CAMBRIAN-ORDOVICIAN CYCLICITY AND ASTRONOMICAL FORCING

Spectral analysis of a gamma-ray log through Late Cambrian and Early Ordovician strata in west-central Utah shows that astronomical forcing was strongly imprinted on some intervals; peak spectral frequencies are interpreted as products of precession, obliquity, and 100-kyr eccentricity cycles. Previous studies of Middle Cambrian cycles by others have used the gamma method, which is based on facies-stacking patterns, to examine signals that can be tested for peak spectra coincident with Milankovitch periodicities. In contrast, assumptions for this approach are based on the stratigraphic distribution of radioactive minerals that presumably are related to climate changes and sea-level fluctuations. Spectral analysis requires a priori knowledge of cyclicity; correlation of outcrops to log using gamma-ray profiles; knowledge of regional patterns of sedimentation; reasonable estimates of accumulation rates; and a notion of the distribution of hiatuses.

Rocks of this study include the Upper Cambrian Corset Spring Shale (Orr Fm.) through the Hellnmaria Member (Notch Peak Fm.), and the uppermost Fillmore Formation and lower Wah Wah Limestone, which are Early Ordovician in age. Peak spectral fall within the expected precessional range at 2.1-3.4 m. Obliquity cycles vary from 6.5-7.3 m. The 99-123-kyr orbital signature is recorded in 20-25 m successions. Broad peaks represent variations in thickness presumably related to different rates of sediment accumulation or variations in cycle periods. Some peaks are associated with harmonics, components of constructive interference. One-hundred-meter cycles exposed in cliff exposures of the Hellnmaria are interpreted as the 400-kyr eccentricity and are beyond the sensitivity of window intervals used in this study.