Two contrasting lower Paleozoic units underlie the region southwest of Big Salmon Lake in south-central Yukon. The lower unit comprises dolomitic quartzite, quartzite, dolostone, dolomitic shale, siltstone and sandstone, and their metamorphosed equivalents. Two-holed crinoid ossicles indicate an Early–Middle Devonian age for the dolostone. These dolomitic rocks are overlain by largely carbonate-free, dark, fine-grained and siliceous strata. Rock types include graphitic phyllite, siltstone, metachert and porphyroblastic metapelitic schist. The two units are correlated with the Askin and Earn groups, respectively. An interval of metabasaltic schist locally marks the boundary between the Askin and Earn groups. Mafic and ultramafic rocks are intermittently exposed beneath the Askin Group in parts of the region.
The lower Paleozoic metasedimentary units are crosscut by deformed Devonian–Mississippian two-mica augen gneiss and by largely undeformed mid-Cretaceous megacrystic biotite granite to monzogranite. Deformation prior to the mid-Cretaceous produced close to tight folds that trend northwest. The associated axial-planar cleavage/schistosity dips northeast at moderate to steep angles, away from the crest of a major antiformal structure.
