LOOKS ARE DECEIVING: STABLE ISOTOPE DATA FROM THE POCATELLO FORMATION CAP-CARBONATE SEQUENCE AND ITS IMPLICATIONS FOR CAP-CARBONATE CORRELATIONS

Global cap-carbonate correlations have important implications for interpreting cap carbonates as discrete global timelines and as representative global events. The cap-carbonate sequence in the Scout Mountain Member of the Pocatello Formation is an excellent location to test the viability of correlation using cap carbonate intervals. Its age is well-constrained within a coherent composite measured section with a maximum depositional age of 665.8±5.7 Ma (U-Pb on detrital zircons in the cap; Dehler et al., 2009) and a minimum age of 667±5 Ma from a reworked tuff ~70 m upsection (U-Pb zircon; Fanning and Link, 2004).

Here we present an expanded carbon and oxygen isotope dataset (n=97 microdrilled samples, dominantly dolomite) mainly from a newly described cap-carbonate succession in the Pocatello Formation. The δ¹³C values range between -1.9 and -5.6 per mil and show a decrease in values upsection over 90 m. The δ¹⁸O values range from -10.2 to -17.4 per mil and also become increasingly negative upsection. Crossplots show minimal correlation between the carbon and oxygen values, suggesting a primary C-isotope signature. A lone 1-m thick limestone ~110 m below the cap-diamictite contact has δ¹³C values of +3.3 to +3.6 per mil and oxygen isotope values of ~-16.5, indicating an overall variability in C-isotope values of ~9 per mil.

C-isotope values are similar to those observed in basal Ediacaran cap carbonates worldwide. For example, the undated Noonday Dolomite in California and the hypothesized correlative ~635 Ma Maieberg cap carbonate of Namibia show similar C-isotope structure (and physical characteristics) as the Pocatello cap carbonate. Although the Noonday Dolomite is interpreted to be basal Ediacaran (Petterson, 2009), it could alternatively be the same age as the Pocatello cap carbonate (i.e., 30 Mys older), thus reinforcing the ideas that: 1) similar cap carbonate features are not unique to a particular time and 2) a third glaciation-cap carbonate formation event occurred at ca. 680-665 Ma.