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The trilobite faunas that occur with the Steptoean Positive Isotope Carbon Excursion (SPICE) at Smithfield Canyon, Utah, have been reported, but not illustrated. Given the importance of the SPICE at this section for international correlations, the trilobites from new collections from the upper Nounan Dolomite to lower St. Charles Formation at Smithfield Canyon are reported herein and integrated with the previously reported taxa. Trilobite assemblages indicate that the upper Cedaria to the Ellipsocephaloides biozones (Miaolingian Series, Guzhangian Stage to Furongian Series, Jiangshanian Stage) are present stratigraphically below or above the SPICE. Some of the taxa reported herein may represent new species, but they are not represented by well-enough preserved specimens and are left in open nomenclature. However, Kingstonia smithfieldensis n. sp. and Bromella utahensis n. sp. are named on the basis of common and well-preserved specimens. New carbon isotope data from Smithfield Canyon from an overlapping section of the lower St. Charles Formation, that add to the overall shape of the SPICE curve, are presented. The new δ13C values above the Elvinia Biozone range from –0.36‰ to +1.5‰, confirming that the SPICE concludes within the Elvinia Biozone. 

Abstract The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent +4–5‰ shift in the Cambrian δ13C record used for global chronostratigraphic correlation. The onset of this excursion is traditionally placed at the base of the Pterocephaliid trilobite biomere (base of the Furongian Series). Recent studies have documented local controls on the expression of the SPICE and emphasize the need for chronostratigraphic standards for these complex biogeochemical signals. We build upon prior work in western Laurentia by integrating δ13C and biostratigraphy with high-precision isotope dilution U-Pb detrital zircon maximum depositional ages that are coincident with the onset, peak, and falling limb of the SPICE. Our study provides the first useful numerical age constraint for the onset of the SPICE and the Laurentian trilobite biozones and requires revision of the late Cambrian geologic time scale boundaries by several million years. 

Abstract Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower diamictite interval of the Scout Mountain Member yield high-precision chemical abrasion isotope dilution U-Pb zircon eruption and depositional ages of 696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new high-precision detrital zircon maximum depositional age of ≤670 Ma for shoreface and offshore sandstones unconformably overlying the lower diamictite, these data are consistent with correlation of the lower diamictite to the early Cryogenian (ca. 717–660 Ma) Sturtian glaciation. These 670–675 Ma zircons persist in beds above the upper diamictite and cap dolostone units, up to and including a purported “reworked fallout tuff,” which we instead conclude provides only a maximum depositional age of ≤673 Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658 Ma provide a maximum depositional age for the upper diamictite and overlying cap dolostone units. This new geochronological framework supports litho- and chemostratigraphic correlations of the lower and upper diamictite intervals of the Scout Mountain Member of the Pocatello Formation with the Sturtian (716–660 Ma) and Marinoan (≤650–635 Ma) low-latitude glaciations, respectively. The Pocatello Formation thus contains a more complete record of Cryogenian glaciations than previously postulated.