Precise U-Pb Age Models Refine Neoproterozoic Western Laurentian Rift Initiation, Correlation, and Earth System Changes
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.
Dehler, Carol; Schmitz, Mark; Bullard, Abigail; Porter, Susannah; Timmons, Mike; Karlstrom, Karl; and Cothren Hannah. (2023). "Precise U-Pb Age Models Refine Neoproterozoic Western Laurentian Rift Initiation, Correlation, and Earth System Changes". Precambrian Research, 396, 107156. https://doi.org/10.1016/j.precamres.2023.107156