STRATIGRAPHIC ANALYSIS OF CARBON ISOTOPE AND GAMMA-RAY RECORDS OF UPPER CAMBRIAN CARBONATE CYCLES FROM UTAH AND NEVADA

Composite geochemical studies of carbonate isotope ratios (δ¹³C) establish that the Cambrian period is punctuated by a series of significant, globally correlated, positive and negative δ¹³C excursions. However, it remains unclear if small scale secular variations in δ¹³C exist superimposed on, or between large scale perturbations in the Cambrian carbon cycle, especially in cyclic successions. Rocks of Steptoean-Sunwaptan (Paibian-Jiangshanian) stage exposed in the Great Basin of Utah and Nevada are an excellent candidate for high-resolution chemostratigraphic analysis. The region is characterized by biomere extinction events, a significant positive carbon isotope excursion (SPICE event) and anomalously uniform meter-scale carbonate cycles. Here, high-resolution δ¹³C data was collected from cyclic sections in Utah and Nevada. In addition, gamma-ray profiles were measured from a total of eight sections across the Great Basin. We find that secular variations of δ¹³C up to +/- 1.33‰ and +/- 0.70‰ exist between correlative 5^th and 6^th order cycles in Utah and Nevada respectively. Carbonate cycles are easily identifiable in plotted δ¹³C data, with more positive values between cycles. Cycles are also identifiable on outcrop gamma ray logs, which closely align with δ¹³C profiles and can be correlated laterally 115km based on key marker beds in the dominantly subtidal succession. Although solutions for the astronomical computation of the orbital motion of the Earth have been only solved for the period spanning 0 to ~250Myr, time-series analysis are underway in order to determine if periodic components exist in δ¹³C and gamma-ray profiles. Despite limitations, spectral analyses of δ¹³C reveals peak frequencies that compare favorably with the precession, obliquity, and eccentricity bands estimated from long-term sedimentation rates. This investigation of Late Cambrian cyclic carbonates provides evidence of allogenic processes controlling cycle formation and putative Milankovitch-style cyclicity recorded in mid-to outer-shelf carbonate deposits of Laurentia.