MILANKOVITCH CYCLICITY IN THE OHIO AND SUNBURY SHALES: ASTRONOMICAL CALIBRATION OF THE LATE DEVONIAN-EARLY CARBONIFEROUS TIMESCALE

The Late Devonian (Famennian) Ohio Shale and Early Carboniferous (Tournaisian) Sunbury Shale of the Appalachian Basin exhibit subtle yet ubiquitous compositional cyclicity at several length scales. We have recorded this cyclicity using an ultra-slow (0.1 m hr^-1) spectral-gamma core logging procedure that yielded reproducible records from a series of four five-cm-diam. drillcores between northern Kentucky and northern Ohio. Evolutionary harmonic analysis was carried out to allow for changes in the length scales of dominant cycles arising from variable sedimentation rates and to assist in identification of gaps/condensed intervals within the sections. The resulting spectra reveal composite cycles with dominant wavelengths at ~0.5 m and 2.5 m in the lower part of the Ohio Shale, shifting to shorter wavelengths upsection but maintaining a ~5:1 length ratio. These cycles likely represent the Earth's orbital ~20-kyr precession and ~100-kyr short eccentricity cycles, yielding an average (compacted) sedimentation rate of ~25 mm kyr^-1 for the lower Ohio Shale. All four drillcores exhibit similar spectral patterns, although with higher sedimentation rates at the environmentally more proximal northern locales. The results of the present study may allow development of "floating" astronomical time scales for portions of the Devonian timescale. Although some problems with the time scale have been resolved by recent high-precision radiometric dates for the Frasnian-Famennian (376.1±1.6 Ma) and Devonian-Carboniferous (360.5±0.8 Ma) boundaries (Tucker et al., 1998; Richards et al., 2002; Kaufmann et al., 2004; Trapp et al., 2004), there is still little agreement about the absolute or relative durations of individual Devonian stages and biozones. Our proposed astronomical calibration can provide minimum durations for each Upper Devonian stage and biozone and, in combination with graphic correlation analysis, may assist in determining the stratigraphic location and duration of intraformational hiati.