A TEST OF THE MAGNETO-STRATIGRAPHIC SUSCEPTIBILITY METHOD: HIGH RESOLUTION CORRELATION OF A WELL-DEFINED MARINE SUCCESSION, THE UPPER ORDOVICIAN (EDENIAN) KOPE FORMATION, KENTUCKY, AND DEVELOPMENT OF FLOATING-POINT TIME SCALES

Magnetic susceptibility (MS) is a measure of the degree to which a substance can acquire a magnetization when placed in a weak magnetic field. The MSEC method (magnetosusceptibility event and cyclostratigraphy) is based on variations of MS in stratigraphic sections. This method requires high-resolution measurements on samples collected at 5 to 10 cm intervals in outcrops and cores. The MSEC signature in marine sediments is dominated by the input of detrital lithogenic material due primarily to factors controlled by climate or eustasy (Ellwood et al., Geology, 2000). This method shows considerable promise for inter-regional and even global correlation and recognition of Milankovitch cyclicity, as demonstrated using MS measurements of unoriented samples from Devonian marine sections, calibrated with good biostratigraphic control, in Morocco, Europe, and North America. However, the efficacy of high-resolution MSEC correlation requires further testing in sections for which detailed correlations have been established independently. The Upper Ordovician (Edenian) Kope Formation in the Cincinnati, Ohio-northern Kentucky area, has been correlated at a bed-by-bed scale using a combination of biostratigraphy, pattern matching, event, and sequence stratigraphy (Holland et al., 2000; Brett et al., 2003). Here we report preliminary MSEC data from two sections of the Kope Fm. in northern Kentucky, each collected at 5 cm intervals. These marine sediments are dominated by repetitious successions of mudstone (70-80%), fossiliferous limestone and calcareous siltstones. Although an AF and thermal demagnetization survey indicates the presence of pervasive remagnetization, as in adjacent areas of the mid-continent, complicating the application of remanence reversal correlations, MSEC data show essentially the same trends and magnitudes at decimeter scale between the two sections. Thermomagnetic measurements indicate that the dominant mineralogy responsible for the MS is a paramagnetic component. Resolution of cycles within the two data sets is excellent and preliminary analysis suggests that these cycles represent climate cyclicity. This result offers the possibility of developing high-resolution floating-point time scales for these and other sections.