Carol M. Dehler

Chuar Group of the Grand Canyon: record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma

The Chuar Group (approximately 1600 m thick) preserves a record of extensional tectonism, ocean-chemistry fluctuations, and biological diversification during the late Neoproterozoic Era. An ash layer from the top of the section has a U-Pb zircon age of 742 +/- 6 Ma. The Chuar Group was deposited at low latitudes during extension on the north-trending Butte fault system and is inferred to record rifting during the breakup of Rodinia. Shallow-marine deposition is documented by tide- and wave-generated sedimentary structures, facies associations, and fossils. C isotopes in organic carbon show large stratigraphic variations, apparently recording incipient stages of the marked C isotopic fluctuations that characterize later Neoproterozoic time. Upper Chuar rocks preserve a rich biota that includes not only cyanobacteria and algae, but also heterotrophic protists that document increased food web complexity in Neoproterozoic ecosystems. The Chuar Group thus provides a well-dated, high-resolution record of early events in the sequence of linked tectonic, biogeochemical, environmental, and biological changes that collectively ushered in the Phanerozoic Eon.

Proterozoic multistage (ca. 1.1 and 0.8 Ga) extension recorded in the Grand Canyon Supergroup and establishment of northwest- and north-trending tectonic grains in the southwestern United States

The Grand Canyon Supergroup records at least two distinct periods of intracratonic extension and sedimentation in the late Mesoproterozoic and Neoproterozoic. New 40 Ar/ 39 Ar age determinations indicate that the Mesoproterozoic Unkar Group was deposited between ca. 1.2 and 1.1 Ga. Basins in which the Unkar Group was deposited and the related northwest-striking faults were created by northeast-southwest extension, which was contemporaneous with regional northwest-southeast “Grenville” contraction. New U-Pb data indicate that the Neoproterozoic Chuar Group was deposited between 800 and 742 Ma. Sedimentary and tectonic studies show that Chuar deposition took place during east-west extension and resulting normal slip across the Butte fault. This event is interpreted to be an intracratonic response to the breakup of Rodinia and initiation of the Cordilleran rift margin. Laramide monoclines of the Grand Canyon region have north and northwest trends, reactivate faults that originated at the time of Unkar and Chuar deposition, and can be traced for great distances (hundreds of kilometers) from the Grand Canyon. We use the distribution of monoclines in the Southwest to infer the extent of Proterozoic extensional fault systems. The 1.1 Ga northwest-trending structures and ca. 800–700 Ma north-trending extensional structures created regional fault networks that were tectonically inverted during formation of the Ancestral Rocky Mountains and Laramide contraction and reactivated during Tertiary extension.

Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia

U-Pb detrital zircon analyses provide a new maximum depositional age constraint on the Uinta Mountain Group (UMG) and correlative Big Cottonwood Formation (BCF) of Utah, and significantly enhance our insights on the mid-Neoproterozoic paleogeographic and tectonic setting of western Laurentia. A sandstone interval of the Outlaw Trail formation with a youngest population (n = 4) of detrital zircons, from a sampling of 128 detrital zircon grains, yields a concordia age of 766 ± 5 Ma. This defines a maximum age for deposition of the lower-middle Uinta Mountain Group in the eastern Uinta Mountains and indicates that the group is no older than middle Neoproterozoic in age (i.e., Cryogenian). These data support a long-proposed correlation with the Chuar Group of Grand Canyon (youngest age 742 Ma ± 6 Ma), which, like the Uinta Mountain Group and Big Cottonwood Formation, records nonmagmatic intracratonic extension. This suggests a ∼742 to ≤766 Ma extensional phase in Utah and Arizona that preceded the regional rift episode (∼670–720 Ma), which led to development of the Cordilleran passive margin. This is likely an intracratonic response to an early rift phase of Rodinia. Further, because the Chuar Group and the Uinta Mountain Group–Big Cottonwood Formation strata record intracratonic marine deposition, this correlation suggests a regional ∼740–770 Ma transgression onto western Laurentia.

High-resolution δ13C stratigraphy of the Chuar Group (ca. 770–742 Ma), Grand Canyon: Implications for mid-Neoproterozoic climate change

A high-resolution C-isotope record based on δ 13 C org from organic-rich shales and δ 13 C carb from dolomites in the ca. 770–742 Ma Chuar Group provides important new data for evaluating the signifi cance of large-magnitude C-isotope anomalies in Neoproterozoic climate change. Three successive, large-magnitude isotopic excursions (8–15‰) are interpreted to represent primary seawater values based on a series of diagenetic tests, and they are not associated with evidence of signifi cant long-term (10 6 –10 7 m.y.) sea-level change nor glaciomarine deposits. Intrabasinal correlation of δ 13 C org values suggests that most Chuar shales record primary values and is consistent with previously reported H/C ratios of >0.49 indicating that Chuar shales experienced minimal thermal alteration. Although some Chuar dolomites reveal early diagenetic alteration, their δ 13 C dol values typically fall near those of coeval “least-altered” dolomites or organic-rich shales (relative to dolomite values). The Chuar carbon record is interpreted to refl ect predominantly primary organic carbon δ 13 C values and contains suf

Synthesis of the 780–740 Ma Chuar, Uinta Mountain, and Pahrump (ChUMP) groups, western USA: Implications for Laurentia-wide cratonic marine basins

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 40 Ar/ 39 Ar 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 δ 13 C org (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 δ 13 C org and δ 13 C carb 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 Visingso Group of Baltica suggests that ChUMP units record continentwide—and perhaps global—evolutionary patterns. The δ 13 C org and δ 13 C carb values in the Chuar Group and its equivalents in Canada and Svalbard show broadly similar trends, including the APCIE, suggesting that δ 13 C org 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 δ 13 C and changes in global carbon balance prior to the onset of snowball Earth.

Coupled Re-Os and U-Pb geochronology of the Tonian Chuar Group, Grand Canyon

Well-preserved strata of the late Tonian Chuar Group exposed in the Grand Canyon host fossil evidence for the development of eukaryotic predation, the presence of unique biomarkers, and large changes in C, S, and Mo isotope chemostratigraphy. Despite the importance of this critical succession, few radioisotopic age constraints are available to place these records into a global context. Here, we couple high-precision U-Pb chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) on zircon crystals with the rhenium-osmium (Re-Os) sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of Earth history. Zircons recovered from a tuff within the uppermost Walcott Member of the Kwagunt Formation yield a weighted mean 206Pb-238U age of 729.0 ± 0.9 Ma (mean square of weighted deviates [MSWD] = 0.86), differing significantly from the previous air-abrasion upper-intercept age of 742 ± 6 Ma on zircons from this same horizon. Organic-rich carbonates from the Carbon Canyon Member of the Galeros Formation yield a model 1 Re-Os age of 757.0 ± 6.8 Ma (MSWD = 0.47, n = 8), and an initial Os isotope (187Os/188Os [Osi]) composition of 1.13 ± 0.02. The radiogenic Osi value from this horizon suggests that the basin was restricted from the open ocean during deposition of the Carbon Canyon Member, in agreement with sedimentological and stratigraphic data. The Re-Os geochronology of marcasite (FeS2) nodules from the Awatubi Member of the Kwagunt Formation yield a model 1 age of 751.0 ± 7.6 Ma (MSWD = 0.37, n = 5), with an Osi of 0.44 ± 0.01. This Re-Os date is interpreted to constrain the growth of the marcasite nodules in the Awatubi Member during deposition. The formation of sulfides and the less radiogenic Osi value are consistent with an influx of sulfate-laden seawater to the basin during deposition of the Kwagunt Formation. Attempts to apply the Re-Os geochronometer to the Walcott and Tanner Members of the Chuar Group failed to yield meaningful ages, despite elevated Re enrichments (>20 ng/g). The Re-Os data from these units yielded negative Osi values, which suggest disturbance to the Re-Os system. The low Os abundances (typically <100 pg/g) relative to the amount expected based on the elevated Re abundances suggest leaching of Os due to oxidative weathering on geologically recent time scales. Finally, the Carbon Canyon Member provides a useful case study for quantifying how input uncertainties in the Re-Os geochronometer propagate into the resulting age uncertainty, and we discuss the protocols that will yield the best improvement in age precision for future studies. The U-Pb and Re-Os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods. Our analysis suggests that for our data, the most efficient way of reducing uncertainties in the presented Re-Os dates is through improved precision of measured Os values.