Abstract

This study resolves fundamental questions concerning the age, provenance, and depositional history of Laberge Group strata in the Whitehorse Trough. The Jurassic Inklin Formation straddles the Stikine and Cache Creek terranes along much of the length of the Whitehorse Trough. Ammonite biochronology indicates an age range of early Sinemurian to late Pliensbachian and provides the temporal framework for interpreting basin history. Strong temporal trends in both paleocurrent patterns and sandstone–conglomerate petrofacies allow definition of three discrete phases in basin-fill history. Stable tectonics characterized by relative volcanic quiescence and low sedimentation rates prevailed during the Sinemurian. Sinemurian sandstone–conglomerate petrofacies record a transitional-arc provenance derived from erosion of the Upper Triassic volcanic pile, flanking coastal sediments, and arc roots of Stikinia to the southwest. During the early Pliensbachian, arc dissection was interrupted by a major magmatic episode with widespread rejuvenated volcanism that caused a strong provenance shift to volcanigenic sources, indicating derivation from a largely undissected Stikinian arc. Southwest-derived, northerly longitudinal paleoflow during the Sinemurian changed to opposed bidirectional radial or transverse paleoflow systems in the early Pliensbachian. Cannibalism of broadly coeval basinal strata and (or) reflected sediment gravity flows were the result of episodic growth of a mobile outer forearc rise, initiating southwest-directed paleoflow systems during the early Pliensbachian and the possible development of a ridged forearc phase. U–Pb dates of graphic and 186 ± 1 Ma from a granite clast and tuff unit, respectively, of the Kunae Zone (early late Pliensbachian) and sandstone–conglomerate petrofacies indicate a late Pliensbachian depositional regime dominated by tectonic controls. The influx of granitic detritus indicates a rapid transition to a fully dissected arc provenance, where accelerated uplift of segments of the arc massif, accompanied by intra-arc strike-slip faulting, resulted in rapid arc dissection and unroofing of comagmatic Pliensbachian plutons.

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