GETTING YOUNGER WITH AGE: MAXIMUM DEPOSTIONAL AGE OF THE STURTIAN SCOUT MOUNTAIN DIAMICTITE AND CAP CARBONATE, POCATELLO FM, IDAHO

Geochronologic constraints on Neoproterozoic diamictites and overlying cap carbonates are vital for understanding the relationship between these seemingly disparate units and their implications for severe global climate change. The upper Scout Mountain Member of the Pocatello Fm. at Portneuf Narrows contains a “conformable” diamictite-cap-carbonate pair that represents a phase of Sturtian glaciation. Recent U-Pb SHRIMP analyses on zircons reveal ages as young as 701±4 Ma on volcanic clasts in the upper diamictite (Fanning and Link, 2008, Geol Soc Aust Abs v. 91) and 667±5 Ma for a reworked tuff ~70 m above the laminated cap dolostone. These age relationships require the upper diamictite and overlying cap dolostone to be no older than 705 Ma and no younger than 662 Ma. A structurally separate diamictite on Oxford Mountain with an underlying tuff is younger than 686±4 Ma.

U-Pb ICPMS analyses were conducted at the Arizona LaserChron lab on 100-grain samples of detrital zircons from the matrix of diamictite directly below the cap-dolostone contact and from thin sandstone beds interbedded with the cap dolostone facies ~4 m above the contact. Both samples yielded maximum depositional ages (i.e. age of youngest zircon population) of about 689-675 Ma. Deposition of these units and the overlying 667 Ma tuff spanned about 10 m.y., supporting one or more unconformities in the 70-m-thick section. The upper Scout Mountain diamictite contains similar zircon populations to ca. 685 Ma diamictite in northern Idaho (Lund et al., 2003). TIMS and SHRIMP analyses are underway to refine the ages of these important units from Utah and SE Idaho.