Visual identification and quantification of Milankovitch climate cycles in outcrop: an example from the Upper Ordovician Kope Formation, Northern Kentucky
Brooks B. Ellwood, Carlton E. Brett, Jonathan H. Tomkin, William D. MacDonald, 2013. "Visual identification and quantification of Milankovitch climate cycles in outcrop: an example from the Upper Ordovician Kope Formation, Northern Kentucky", Magnetic Methods and the Timing of Geological Processes, L. Jovane, E. Herrero-Bervera, L.A. Hinnov, B. Housen
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Applying time-series analyses using Fourier transform and multi-taper methods to low-field, mass-specific magnetic susceptibility (χ) measurements on marine samples from well-studied shale and limestone outcrops of the Upper Ordovician (Edenian Stage; Upper Katian) Kope Formation, northern Kentucky, corroborates direct visual identification in outcrops of Milankovitch eccentricity (c. 405 and 100 ka), obliquity and precessional climate cycles. Because individual outcrops were too short and deposition too chaotic to yield significant time-series results, it was necessary to build a c. 50 m thick composite sequence from three well-correlated outcrops to quantify the cyclicity. Time-series analysis was then performed using χ measured for 1004 closely spaced samples covering the section. Milankovitch bands are recorded in the time-series data from the composite. We tested this result by comparison of these bands to cyclic packages in outcrop, which correspond to thicknesses represented in the time-series datasets. This is particularly well defined for the eccentricity and obliquity cycles, with precessional bands being evident but as less well-defined packages of beds.
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Magnetostratigraphy is best known as a technique that employs correlation among different stratigraphic sections using the magnetic directions defining geomagnetic polarity reversals as marker horizons. The ages of the polarity reversals provide common tie points among the sections, allowing accurate time correlation. Recently, studies of magnetic methods and the timing of geological processes have acquired a broader meaning, now referring to many types of magnetic measurements within a stratigraphic sequence. Many of these measurements provide correlation and age control not only for the older and younger boundaries of a polarity interval, but also within intervals. Thus, magnetostratigraphy no longer represents a dating tool based only on geomagnetic polarity reversals, but comprises a set of techniques that includes measurements of geomagnetic field parameters, environmental magnetism, rock-magnetic properties, radiometric dating and astronomically forced palaeoclimatic change recorded in sedimentary rocks, and key corrections to magnetic directions related to geodynamics, palaeocurrents, tectonics and diagenetic processes.