TESTING THE SENSITIVITY OF MAGNETIC SUSCEPTIBILITY TO UNCONFORMITIES, FLOODING SURFACES, AND REDOX RHYTHMS: SALUDA THROUGH LAUREL FORMATIONS (UPPER ORDOVICIAN THROUGH MIDDLE SILURIAN) OF WESTERN KENTUCKY

Magnetic susceptibility (MS) is increasingly recognized as an effective and powerful tool for stratigraphic and paleoenvironmental investigation of Paleozoic marine successions. This is due primarily to the fact that the magnetic carriers in such strata are typically paramagnetic detrital particles, allowing MS to serve as a reliable indicator of siliciclastic clay influx. The MS signal of an Upper Ordovician (Katian: Richmondian) through middle Silurian (Wenlock: Sheinwoodian) section near Mt. Washington, Bullitt County, western Kentucky, was studied to document patterns across sequence stratigraphic surfaces and to test predictions related to sedimentological influences on MS. The section consists of, in ascending order, the Saluda, ‘golden Brassfield,’ Lee Creek, Osgood, Lewisburg, Massie, and Laurel formations. Siliciclastic mudrocks (lower Osgood, lower Massie), silty, calcareous mudrocks (upper Osgood, upper Massie), and carbonates (‘golden Brassfield’, Saluda, Lee Creek, Lewisburg, Laurel) are present; key surfaces encompass four erosional sequence boundaries, flooding surfaces, and forced regression surfaces. Key findings are: (1) MS values are universally within the range expected for Paleozoic marine strata; (2) erosional unconformities are less pronounced here than otherwise expected due to their occurrence between regressive silt-rich units and transgressive carbonate units, both of which are characterized by low MS; (3) an exception to the previous point is represented by the Saluda-‘golden Brassfield’ contact because of the dramatically lower MS values of the ‘golden Brassfield’s’ crinoidal grainstone lithology; (4) major flooding surfaces and forced regression surfaces show up as sharp, prominent increases and decreases, respectively, in MS, reflecting the sharp contrasts between transgressive carbonates, highstand clay-dominated mudrocks, and falling stage siltier lithologies; and (5) the lower Osgood is a lithologically homogenous claystone except for red-green color alternations, which show up surprisingly clearly as relatively elevated values for red samples. Collectively, this study highlights the utility of MS as a stratigraphic and sedimentological tool in lithologically diverse successions recording eustatic fluctuations.