GETTING BETTER WITH AGE: NEW U-PB SHRIMP DATA FROM THE “STURTIAN” SCOUT MOUNTAIN DIAMICTITE-CAP-CARBONATE SEQUENCE, POCATELLO FM, IDAHO

The diamictite- and cap-carbonate-bearing Scout Mountain Member (SMM; >450 m thick) is the middle member of the Pocatello Fm. (1.3 km thick) of southeast Idaho, which records continental rifting and glaciogenic sedimentation during “Sturtian” time. SHRIMP U-Pb age constraints for the Pocatello basin range from >717+/- 4 Ma (volcanic clast in upper diamictite, SMM) to 667+/-5 Ma (reworked tuff 70 m above the cap-diamictite contact near top of SMM) (Fanning and Link, 2004; 2008).

Three new U-Pb SHRIMP analyses from samples in an upright, coherent stratigraphic section of the SMM indicate that this member, up to the upper diamictite, can be no older than ~680 Ma, and the overlying cap-carbonate sequence and above is ~665 Ma. The samples include a plagioclase-rich sandstone 110 m above the base of the SMM, matrix from the upper diamictite just below the sharp contact with the overlying cap sequence (390 m from base of SMM), and thin sandstone beds interbedded with the cap dolostone facies (4 m above the diamictite-cap sequence contact).

Euhedral, zoned igneous zircons with pyramidal terminations (i.e., least transported) were analyzed. The plagioclase-rich sandstone and diamictite samples yielded maximum depositional ages (i.e. age of youngest zircon population) of about 680 Ma; these zircon populations are similar to those found in the ca. 685 Ma diamictite in northern Idaho (Lund et al., 2003). The sandstone sample from the cap dolostone facies yielded a maximum depositional age of 666 +/- 6 Ma, which overlaps in age with the 667+/-5 Ma reworked tuff 70 m above and indicates that the cap-carbonate sequence was likely deposited rapidly at ~666 Ma. The cap-carbonate sequence overlaps in age with the 659 +/- 6 Ma glaciogenic interval of the Wilyerpa Fm of southern Australia (Fanning and Link, 2008), the 657 +/- 5 Ma post-glacial black shale of the Aralka Fm., central Australia (Kendall et al., 2006), and the 663 +/-4 Ma cap carbonate in the Datangpo Fm., south China (Zhou et al., 2004).

Deposition of the SMM spanned <15 m.y., and leaves in question whether unconformities are present. TIMS and further SHRIMP analyses are underway to better evaluate the tempo of SMM deposition and how it relates to rift basin development, glaciation, and cap carbonate deposition.