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We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The formation consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meter-scale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.
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The Road River Group of northern Yukon, Canada: early Paleozoic deep-water sedimentation within the Great American Carbonate Bank
Cambrian–Devonian sedimentary rocks of the northern Canadian Cordillera record both the establishment and demise of the Great American Carbonate Bank, a widespread carbonate platform system that fringed the ancestral continental margins of North America (Laurentia). Here, we present a new examination of the deep-water Road River Group of the Richardson Mountains, Yukon, Canada, which was deposited in an intra-platformal embayment or seaway within the Great American Carbonate Bank called the Richardson trough. Eleven detailed stratigraphic sections through the Road River Group along the upper canyon of the Peel River are compiled and integrated with geological mapping, facies analysis, carbonate and organic carbon isotope chemostratigraphy, and new biostratigraphic results to formalize four new formations within the type area of the Richardson Mountains (Cronin, Mount Hare, Tetlit, and Vittrekwa). We recognize nine mixed carbonate and siliciclastic deep-water facies associations in the Road River Group and propose these strata were deposited in basin-floor to slope environments. New biostratigraphic data suggest the Road River Group spans the late Cambrian (Furongian) – Middle Devonian (Eifelian), and new chemostratigraphic data record multiple global carbon isotopic events, including the late Cambrian Steptoean positive carbon isotope excursion, the Late Ordovician Guttenberg excursion, the Silurian Aeronian, Valgu, Mulde (mid-Homerian), Ireviken (early Sheinwoodian), and Lau excursions, and the Early Devonian Klonk excursion. Together, these new data not only help clarify nomenclatural debate centered around the Road River Group, but also provide critical new sedimentological, biostratigraphic, and isotopic data for these widely distributed rocks of the northern Canadian Cordillera.
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- PAR ID:
- 10174477
- Date Published:
- Journal Name:
- Canadian Journal of Earth Sciences
- ISSN:
- 0008-4077
- Page Range / eLocation ID:
- 1 to 27
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Neoproterozoic–Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian–lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ13Ccarb values as low as –13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin.more » « less
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null (Ed.)The extent to which Paleozoic oceans differed from Neoproterozoic oceans and the causal relationship between biological evolution and changing environmental conditions are heavily debated. Here, we report a nearly continuous record of seafloor redox change from the deep-water upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. Bottom waters were largely anoxic in the Richardson trough during the entirety of Road River Group deposition, while independent evidence from iron speciation and Mo/U ratios show that the biogeochemical nature of anoxia changed through time. Both in Yukon and globally, Ordovician through Early Devonian anoxic waters were broadly ferruginous (nonsulfidic), with a transition toward more euxinic (sulfidic) conditions in the mid–Early Devonian (Pragian), coincident with the early diversification of vascular plants and disappearance of graptolites. This ~80-million-year interval of the Paleozoic characterized by widespread ferruginous bottom waters represents a persistence of Neoproterozoic-like marine redox conditions well into the Phanerozoic.more » « less
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null (Ed.)The age and nature of the Neoproterozoic – early Paleozoic rift–drift transition has been interpreted differently along the length of the North American Cordillera. The Ediacaran “upper” group (herein elevated to the Rackla Group) of the Coal Creek inlier, Yukon, Canada, represents a key succession to reconstruct the sedimentation history of northwestern Laurentia across the Precambrian–Cambrian boundary and elucidate the timing of active tectonism during the protracted breakup of the supercontinent Rodinia. These previously undifferentiated late Neoproterozoic – early Paleozoic map units in the Coal Creek inlier are herein formally defined as the Lone, Cliff Creek, Mount Ina, Last Chance, Shade, and Shell Creek formations. New sedimentological and stratigraphic data from these units is used to reconstruct the depositional setting. In the Last Chance Formation, chemostratigraphic observations indicate a ca. 5‰ δ 13 C carb gradient coincident with the globally recognized ca. 574–567 Ma Shuram carbon isotope excursion. Map and stratigraphic relationships in the overlying Shell Creek Formation provide evidence for latest Ediacaran – middle Cambrian tilting and rift-related sedimentation. This provides evidence for active extension through the Cambrian Miaolingian Series in northwestern Canada, supporting arguments for a multiphase and protracted breakup of Rodinia.more » « less
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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.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1139/cjes-2020-0017
