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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 [...]

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]

The rise of animals occurred during an interval of Earth history that witnessed dynamic marine redox conditions, potentially rapid plate motions, and uniquely large perturbations to global biogeochemical cycles. The largest of these perturbations, the Shuram carbon isotope excursion, has been invoked as a driving mechanism for Ediacaran environmental change, possibly linked with evolutionary innovation or extinction. However, there are a number of controversies surrounding the Shuram, including its timing, duration, and role in the concomitant biological and biogeochemical upheavals. Here we present radioisotopic dates bracketing the Shuram on two separate paleocontinents; our results are consistent with a global and synchronous event between 574.0 ± 4.7 and 567.3 ± 3.0 Ma. These dates support the interpretation that the Shuram is a primary and synchronous event postdating the Gaskiers glaciation. In addition, our Re-Os ages suggest that the appearance of Ediacaran macrofossils in northwestern Canada is identical, within uncertainty, to similar macrofossils from the Conception Group of Newfoundland, highlighting the coeval appearance of macroscopic metazoans across two paleocontinents. Our temporal framework for the terminal Proterozoic is a critical step for testing hypotheses related to extreme carbon isotope excursions and their role in the evolution of complex life. [ABSTRACT FROM AUTHOR]