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

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