Abstract

The upper Tonian Chuar, Uinta Mountain, and middle Pahrump (ChUMP) groups of present-day western Laurentia collectively record the early breakup of Rodinia, large-scale perturbations in the carbon cycle, and eukaryotic evolution, all of which preceded the onset of global glaciation by tens of millions of years. The spectacularly preserved and shale-rich Chuar Group of the Grand Canyon Supergroup stands out as one of the best global records of this time period, particularly for paleobiology. A new U-Pb age of 782 Ma on detrital zircons (n = 14 young grains) from the underlying Nankoweap Formation refines the Chuar Group’s maximum depositional age to younger than 782 Ma. A new 40Ar/39Ar age of 764 ± 16 Ma (2σ) from K-feldspar within early diagenetic marcasite nodules from the upper Chuar Group (Awatubi Member) helps calibrate the rich Chuar microfossil record and constrain the large-magnitude shift in δ13Corg (up to 18‰; referred to here as the Awatubi positive carbon-isotope excursion or APCIE) to between ca. 764 and ca. 742 Ma, the date of an ash near the top of the Chuar Group.

In addition to the maximum depositional age of ca. 782 Ma, U-Pb detrital zircon analyses (n = 826 grains) on sandstone beds from the underlying Nankoweap Formation indicate the presence of multiple older Laurentian age peaks. The similarity of detrital zircon populations and sedimentary character to that of the overlying Chuar Group (n = 764 grains) suggests that the Nankoweap Formation should be included as the lowermost unit in the Chuar Group. This revised geochronological framework indicates a 300 Ma unconformity between the Chuar Group (including the Nankoweap Formation) and the underlying 1.1 Ga Cardenas Basalt of the Unkar Group.

Chuar Group detrital zircon populations share similarities with those of the Uinta Mountain Group and especially the middle Pahrump Group, including ca. 780 Ma grains. Biostratigraphic correlation using microfossils enhances the ChUMP connection and shows a trend of higher acritarch diversity in the lower Chuar and Uinta Mountain groups, and the presence of vase-shaped microfossils in the upper intervals of all three ChUMP units. Comparisons of δ13Corg and δ13Ccarb among ChUMP successions suggest a combination of local and regional controls. Thus, ChUMP successions are coeval within the 780–740 Ma range, show similar fossil and C-isotope trends, and derived sediments from similar Laurentian sources or source types.

In light of recent age constraints and compiled paleontology in other Neoproterozoic basins, our high-resolution correlation of ChUMP successions can be extended to the Callison Lake dolostone of NW Canada and the Akademikerbreen-Polarisbreen groups of Svalbard.

Biostratigraphic correlation with poorly age-constrained strata such as the Akademikerbreen-Polarisbreen groups and, farther afield, the Visingsö Group of Baltica suggests that ChUMP units record continentwide—and perhaps global—evolutionary patterns. The δ13Corg and δ13Ccarb values in the Chuar Group and its equivalents in Canada and Svalbard show broadly similar trends, including the APCIE, suggesting that δ13Corg values from organic-rich shale record variations in the C-isotope composition of late Tonian oceans. Intracratonic basins and contiguous rift margins of ChUMP age are inferred to have been important locations for microbial productivity and significant organic carbon burial that induced large positive shifts in δ13C and changes in global carbon balance prior to the onset of snowball Earth.

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