40 NORTH AMERICA/Northern Cordillera Figure Distribution, origins, and times of accretion of terranes Terrane names, abbreviated herein as AG, BR, etc., and brief terrane descriptions are in Appendix B Palaeoproterozoic (2.3–1.8 Ga) magmatic arcs and accretionary wedges Although mostly buried, it is exposed locally in south-eastern British Columbia (Monashee Complex) and northern Idaho (Priest River Complex) The continental basement slopes beneath the eastern Rocky Mountains System as a layer about 35 km thick, but thins abruptly and appears to be no more than a few kilometres thick beneath the southern Intermontane Plateau System, and is absent below the Pacific Mountains System Palaeoproterozoic (1.8–1.7 Ga) basinal to platformal deposits (the Wernecke Supergroup) in northern Canada and east-central Alaska (Tindir Group) are rift-related and as much as 13 km thick However, it is uncertain whether they formed in an intercontinental basin or during continental break-up An angular unconformity between folded and slightly metamorphosed rocks of the Wernecke Supergroup and younger (1.4– 1.0 Ga) Mesoproterozoic basinal to platformal deposits brackets the age of the Racklan Orogeny Mesoproterozoic (1.5–1.4 Ga) strata (Belt-Purcell Supergroup) in Montana, Idaho, and adjoining parts of British Columbia and Alberta form a north-west-trending prism up to 20 km thick The lower part consists of deep-water turbiditic quartzite, siltstone and argillite, and voluminous mafic sills; the upper part comprises shallowwater siliciclastics and stromatolitic carbonates deposited in intertidal and floodplain settings The rocks apparently accumulated within and near an intracontinental rift basin Neoproterozoic (780–550 Ma) strata are the oldest rocks to occur more or less continuously along the entire northern Cordillera Glaciogenic deposits (Sturtian?) at the base of the Windermere Supergroup contain mafic volcanic rocks with ages of 780–760 Ma These date the initiation of rifting that led eventually to dispersal of components of the Neoproterozoic Rodinian supercontinent and creation of what became the margin of Laurentia The overlying succession, up to 10 km thick, consists of shale with intercalated quartz-and-feldspar ‘grit’ turbidites, and grades upwards into slope deposits, carbonate, and evaporite shelf deposits