Ordovician and Silurian

Abstract Most Ordovician and Silurian rocks in western and northwestern Canada were deposited on the North American craton and its western continental margin. Strata of these ages have also been recognized in three allochthonous terranes in the Cordillera, described in the companion volume on the Cordilleran Orogen (Gabrielse and Yorath, 1991). The continental margin is equated with the Cordilleran Miogeocline of western Canada. The miogeocline/craton boundary is along a hinge line west of which the rate of thickening of Paleozoic strata increases significantly. On the basis of initial 87Sr/86Sr ratios for Mesozoic and younger plutonic rocks (Armstrong, 1979) that intrude them, all autochthonous Ordovician and Silurian strata, of both platformal and basinal facies, are inferred to be underlain by attenuated continental crust, and are thus in the miogeocline. Oceanic rocks are virtually unknown, and if present are either buried beneath younger strata, masked by metamorphism, and/or transported northwestward by Mesozoic and Cenozoic strike-slip faults. The only strata of this age that may be oceanic are rocks of the Kootenay Terrane in southernmost British Columbia. The Cordilleran Miogeocline formed in Late Proterozoic to Early Cambrian time (see Bond and Kominz, 1984). It is considered to be a continental margin of Atlantic type that was affected by several periods of renewed extensional tectonism, one of which occurred during late Early Ordovician and Middle Ordovician time. The eastern part of this ancient continental margin and adjacent craton were characterized by shallow-water, carbonate platforms and/or land areas composed mostly of exposed older Paleozoic and Precambrian