TESTING A MARINE SIGNATURE FOR THE NEOPROTEROZOIC CHUAR GROUP OF GRAND CANYON USING SR ISOTOPIC DATA
We analyzed 36 samples for their elemental concentrations to determine if they had been significantly altered. Samples with high Sr and low Fe/Sr, Mn/Sr and Mg/Sr were considered least altered and used for 87Sr/86Sr analysis. 19 of the least altered samples were then analyzed to find their 87Sr/86Sr ratios. In the upper Chuar (Kwagunt Formation) we found what we interpret to be primary Neoproterozoic marine 87Sr/86Sr values in 7 samples ranging from 0.706961 to 0.707219. We also found 4 samples that had radiogenic 87Sr/86Sr values from 0.702705-0.7084803 that we interpret to be altered based on the elemental concentrations. In contrast, in the lower Chuar (Galeros Formation) all 8 of our samples had apparently non-marine 87Sr/86Sr values from 0.7078137-0.7099816. These radiogenic samples from the lower Chuar have elemental concentrations similar to the samples with primary marine signatures in the upper Chuar suggesting that their 87Sr/86Sr values may be primary and non-marine. We conclude that lower Chuar Group rocks may be non-marine or had an intermittent marine connection whereas the upper Chuar Group is marine, making the large δ13C excursion in the upper Chuar as a likely, in part, marine signal.