TESTING REDOX HYPOTHESES FOR THE ORDOVICIAN JUAB AND KANOSH FORMATIONS AT FOSSIL MOUNTAIN, UT, USING CARBONATE ASSOCIATED SULFATE

The Middle Ordovician Juab Limestone and shale-dominated Kanosh Formation at Fossil Mountain, UT, are well-studied because of their importance for understanding the great Ordovician biodiversification event. Based in part on the occurrence of black-colored shales in the Kanosh Formation, Ross et al. (1989) interpreted the Kanosh Formation to have been deposited in a dysaerobic to anaerobic setting, with calcareous shell beds deposited during storm events. However, based on the occurrence of in-situ attachment structures as well as the abundance of fossils and trace fossils, Wilson et al. (1992) hypothesized that the Kanosh Formation was deposited under normal marine, well-oxygenated conditions. Here we present a geochemical test of the Ross et al (1989) hypothesis using sulfur and carbon geochemistry.

We measured the abundances of total sulfur, organic carbon, and carbonate associated sulfate ([S_total], [C_org], [CAS]) as well as δ³⁴S_CAS from carbonates at Fossil Mountain. [S_total] and [C_org] abundances were very low (average 0.02 and 0.2 wt.%, respectively) and agree with previously-reported results from shales at this locality. Such low abundances are not indicative of deposition under anoxic conditions because anoxia should have resulted in increased sulfide production and organic carbon preservation. In a restricted basin, an up-section increase in δ³⁴S_CAS coupled with a decrease in [CAS] would indicate prolonged anoxia in the basin. Rather, our results indicate an up-section decrease followed by an increase in δ³⁴S_CAS. Our δ³⁴S_CAS results are similar to those reported by Thompson and Kah (2012) in rocks of this age from Newfoundland and South America, indicating a global, not restricted, signature. [CAS] results show a weak (R²=0.4) negative correlation with δ³⁴S_CAS, which might be suggestive of a restricted basin. However, we interpret the trends in [CAS] to reflect changing fossil abundances up-section because biogenic carbonates tend to have much higher [CAS]. Although our results suggest that the succession at Fossil Mountain was not deposited under anoxic conditions, the relatively elevated δ³⁴S_CAS values (from +21 to +34 ‰ VCDT) support the interpretation of Thompson and Kah (2012) that the Ordovician was a time of deep-ocean anoxia.