PRELIMINARY SPECTRAL ANALYSIS OF THE SALONA-COBURN (UPPER ORDOVICIAN) INTERVAL FROM CENTRAL PA USING BED BY BED THICKNESS MEASUREMENTS

In Central PA, the Upper Ordovician (Mohawkian Series) Salona and Coburn Formations (SCF) of the Trenton consist of interbedded limestones/shales deposited in a carbonate ramp on the margin of the Taconic Foreland Basin. These were deposited during the Vermontian tectophase of the Taconic Orogeny. The Salona contains the M4/M5 sequence boundary, which lies below the Late Ordovician Guttenberg Carbon Isotope Excursion. The Coburn is overlain by the Antes Shale (Utica of NY). The carbonate-shale package of the SCF (and coeval strata) has been noted by previous workers to show spatio-temporal cycles on the basis of cycle thickness, and repeating lithologies, which have been attributed to Milankovich cycles.

In this study, we measured bed thickness in outcrops (Union Furnace, Lemont, and Reedsville, PA) and core (Mackeyville, PA) to the nearest 0.5cm to use spectral analysis in the SCF interval to calculate statistically significant cycle frequencies. Our data provides evidence of multiple cycles within the SCF, especially at Reedsville and in core from Mackeyville. Lomb-Scargle periodogram and REDFIT analyses identified several statically significant frequencies. Cycle thicknesses were identified by significant peaks in power spectrum scores and were compared to predicted nested ratios of 1:4, and 1:5, as per Milankovitch predictions.

Initial results show that ratios of ~1:4 were identified within the Reedsville cut where significant cycle thicknesses appeared for sub-decameter-scale, sub-five meter, two-meter, and 1 meter scales. The core demonstrated decameter and two-meter scale cycles. These data suggest multiple temporal scales are at play and need to be resolved. When compared to dated K-bentonites, at least some of the cycles observed at Reedsville and in the core might indicate harmonic modulation between high-frequency precession and low-frequency eccentricity cycles. These concur with the suggestions of other studies. In addition, other cycles with multiples of three were noted in the core as statistically significant. These were observed between the two-meter cycles and even higher frequency cycles ranging down to 27 and 12 cm. Although more work remains, the multiple of three ratios at this thickness scale are likely attributed to processes that result in turbidite formation.