700 MA U-PB SHRIMP AGE FOR STURTIAN (!) DIAMICTITES OF THE POCATELLO FORMATION, SOUTHEASTERN IDAHO

The Snowball Earth model predicts isochronous global cryogenic events associated with the Neoproterozoic Sturtian glacial period. A number of geochronological studies document that on several continents this inferred Sturtian glacial 'event' is diachronous and may have extended over a period of some 50-70 Ma.

We have sampled three stratigraphically constrained igneous rocks from the Pocatello Formation, southern Idaho, traditionally correlated with Sturtian glaciation. Zircons from an epiclastic plagioclase-phyric tuff-breccia in the Bannock Volcanic Member, immediately below Scout Mountain Member diamictite horizons on Oxford Mountain, 60 miles south of Pocatello, Idaho, yield a SHRIMP U-Pb concordia age of 709 ± 5 Ma for all 18 grains analyzed. A porphyritic rhyolite clast from within the upper Scout Mountain diamictite at Portneuf Narrows, south of Pocatello, yields a concordia age of 717 ± 4 Ma. The simple igneous zircon population from a 2-3 cm wide green fallout tuff band 20 m above the upper diamictite and its cap carbonate north of Portneuf Narrows has a concordia age of 667 ± 5 Ma.

These SHRIMP U-Pb zircon data show that the upper diamictite and its cap carbonate are younger than ca.710 to ca.715 Ma and older than 667 ± 5 Ma. They support the previous interpretation of Scout Mountain Member glaciation of nearby young (rift-related) Bannock Volcanic highlands. The Scout Mountain cap carbonate and other thin stratigraphically younger carbonates, that bracket the 667 ± 5 Ma tuff bed, have negative carbon-13 isotopic values, and are correlated with post-Sturtian carbonates worldwide (Lorentz et al., 2003, GSA Abstract). The new Bannock Volcanic ages are close to, but distinctly older than 685 Ma ages from correlative Edwardsburg Formation in central Idaho (Lund et al., 2003, GSAB) suggesting that the major rifting phase in this part of western Laurentia spanned ages between 710 and 685 Ma.

Further these data suggest that on the global scale, the Sturtian glacial epoch lasted at least 75 Ma, from about 745 Ma to 670 Ma or ~75 Ma, with distinct phases represented on different continents. This implies diachronous glacial periods on various continents, counter to Snowball Earth predictions.