TESTING A MARINE SIGNATURE FOR THE NEOPROTEROZOIC CHUAR GROUP OF GRAND CANYON USING SR ISOTOPIC DATA

The Chuar Group was deposited during a dynamic time in Earth’s history between 780 and 742 Ma. This was during the breakup of the supercontinent of Rodinia, and before the onset of the Snowball Earth episodes and the following commencement of multicellular life. The Chuar Group is a 1600-meter-thick, apparently conformable, fossiliferous, succession that comprises dominantly shale with subordinate, but recurring, sandstone and carbonate beds. Carbon isotope studies have documented large (up to 15 ‰) excursions in organic carbon in organic-rich shale that, if marine, represent some of the largest positive excursions in Earth history. This makes determining the depositional environment of the Chuar critical. If all or part of it was not deposited in the Neoproterozoic ocean, then any data recovered may not be relevant to the ocean chemistry of this time. Previous studies inferred a shallow marine depositional environment based on sedimentary structures, marine microfossils and large swings in δ13C, similar to other Neoproterozoic sections worldwide.

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.