Timing of Cache Creek Ocean closure: insights from new Jurassic radiolarian ages in British Columbia and Yukon and their significance for Canadian Cordillera tectonics

The closure of the Cache Creek Ocean during the Jurassic led to the structuration of the Canadian Cordillera. To reassess the timing of this major tectonic event, this study proposes a biochronological review of all the youngest radiolarian-bearing localities of the Cache Creek Terrane in British Columbia and Yukon using updated radiolarian biozonations and taxonomy, complemented by new data from the type locality. The results show that all ages are Early Jurassic and fairly homogenous along the entire terrane. All the youngest siliceous rocks are restricted to the late Sinemurian - early Toarcian interval (similar to 195-180 Ma). The youngest well-dated pelagic cherts are early Pliensbachian, with slightly younger siliceous argillites of Pliensbachian and possibly early Toarcian age. These results suggest that Cache Creek deep pelagic sedimentation ceased fairly synchronously along the entire belt, probably during the Pliensbachian (similar to 191-183 Ma), predating the final closure of the basin when Cache Creek units were thrust over Stikinia and Quesnellia in the Middle and Late Jurassic. This apparent synchronicity should result in re-evaluating the tectonic mechanism of the Cache Creek entrapment model in its present form. The study also shows that pelagic sedimentation ceased 22-31 million years earlier in the Cache Creek Terrane than in the Bridge River Terrane, confirming the stratigraphic discrepancy between the two terranes during the Jurassic.