SPECTRAL ANALYSIS OF LATE CAMBRIAN CYCLIC CARBONATES AND ASTRONOMICAL FORCING

The forcing mechanism for Cambrian sea-level changes is a matter of long-standing debate: are sea-level fluctuations related to climate changes or changes in the rate of sea-floor spreading? Spectral analysis of a gamma-ray log through a thick succession of Late Cambrian miogeoclinal platform carbonates in western Utah shows peak frequencies that can be correlated with Milankovitch periodicities, especially those associated with the precessional and eccentricity spectra.

Stratigraphic units in the American Quasar Horse Heaven-State 16-21A borehole, located on the crest of the Confusion Range, can be correlated confidently with outcrops based on similarities of the gamma-ray log with gamma-ray profiles of stratigraphic sections in House Range. The highly radioactive Corset Spring Shale Member (Orr Fm.) and less radioactive Red Tops Member (Notch Peak Fm.) bracket a thick carbonate lithosome. In the lower part of this succession, the Sneakover Limestone (Orr Fm.) and "lower map unit" of the Hellnmaria Member (Notch Peak Fm.) are more than 550 ft (150 m) thick, with beds ranging from 1.5-11 ft (0.45-3.3 m) thick. These strata accumulated during a major eustatic rise that began during Elvinia chron. In outcrop, both the Sneakover and lower Hellnmaria exhibit prima facie evidence of cyclicity. Conspicuously uniform bedding and repetitive variations in lithofacies were the impetus for previous investigations of stacking patterns using Fisher plots.

Spectral analysis of the Corset Spring through Red Tops log intervals provides an independent a posteriori means for examining patterns of cyclicity. Subtle variations in clay content likely are responsible for the cyclic gamma-ray signal. Using a sedimentation rate of 0.43 ft/kyr (0.13 m/kyr), estimated from geochronologic ages (Bowring et. al. 1999), this study shows that climate oscillations played a key role in influencing sea-level fluctuations, but the contribution of sea-floor spreading rates remains unconstrained.