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.

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