian) and gross lithology (siliciclastic mudrocks and argillaceous carbonates). Redox rhythms must therefore be considered in evaluation of relevant MS profiles in future studies. Moreover, this study provides further evidence of the value of MS analysis in paleoenvironmental analysis and stratigraphic study of rhythmic/cyclic facies successions.