Early Neoproterozoic Basin Formation in Yukon, Canada: Implications for the make-up and break-up of Rodinia

SUMMARY: Geological mapping and stratigraphic anaylsis of the early Neoproterozoic Fifteenmile Group in the western Ogilvie Mountains of Yukon, Canada, has revealed large lateral facies changes in both carbonate and siliciclastic strata.  Syn-sedimentary NNW-side-down normal faulting during deposition of the lower Fifteenmile Group generated local topographic relief and wedge-shaped stratal geometries. These strata were eventually capped by platformal carbonate after the establishment of a NNW-facing stromatolitic reef complex that formed adjacent to the coeval Little Dal Group of the Mackenzie Mountains, Northwest Territories.  Correlations between specific formations within these groups are tested with carbon isotope chemostratigraphy.  As there are no known 830-780 Ma stratigraphic successions south of 62°N, the basin-forming event that accommodated the Fifteenmile and Little Dal Groups of the Ogilvie and Mackenzie Mountains and equivalent strata in the Shaler Supergroup of Victoria Island was restricted to the northwest margin of Laurentia. Therefore, this event does not represent widespread rifting of the entire western margin of Laurentia and instead we propose that these strata were accommodated in a failed rift generated by localized subsidence associated with the emplacement of the coeval Guibei (China) and Gairdner (Australia) large igneous provinces.  The northern margin of Laurentia was reactivated by renewed extension at ca. 720 Ma associated with the emplacement of the Franklin large igneous province.  Significant crustal thinning and generation of a thermally subsiding passive margin on the western margin of Laurentia may not have occurred until the late Ediacaran.RÉSUMÉLe cartographiage géologique et l’analyse stratigraphique du groupe néoprotézoïque Fifteenmile situé à l’ouest des montagnes Ogilvie du Yukon, Canada, ont révélé de grands changements latéraux de faciès à la fois pour les strates carbonatées et silicoclastiques. La mise en place des failles normales syn-sédimentaires inclinées vers le NNW au cours du dépôt du groupe Fifteenmile inférieur, a entrainé la formation locale d’un relief topographique et une prisme des strates. Ces dernières ont finalement été recouvertes de carbonates de plate-forme issus de la mise en place d’un complexe récifal stromatolitique exposé NNW contigu à la formation de même âge du groupe Little Dal des montagnes Ogilvie, en Territoires du Nord-Ouest. Les corrélations existant entre des formations spécifiques de chacun de ces groupes, sont testées grâce à la chimiostratigraphie des isotopes du carbone. Aucunes successions stratigraphiques agées de 830-780 Ma n’étant connues au sud de 62° N, la formation du bassin où sont accumulés les groupes Fifteenmile et Little Dal des massifs Ogilvie et Mackenzie, ainsi que les strates analogues du supergroupe Shaler de l’île Victoria, était restreinte à la bordure nord-ouest de la Laurentie. De ce fait, cet événement ne correspond pas au large rifting s’étendant sur l’entière bordure ouest de la Laurentie et nous proposons à la place, que ces strates ont été localisées au cours d’un rift avorté généré par la mise en place simultanée des larges provinces ignées Guibei (Chine) et Gairdner (Australie). La bordure nord de la Laurentie a été réactivée par une nouvelle phase d’extension à ca. 720 Ma associée à l’emplacement de la province ignée Franklin. L’amincissement crustal et la formation d’une marge passive thermiquement subsidente le long de la bordure ouest de la Laurentie ne se sont certainement pas produits avant l’Édiacarien supérieur

Proterozoic prism arrests suspect terranes: Insights into the ancient Cordilleran margin from seismic reflection data: Comment

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New Occurrences of \u3cem\u3eOldhamia\u3c/em\u3e in Eastern Yukon, Canada: Stratigraphic Context and Implications for Cambrian Deep-Marine Biostratigraphy

For over a century, the ichnogenus Oldhamia has been used as an index fossil for Cambrian deep-marine siliciclastic successions. New specimens from the Cambrian of Selwyn Basin, eastern Yukon, Canada, provide an opportunity to constrain the chronostratigraphic range of Oldhamia and test its potential for more detailed biozonation. The upper part of Narchilla Formation (Arrowhead Lake Member) yields O. antiqua, O. curvata, O. flabellata, and O. radiata. The overlying Gull Lake Formation yields a less diverse assemblage of O. antiqua, O. curvata, and O. flabellata. The upward disappearance of O. radiata is consistent with models that infer increasing complexity of Oldhamia ichnospecies through time but may also be a function of limited sampling. Regional stratigraphic correlations suggest Oldhamia appeared in Arrowhead Lake Member during Cambrian Stage 3. Archaeocyathans and trilobites from Gull Lake Formation confirm that Oldhamia was present during Cambrian Stages 3 to 4. A younger age limit on Oldhamia in Selwyn Basin is provided by syn-volcanic detrital zircons recovered from the base of the overlying Old Cabin Formation, which yield an age of 499.89 ± 0.14 Ma (earliest Guzhangian). Oldhamia appears to be a reliable index fossil for Cambrian Series 2 and Series 3 in Selwyn Basin

Early Neoproterozoic Basin Formation in Yukon, Canada: Implications for the Make-Up and Break-Up of Rodinia

Geological mapping and stratigraphic anaylsis of the early Neoproterozoic Fifteenmile Group in the western Ogilvie Mountains of Yukon, Canada, has revealed large lateral facies changes in both carbonate and siliciclastic strata. Syn-sedimentary NNW-side-down normal faulting during deposition of the lower Fifteenmile Group generated local topographic relief and wedge-shaped stratal geometries. These strata were eventually capped by platformal carbonate after the establishment of a NNW-facing stromatolitic reef complex that formed adjacent to the coeval Little Dal Group of the Mackenzie Mountains, Northwest Territories. Correlations between specific formations within these groups are tested with carbon isotope chemostratigraphy. As there are no known 830-780 Ma stratigraphic successions south of 62°N, the basin-forming event that accommodated the Fifteenmile and Little Dal Groups of the Ogilvie and Mackenzie Mountains and equivalent strata in the Shaler Supergroup of Victoria Island was restricted to the northwest margin of Laurentia. Therefore, this event does not represent widespread rifting of the entire western margin of Laurentia and instead we propose that these strata were accommodated in a failed rift generated by localized subsidence associated with the emplacement of the coeval Guibei (China) and Gairdner (Australia) large igneous provinces. The northern margin of Laurentia was reactivated by renewed extension at ca. 720 Ma associated with the emplacement of the Franklin large igneous province. Significant crustal thinning and generation of a thermally subsiding passive margin on the western margin of Laurentia may not have occurred until the late Ediacaran.Earth and Planetary Science

New Occurrences of <i>Oldhamia</i> in Eastern Yukon, Canada: Stratigraphic Context and Implications for Cambrian Deep-Marine Biostratigraphy

<p>For over a century, the ichnogenus <i>Oldhamia</i> has been used as an index fossil for Cambrian deep-marine siliciclastic successions. New specimens from the Cambrian of Selwyn Basin, eastern Yukon, Canada, provide an opportunity to constrain the chronostratigraphic range of <i>Oldhamia</i> and test its potential for more detailed biozonation. The upper part of Narchilla Formation (Arrowhead Lake Member) yields <i>O. antiqua, O. curvata, O. flabellata</i>, and <i>O. radiata</i>. The overlying Gull Lake Formation yields a less diverse assemblage of <i>O. antiqua, O. curvata</i>, and <i>O. flabellata</i>. The upward disappearance of <i>O. radiata</i> is consistent with models that infer increasing complexity of <i>Oldhamia</i> ichnospecies through time but may also be a function of limited sampling. Regional stratigraphic correlations suggest <i>Oldhamia</i> appeared in Arrowhead Lake Member during Cambrian Stage 3. Archaeocyathans and trilobites from Gull Lake Formation confirm that <i>Oldhamia</i> was present during Cambrian Stages 3 to 4. A younger age limit on <i>Oldhamia</i> in Selwyn Basin is provided by syn-volcanic detrital zircons recovered from the base of the overlying Old Cabin Formation, which yield an age of 499.89 ± 0.14 Ma (earliest Guzhangian). <i>Oldhamia</i> appears to be a reliable index fossil for Cambrian Series 2 and Series 3 in Selwyn Basin.</p

Calibrating the Cryogenian

The Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.Earth and Planetary SciencesEngineering and Applied Science

The evolution of the image of China in the United States during the cold war

This article describes the evolution of the image of China in the United States (US) during the Cold War. Aware that China-US bilateral relations have been influenced by periods of conflict and animosity, harmony and détente, the article argues that the image of China has always been embedded in a network of events - birth of the PRC (People's Republic of China [PRC], Korean War, Sino-Soviet alliance) that prevented American public opinion from developing an objective and unbiased picture of the PRC. Since 1950s, China has always been linked to the idea of the 'Yellow Peril'. Moreover, the lack of a direct contact between China and America further thwarted the opportunity of shaping a fair picture of the PRC. Although recognising that during the Cold War American political parties played a significant role in conveying a negative image of China, this article shows how media helped in strengthening Chinese stereotypes among the American public. Analysing all articles published by Time, National Geographic and Readers' Digest from 1949 to 1972, the article highlights both similarities and differences of the way in which these magazines introduced China to their readers. © Sage Publication 2009.link_to_subscribed_fulltex