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Tectonic models for development of the Svalbard Caledonides depend on reliable assessment of the metamorphic evolution of the various basement provinces involved. The Mosselhalvoya Group (MG) and the Atomfjella Complex (AC) have previously been assigned to the Nordaustlandet and West Ny-Friesland terranes, respectively. New analytical data and petrographic observations indicate that both units experienced two-stage metamorphism under similar pressure-temperature (P-T) conditions. Two stages of amphibolite facies metamorphism (Ml and M2) are clearly recorded by garnet and staurolite porphyroblast textures. The results of thermodynamic phase equilibrium modeling indicate that peak M2 metamorphism occurred at ~ 7-7.5 kbar and 590-600 [degrees]C in both units. Zirconium-in-rutile trace element thermometry confirms the temperature estimates for Ml and M2 stages of metamorphism. Monazite chemical Th-U-Pb dates from the MG resolve a two-stage garnet growth at 444 [+ or -] 7 Ma (Ml) and 423 [+ or -] 6 Ma (M2). In contrast, monazite dated in the AC defines a single age of 420 [+ or -] 4 Ma interpreted as M2 growth. We suggest M2 was coeval with early strike-slip motion along the Billefjorden Fault Zone, whereas Ml reflects initial tectonic burial of the studied units. The similarity in metamorphic history between the both units suggests that the boundary between them is a subordinate thrust fault within the Atomfjella thrust stack rather than a major boundary separating the Nordaustlandet and West Ny-Friesland terranes. The MG should be included within the West Ny-Friesland terrane and the tectonic boundary with the Nordaustlandet terrane is likely the Eolussletta Shear Zone. Key words: Mosselhalvoya Group, Atomfjella Complex, West Ny-Friesland terrane, Nordaustlandet terrane, thermodynamic phase equilibrium modelling, monazite, 1. Introduction The Svalbard Caledonides are traditionally divided into three main basement blocks, terranes, or provinces with differing structural history, sedimentary record, and tectono-thermal evolution (Harland 1997). The Northeastern, Northwestern, [...]

Theageand natureof 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[per thousand] [[delta].sup.13][C.sub.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. Key words: Rodinia, Yukon, Shuram carbon isotope excursion, Rackla Group, Coal Creek inlier. Les interpretations different quant a l'age et la nature de la transition de rifting a derive, du Neoproterozoique au Paleozoique precoce, le long dela cordillere nord-americaine. Le groupe d'Ediacara [much less than] superieur [much greater than] (eleve dans le present article au Groupe de Rackla) de l'enclave de Coal Creek (Yukon, Canada) represente une succession cle pour reconstituer l'histoire sedimentaire du nord-ouest de la Laurentie de part et d'autredela limite Precambrien-Cambrien et elucider la chronologie de l'activite tectonique durant la longue dislocation du supercontinent Rodinia. Ces unites cartographiques d'age neoproterozoique tardif a paleozoique precoce dans l'enclave de Coal Creek, qui n'etaient pas differenciees aupara-vant, sont formellement definies comme etant les Formations de Lone, de Cliff Creek, de Mount Ina, de Last Chance, de Shade et de Shell Creek. De nouvelles donnees sedimentologiques et stratigraphiques sur ces unites sont utilisees pour reconstituer le contexte sedimentaire. Dans la Formation de Last Chance, des observations chimiostratigraphiques indi-quent un gradient du [[delta].sup.13][C.sub.carb] d'environ 5 [per thousand] coincidant avec l'excursion des isotopes de carbone de Shuram vers 574-567 Ma, reconnue a l'echelle planetaire. Les relations cartographiques et stratigraphiques dans la Formation de Shell Creek sus-jacente fournissent des preuves d'un basculement et d'une sedimentation reliee au rifting de la fin de l'Ediacara au Cambrien moyen. Cela indiquerait une extension active pendant la serie miaolingienne du Cambrien dans le nord-ouest du Canada, ce qui argue en faveur d'une dislocation de Rodinia en plusieurs phases et de longue duree. [Traduit par la Redaction] Mots-cles: Rodinia, Yukon, excursion des isotopes de carbone de Shuram, Groupe de Rackla, enclave de Coal Creek., Introduction Neoproterozoic and lower Paleozoic strata of the western United States and Canada generally record a transition from continental rifting to passive margin sedimentation during the breakup of the supercontinent [...]

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. Key words: Road River Group, Yukon, stratigraphy, chemostratigraphy, biostratigraphy. Des roches sedimentaires du Cambrien au Devonien du nord de la Cordillere canadienne temoignent de l'etablissement et de la fin du grand banc de carbonates americain, un vaste systeme de plateformes carbonatees qui bordait les marges continentales ancestrales de l'Amerique du Nord (Laurentie). Nous presentons les resultats d'un nouvel examen du Groupe de Road River, une unite d'eau profonde, des monts Richardson (Yukon, Canada), qui a ete depose dans un enfoncement ou un bras de mer intra-plateforme au sein du grand banc de carbonates americain appele la fosse de Richardson. Onze coupes stratigraphiques detaillees a travers le Groupe de Road River le long du canyon superieur de la riviere Peel sont compilees et integrees a la cartographie geologique, l'analyse des facies, la chimiostratigraphie des carbonates et des isotopes de carbone organique et de nouveaux resultats de biostratigraphie, permettant de definir quatre nouvelles formations dans la region type des monts Richardson (Cronin, Mount Hare, Tetlit et Vittrekwa). Nous relevons neuf associations de facies mixtes carbonates et silicoclastiques d'eau profonde dans le groupe de Road River et proposons que ces strates ont ete deposees dans des milieux allant de fonds de bassin a des talus. De nouvelles donnees biostratigraphiques donnent a penser que le Groupe de Road River s'etalerait du Cambrien tardif (Furongien) au Devonien moyen (Eifelien), et de nouvelles donnees chimiostratigraphiques font ressortir plusieurs episodes d'envergure planetaire du registre des isotopes du carbone, dont l'excursion isotopique positive steptoeenne au Cambrien tardif, l'excursion de Guttenberg a l'Ordovicien tardif, les excursions aeronien, de Valgu, de Mulde (Homerien intermediaire), d'Ireviken (Sheinwoodien precoce) et de Lau au Silurien et l'excursion de Klonk au Devonien precoce. Collectivement, ces nouvelles donnees aident non seulement a eclairer des debats relatifs a la nomenclature centres sur le Groupe de Road River, mais fournissent aussi de nouvelles donnees sedimentologiques, bio stratigraphiques et isotopiques cles sur ces roches largement repandues du nord de la Cordillere canadienne. [Traduit par la Redaction] Mots-cles: Groupe de Road River, Yukon, stratigraphie, chimiostratigraphie, biostratigraphie., Introduction The early Paleozoic witnessed major evolutionary radiations and biotic crises that dramatically altered animal ecosystems (Bambach et al. 2002; Alroy et al. 2008; Servais et al. 2008; Erwin et [...]

After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post-Snowball Earth hothouse.

Ediacara‐type macrofossils appear as early as ~575 Ma in deep‐water facies of the Drook Formation of the Avalon Peninsula, Newfoundland, and the Nadaleen Formation of Yukon and Northwest Territories, Canada. Our ability to assess whether a deep‐water origination of the Ediacara biota is a genuine reflection of evolutionary succession, an artifact of an incomplete stratigraphic record, or a bathymetrically controlled biotope is limited by a lack of geochronological constraints and detailed shelf‐to‐slope transects of Ediacaran continental margins. The Ediacaran Rackla Group of the Wernecke Mountains, NW Canada, represents an ideal shelf‐to‐slope depositional system to understand the spatiotemporal and environmental context of Ediacara‐type organisms' stratigraphic occurrence. New sedimentological and paleontological data presented herein from the Wernecke Mountains establish a stratigraphic framework relating shelfal strata in the Goz/Corn Creek area to lower slope deposits in the Nadaleen River area. We report new discoveries of numerous Aspidella hold‐fast discs, indicative of frondose Ediacara organisms, from deep‐water slope deposits of the Nadaleen Formation stratigraphically below the Shuram carbon isotope excursion (CIE) in the Nadaleen River area. Such fossils are notably absent in coeval shallow‐water strata in the Goz/Corn Creek region despite appropriate facies for potential preservation. The presence of pre‐Shuram CIE Ediacara‐type fossils occurring only in deep‐water facies within a basin that has equivalent well‐preserved shallow‐water facies provides the first stratigraphic paleobiological support for a deep‐water origination of the Ediacara biota. In contrast, new occurrences of Ediacara‐type fossils (including juvenile fronds, Beltanelliformis, Aspidella, annulated tubes, and multiple ichnotaxa) are found above the Shuram CIE in both deep‐ and shallow‐water deposits of the Blueflower Formation. Given existing age constraints on the Shuram CIE, it appears that Ediacaran organisms may have originated in the deeper ocean and lived there for up to ~15 million years before migrating into shelfal environments in the terminal Ediacaran. This indicates unique ecophysiological constraints likely shaped the initial habitat preference and later environmental expansion of the Ediacara biota. [ABSTRACT FROM AUTHOR]

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