Testing a Marine Signature for the Neoproterozoic Chuar Group of Grand Canyon Using Sr Isotopic Data

Abstract

The goal of this research is to further test the depositional environment of the Grand Canyon's Chuar Group through 87Sr/86Sr analyses. The Chuar Group was deposited during a dynamic time in Earth's history between 780 and 742 Ma. This was just before some of the most dramatic changes in geology and biology in Earth's history: the breakup of the supercontinent of Rodinia, the Snowball Earth episodes, and the evolutionary commencement leading to multicellular life. The Chuar Group is a 1600-meter-thick, apparently conformable, microfossiliferous, unmetamorphosed succession that is composed of dominantly mudrock with subordinate but recurring sandstone and carbonate beds. Carbon isotopic studies have documented large 15 ‰ excursions in organic carbon in black mudrocks that, if marine, represent some of the largest positive excursion in Earth history. This is why it is critical to determine the depositional environment of the Chuar. If all or part of it was not deposited in the Neoproterozoic ocean then any data recovered from its sediments may not be relevant to the ocean chemistry of this time. Previous studies by Carol Dehler have inferred a shallow marine depositional environment based on sedimentary structures, marine microfossils and large swings in δ13C, which are recognized in all Neoproterozoic sections worldwide. To test for diagenesis we found the elemental concentrations of Sr, Fe, Mn and Mg for each sample. Samples with high Sr and low Fe/Sr, Mn/Sr and Mg/Sr were considered least altered and used for 87Sr/86Sr analysis.We analyzed 36 samples for their elemental concentrations to determine if they had been significantly altered. 19 of the least altered samples were then analyzed to find their 87Sr/86Sr ratios. In the upper Chuar (Wallcott and Awatubi members) 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 (Tanner, Jupiter and Carbon Canyon Members) 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 (Galleros Formation) may be non-marine or had an intermittent marine connection whereas the upper Chuar Group (Kwagunt Formation) is marine.