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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. 

The upper Tonian ChUMP (Chuar-Uinta Mountains-Pahrump) strata of the southwestern U.S.A. are hypothesized to be regional correlatives and to record a time of rift basin evolution commencing at ca. 770 Ma in western Laurentia (modern-day coordinates). We test this correlation using U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on detrital zircon grains from basal units within these successions. ChUMP units yield CA-ID-TIMS maximum depositional ages (MDA) between 775 and 766 Ma: the Chuar Group of AZ has an MDA of 770.1 ± 0.5 Ma (n = 1) and an additional young zircon mode at 775.7 ± 0.3 Ma (n = 11); the Uinta Mountain Group of northern UT has an MDA of 766.3 ± 0.5 Ma (n = 5) and contains a second young mode at 775.1 ± 0.7 Ma (n = 3); and the basal Horse Thief Springs Formation of the middle Pahrump Group CA has an MDA of 775.4 ± 0.7 Ma (n = 3). The ca. 775 and 770 Ma grains are interpreted to be from zircon-bearing mafic sources related to the 770–778 Ma Gunbarrel Large Igneous Province of Yukon and NW U.S.A. The 766 Ma population was either derived from the Mt Rogers complex of eastern Laurentia or could have come from conjugate margins that were in the process of rifting away, such as Tasmania. The CA-ID-TIMS dates on the Chuar Group in Grand Canyon anchor a Bayesian age model for evaluating late Tonian Earth systems. Faster sediment accumulation rates (80 + 150/-44 m/My) in the lower Chuar Group are consistent with the inception of an extensional basin related to Rodinia breakup; slower rates in the upper Chuar Group (25 + 12/-5 m/My) record are associated with relatively deeper water sedimentation and concomitant organic carbon burial during marine transgression. The model also constrains the timing of several biological events recorded in the Chuar Group, including eukaryovorous predation (>767 Ma), the first appearance of vase-shaped microfossils (∼741 Ma), and the ranges of Cerebrosphaera globosa (=C. buickii; 800–743 Ma) and Lanulatisphaera laufeldii. (766–740 Ma), both proposed as possible marine index fossils for late Tonian time. Finally, the model can also be used to search for stratigraphic evidence of a purported glaciation at ca. 751 Ma. 

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.