The Whitehorse Trough, south-central Yukon, originated as a Mesozoic fore-arc basin separating the allochthonous Stikine Terrane to the west from the North American craton. Late Triassic erosion of a volcanic arc supplied detritus to the basin. Subsequent cessation of volcanism, unroofing and deep erosion of the arc into the Middle Jurassic resulted in a progressive increase in granodioritic sediment. Late Triassic-Jurassic Laberge conglomerate within the Whitehorse Trough are coarse, polymictic and typically massive. Inverse or normal grading, planar stratification and cross-bedding are less common. Conglomerates are debris flow, sheet-flood and bar deposits of braided alluvial fan-deltas. These conglomerates usually overlie and grade basinward into feldspathic graywacke or arkosic sandstone. Crystal tuffs grade laterally into sandstone and occur as interbeds as well. Sandstones commonly display trough cross-bedding or planar stratification. Hummocky cross-stratification rarely occurs in sandstones interbedded with bioturbated silty mudstone. Other facies include graded sandstone-mudstone with Bouma BC(E) sequences; float-stone/micritic limestone and rare calcarenite/rudite. Sandstone-conglomerate facies transitions indicate a vertical progression from shallow marine and shoreface sedimentary strata of Late Triassic age to coarse alluvial fan conglomerates of Jurassic age, reflecting progradation of fan-delta systems with progressive infilling of the basin. The Stikine Terrane accreted to North America in the Late Jurassic with basin shallowing and closure reflected by changes in the sedimentary sequences.
