John R. Dickie

The Whitehorse Trough, south-central Yukon, is a remnant Mesozoic fore-arc basin. The Carnian-Norian (and Rhaetian) Lewes River and Hettangian-Bajocian Laberge Groups were deposited during the final stage of a Late Triassic arc-continent collision. Convergence between arc and craton preceded the eventual obduction of the Lewes River Arc across the ancient rifted margin of North America. During oblique convergence, dextral transpressive strain in the frontal arc may have migrated toward the arc core zone, resulting in arc dissection. Exhumation and erosion of arc plutons continued into the Middle Jurassic. Collision-induced flexure of the adjacent, subsiding Whitehorse Trough created an elongate, arc-parallel depocentre. Coarse clastic sediment was shed from the eroding arc into this basin, providing a record of convergence and basin closure.

Polymictic cobble to boulder conglomerates of the Laberge Group suggest highly competent flows drained a lithologically diverse source terrane. Facies stacking trends, clast provenance and basin fill petrographic evolution combine with dispersal trends to indicate a western (arc) source. Steep gradient, mass-flow dominated fan-deltas prograded eastward across a drowned carbonate platform (Lewes River Group). Gravel was cannibalized at coastal fans by marine transgression (subsidence-driven) and transported to submarine gravel cones along the slope. Submarine chute and gully networks resedimented fan gravel to the slope as arc-flanking lobes, locally coalescing into a slope apron. Most Laberge conglomerates are deep marine in origin.

Conglomerates prograded from an uplifted, dissected and eroding volcanic arc. Collision and the initiation of arc terrane obduction led to basin shoaling, a provenance shift to the northeast (?) and termination of the deep marine Whitehorse Trough seaway. Asymmetric obduction (parallel to the arc) created diachronous basin shoaling during the final stages of Laberge Group deposition.

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Supervisor:   F. J. Hein