A MORE COMPLETE CRYOGENIAN RECORD IN SE IDAHO? EVIDENCE FOR BOTH SNOWBALL EARTH GLACIATIONS IN THE POCATELLO FORMATION

Recent studies of the Pocatello Formation in SE Idaho, which contains a lower diamictite, arkosic interval, upper diamictite, and cap dolostone, suggest that strata from the Sturtian and Marinoan glaciations and the Marinoan deglaciation are present, rather than two stades of Sturtian glaciation.

At Scout Mountain, the lower diamictite, interbedded with 696 Ma rhyolitic pyroclastic flows, is a massive to crudely bedded glacial deposit with polymictic clasts that are faceted and rarely striated. Sharply overlying the lower diamictite is a non-glacial package of siltstone that coarsens upward to arkosic arenite and displays hummocky cross-stratification, ripple marks, and crossbeds. Detrital zircon (DZ) samples from both intervals were analyzed for U-Pb ages using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS).

The resultant DZ U-Pb age distributions of the two samples are in sharp contrast. The diamictite matrix sample has age peaks at 1.4, 1.7, and 2.45 Ga, with youngest Neoproterozoic grains ranging from 673 – 699 Ma (n=14). The arkosic sample has age peaks at 1.1-1.3, 1.4 and 1.7 Ga with youngest Neoproterozoic grains ranging from 668 – 699 Ma (n=13). The Neoproterozoic zircon populations overlap and will require higher precision geochronology through chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) to resolve age differences. Similar DZ results using this tandem in situ-isotope dilution approach have been documented in correlative facies at Oxford Peak and Portneuf Gap, respectively, with the lower diamictite yielding a young DZ age group at 685.5 ± 0.4 Ma and the arkosic sample showing DZs at ≤675 Ma.

The contrasting DZ age distributions from Scout Mountain, in combination with a sharp shift in facies, and TIMS ages of correlative facies, define a ≥10 Ma disconformity at the top of the Sturtian diamictite. The succession above the unconformity includes ~450m of sandstone, siltstone, and rare limestone, overlain by the upper diamictite and a cap carbonate with features akin to other Marinoan caps. Lithologic relations and available geochronologic data in the Pocatello Formation suggest that both Sturtian and Marinoan glaciations and Marinoan cap dolostone are present, making this a critical area to study Snowball Earth glaciations, but more age control is needed.