Late Precambrian to Triassic weakly metamorphosed sedimentary rocks that underlie most of Nahanni map area form three sequences: 1.) From the late Precambrian to Middle Devonian two northwest-trending facies belts developed consisting of shallow water carbonate and sandstone (Mackenzie Platform) on the northeast and time-equivalent turbiditic sandstone, deep water limestone, shale, and chert (Selwyn Basin) on the southwest. Euxinic shale of Early Silurian age is host to important stratiform lead-zinc deposits. The aggregate thickness of Lower Cambrian to Middle Devonian rocks ranges from 4200 m for platform and near-platform strata to 1600 m for outer basin strata. 2.) In the Late Devonian and Mississippian, shale was deposited across the Mackenzie Platform, whereas to the southwest, turbiditic quartz-chert sandstone and chert-pebble conglomerate (900+? m) were deposited, derived from elevated fault blocks of Selwyn Basin strata. Stratiform barite and lead-zinc-barite deposits occur within siliceous shale of Middle to Late Devonian age. 3.) Early Mississippian to Triassic sedimentation was dominated by shale, chert, minor sandstone, and siltstone (1700 m) deposited on a shallow marine shelf. Regional Jura-Cretaceous deformation resulted in decollement style northwest-trending folds and minor thrust faults. Competent strata of Mackenzie Platform formed large-scale open folds. The incompetent strata of the Selwyn Basin area were deformed into small- to large-scale, open to tight folds with associated axial-plane cleavage. Granitic intrusions of the mid-Cretaceous (100 Ma) Selwyn Plutonic Suite crosscut the regional structure. Tungsten skarn deposits are preferentially associated with biotite-muscovite-bearing plutons where they contact argillaceous limestone of various ages.

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Late Precambrian to Cretaceous, weakly metamorphosed strata of the ancestral North American margin comprise four sequences: Upper Precambrian to Middle Devonian (more than 3000 m) turbiditic sandstone, deep-water limestone, shale, and chert (Selwyn Basin), flanked southwesterly in the Siluro-Devonian by shallow-water carbonate and clastic sediments (McEvoy Platform). Early Cambrian pelite hosts deposits of stratiform lead-zinc. Upper Devonian and Mississippian turbiditic quartz-chert sandstone and chert-pebble conglomerate (2000 m) shed from elevated fault blocks of Selwyn Basin strata. Stratiform barite occurs within siliceous shale of mid- to Upper Devonian age. Mississippian to Triassic shale, chert, limestone, minor sandstone, and siltstone (1700 m) deposited on a shallow-marine shelf; and Lower Cretaceous clastic sediments (120+ m) derived from Jura-Cretaceous deformation and uplift. Regional Jura-Cretaceous deformation formed décollement style, northwest-trending folds and large, shallow-dipping thrust faults. Incompetent Cambro-Ordovician to Lower Devonian strata are complexly deformed above a regional flat-lying detachment. Deformation was accompanied by obduction of oceanic ultramafite, basalt, chert, and carbonate (Slide Mountain Terrane) as well as siliceous mylonite and schist (Yukon-Tanana Terrane). Granitic intrusions of the mid-Cretaceous (100 Ma) Selwyn Plutonic Suite crosscut regional structure and are responsible for small skarn and base-metal vein deposits. Coeval pyroclastic rocks of the South Fork volcanics are preserved within huge calderas. Cretaceous-Tertiary dextral slip along Tintina Fault has offset geological elements at least 430 km and formed pull-apart basins along and near the fault. Eocene fill of fluvial clastic and bimodal volcanic rocks host epithermal precious-metal veins.

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