The Anvil Mining District contains the most westerly exposures of the off-shelf basinal facies (Selwyn Basin) of the Cordilleran miogeocline, a prism of sedimentary rocks of Precambrian to Jurassic age deposited along the relatively stable, passive continental margin of western North America. Anvil District is immediately northeast of the Slide Mountain and Yukon-Tanana Terranes, the most easterly of the allochthonous suspect terranes which were amalgamated with North America starting in Jurassic time. The total interpreted stratigraphic thickness of the metasedimentary rocks of North American affinity is greater than 7400 m, ranging in age from latest Precambrian or earliest Cambrian through Devonian. These metasedimentary rocks consist predominantly of fine clastic sediments deposited in an off-shelf marine basin with local occurrence of euxinic carbonaceous shales and coarser sandstones and conglomerates. Extensive Ordovician within-plate basaltic submarine volcanic rocks and associated epiclastic breccias are indicative of localized rifting along the continental margin.