Eocene-Oligocene strike-slip faulting along the Denali fault system in Yukon Territory produced several local strike-slip basins that were filled by coarse-grained alluvial, fluvial, and lacustrine sediment referred to as the Amphitheatre Formation. The Amphitheatre Formation provides an excellent opportunity to study in detail the composition of siliciclastic sediment in a major, accretionary strike-slip orogen. Petrographic analyses of sandstones and clast counts of conglomerates reveal a lithologically diverse provenance. On standard QFL and QmFLt diagrams, sandstones from the Burwash and Bates Lake basins are arkosic, overlapping the uplifted basement and dissected magmatic arc provenance fields of Dickinson and Suczek (1979). Conglomerates were derived from volcanic, plutonic, and medium- to low-grade metasedimentary rocks that currently are found in several accreted terranes associated with the Denali fault system. Wrangellia and the Alexander terrane provided voluminous volcanic, metavolcanic (greenstone), sedimentary, and low-grade metasedimentary material. The Yukon Crystalline terrane provided medium-grade (gneissic and schistose) material and plutonic material, and the Gravina-Nutzotin belt provided pelitic and metasedimentary material. These data support the contention that the Amphitheatre Formation was derived mainly from local, high-relief sources associated with major uplift in the eastern St. Elias Mountains, as well as the southern Yukon Crystalline terrane. The Eocene-Oligocene climatic event apparently had no significant effect on Amphitheatre sandstone composition. This is attributed to the proximal character of the sandstones studied; steep local relief kept soil residence times to a minimum in spite of climatic change. In addition, the persistence of coal- and stream-dominated alluvial fan facies throughout the entire stratigraphic section indicates that climate remained humid in southwestern Yukon Territory, possibly owing to local orographic effects. It is proposed that, in general, climate should not have a major impact on detrital sand composition in large strike-slip orogens, where sediment accumulates within the orogen itself and is subject neither to long distances of transport nor to long-term residence in lowland soil profiles where most compositional modification is expected.