Abstract

Periodicities of 15 cyclic sections of the Cisco Group are tested through gamma and spectral analyses. The sections are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the Eastern Shelf, north-central Texas, in Late Pennsylvanian and Early Permian time. Gamma analysis generates facies-dependent thickness-time conversion factors (gamma 's) to construct gamma -corrected time series, which are more realistic than the uncorrected series obtained by proportional conversion. Positive and stable gamma 's and improved resolution of the corrected spectra indicate successful gamma tuning. On the Cisco spectra, a short-eccentricity peak (95-131 ky) is the most prominent and persistent; long-eccentricity (413 ky) and obliquity (34 and 43 ky) peaks are persistent, and precessional index peaks (17 and 21 ky) are less persistent. A persistent peak of period approximately 50-80 ky may be a combination tone between obliquity and precessional index cycles. Non-Milankovitch peaks are sporadic. The validity of gamma and spectral results is indicated by consistent g-estimated ages among the Cisco sections and reasonable effective sedimentation rates calculated on the basis of Milankovitch spectral calibrations. The results, combined with geologic evidence, suggest that the Cisco cyclicity was controlled dominantly by Milankovitch climatic forcing.

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