U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho

Abstract

Three stratigraphically well defined rocks from the glaciogenic Scout Mountain Member, Neoproterozoic Pocatello Formation, southern Idaho, yielded sensitive, high-resolution ion-microprobe (SHRIMP) U-Pb zircon ages that constrain the age of the upper diamictite and its cap carbonate to between ca. 710 and 667 Ma. (1) Zircons from an epiclastic plagioclase-phyric tuff breccia immediately below glaciogenic Scout Mountain Member diamictite on Oxford Mountain, just north of the Utah border, yield a SHRIMP U-Pb concordia age of 709 ± 5 Ma. (2) A porphyritic rhyolite clast from the upper Scout Mountain Member diamictite at Portneuf Narrows, south of Pocatello, yields a concordia age of 717 ± 4 Ma. (3) The simple igneous zircon population from a reworked fallout tuff bed in the uppermost Scout Mountain Member, 20 m above the upper diamictite and its cap carbonate and immediately below a second cap-like carbonate, has a concordia age of 667 ± 5 Ma. These data support previous interpretations that the Scout Mountain Member glaciation scoured nearby volcanic highlands composed of the bimodal Bannock Volcanic Member and suggest that the volcanism was 717 ± 4 Ma. This age is close to, but distinctly older than, ca. 685 Ma U-Pb SHRIMP ages from the lithostratigraphically correlative Edwardsburg Formation in central Idaho. These data imply that the major rifting phase in this part of western Laurentia spanned 717-685 Ma rather than 800-750 Ma, as previously suggested. Further, because the Scout Mountain succession has been correlated with the Sturtian glacial phase on the basis of lithostratigraphy plus C and Sr isotope values in the carbonates, these data suggest that the Sturtian glacial epoch may have lasted until 670 Ma.