Abstract

The accuracy of published Jurassic time scales is compromised by a paucity of sufficiently accurate and stratigraphically well-constrained isotopic dates. U–Pb dating of volcaniclastic rocks within fossiliferous marine successions is a method suitable of providing additional useful calibration points. Stikinia and Wrangellia, two of the major accreted terranes of the Canadian Cordillera, comprise volcanosedimentary assemblages of magmatic arc origin. Within them we selected sections from which we report 14 new U–Pb zircon ages that are integrated with ammonite biochronology at the zonal level. Poor zircon recovery and complex U–Pb systematics made it difficult to obtain precise ages for many of the samples. The following ages were determined (with 2σ errors): forumla Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian), 191.5 ± 0.8 Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian) to the Whiteavesi Zone (lower Pliensbachian), forumla Ma from the Freboldi Zone (lower Pliensbachian), 184.7 ± 0.9 Ma from the Kunae Zone (upper Pliensbachian), forumla Ma from the Kanense to Planulata zones (lower to middle Toarcian), 179.8 ± 6.3 Ma from the Yakounensis Zone (upper Toarcian) to the lower Bajocian, forumla Ma from the Rotundum Zone (upper Bajocian), forumla Ma from the upper Bathonian, and forumla from the upper Bathonian to lower Callovian. Five other dates are inferior in precision, but still provide some constraints for time-scale calibration. The U–Pb dates reported here together with a wealth of other recently obtained Cordilleran isotopic dates will be used for a significant revision of the Jurassic time scale.

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