Ordovician-Silurian Stratigraphic Framework, Macdonald Platform to Ospika Embayment Transect, Northeastern British Columbia

ABSTRACT Strata across a northeast-southwest platform-to-basin transect, northeastern British Columbia, record the tectonic evolution of the rifted margin of Laurentia from the Early Ordovician to Early Silurian. The transect spans over 70 km and contains facies transitions of the Macdonald Platform to the shelf-break and basinal facies of the Ospika Embayment, the southern extension of the Kechika Trough. Only the general stratigraphic framework is understood from previous regional mapping. This study provides detailed stratigraphic descriptions of platformal units that include the uppermost Kechika, Skoki, Beaverfoot and Nonda formations. A new platformal unit, the McCusker Formation, lies stratigraphically between the Beaverfoot and Nonda formations. Three new formations of the Road River Group, the Robb, Kenny and Laurier formations are transitional facies and off-shelf equivalents of the Beaverfoot, McCusker and Nonda formations. Two new members are recognized within the Robb Formation and formally named the Sidenius and Calnan members. Basinal facies of the Road River Group include the Ospika and Kwadacha formations. The abrupt lateral facies changes and occurrence of volcanics within the Skoki (platform) and Ospika (basinal) formations record renewed phases of extension of the continental margin in Ordovician and Silurian time. Throughout much of northwestern Alberta and northeastern British Columbia, the Ordovician-Silurian succession is missing beneath the sub-Devonian unconformity, and prominent unconformities punctuate the stratigraphic succession preserved in the study area. The detailed stratigraphic and conodont biostratigraphic work provides temporal constraints of platformal and slope units and scales of hiatuses during the early Paleozoic post-initial rift phase for this part of the Cordilleran margin of Laurentia. RESUME Les strates d'un transect nord-est-sud-ouest, de la plate-forme au bassin du nord-est de la Colombie-Britannique, enregistrent l'evolution tectonique de la marge de rift de Laurentia, de l'Ordovicien inferieur au Silurien inferieur. Le transect s'etire sur plus de 70 km et contient des transitions de facies de la plate-forme de Macdonald a la bordure de la plate-forme et les facies de bassin de la baie d'Ospika, l'extension sud de la fosse de Kechika. Seul le cadre stratigraphique a ete compris a partir de la cartographie regionale anterieure. Cette etude fournit des descriptions stratigraphiques de detail des unites de plate-forme qui incluent les formations sommitales de Kechika, de Skoki, de Beaverfoot et de Nonda. Une nouvelle unite de plate-forme, la Formation de McCusker, git stratigraphiquement entre les formations de Beaverfoot et de Nonda. Ces trois nouvelles formations du Groupe de Road River, soit les formations de Robb, de Kenny et de Laurier sont de facies transitionnels et des equivalents d'en dehors de la plate-forme des formations de Beaverfoot, de McCusker et de Nonda. Deux nouveaux membres sont reconnus a l'interieur de la Formation de Robb et sont formellement nommes les membres de Sidenius et de Calnan. Les facies de basin du Groupe de Road River incluent les formations de Ospika et de Kwadacha. L'abrupt changement lateral de facies et l'occurrence de volcanites a l'interieur des formations de Skoki (plate-forme) et de Ospika (bassin), enregistre des phases de renouvellement de l'extension de la marge continentale aux periodes de l'Ordovicien et du Silurien. Dans une grande partie du nord-ouest de l'Alberta et du nord-est de la Colombie-Britannique, la succession de l'Ordovicien-Silurien est manquante sous la discordance sub-devonienne et des discordances proeminentes ponctuent la succession stratigraphique preservee dans la region d'etude. Les travaux stratigraphiques et la biostratigraphie des conodontes de detail fournissent les contraintes temporelles des unites de plate-forme et de pente et les echelles des hiatus durant la phase posterieure au rift initial du debut du Paleozoique pour cette partie de la marge de Laurentia de la Cordillere. Traduit par Lynn Gagnon End_Page 513------------------------

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