Stratigraphic, structural and U-Pb geochronologic investigation of Lower Paleozoic eugeoclinal strata in the Kootenay Arc, NE Washington and SE British Columbia

Abstract

The Kootenay Arc in northeastern Washington and southeastern British Colunbia is transitional between: (1) lower Paleozoic autochthonous miogeoclinal strata and Paleozoic to Mesozoic eugeoclinal terranes of uncertain paleogeographic affinity (e.g., the Quesnel terrane): and (2) areas where Devono-Mississippian tectonism was of a compressional nature (Antler Orogeny in central Nevada), and areas in northern British Columbia and southern Yukon Territory where coeval deformation was evidently of an extensional nature. The nature of these transitions was addressed by study of lower Paleozoic eugeoclinal strata comprising the portions of the Kootenay terrane, the most inboard of the terranes in the Canadian Cordillera, and it's contact relations with miogeoclinal strata. Stratigraphic, structural, and U-Pb geochronologic studies focussed primarily on portions of the Lardeau Group in the Trout Lake area in the northern Kootenay Arc and the Covada Group in the southern Kootenay Arc. As a result of these studies, the following concepts are proposed: (1) lower Paleozoic eugeoclinal strata can be correlated along the length of the Arc; (2) these strata are in fault contact with miogeoclinal strata along the length of the Arc: (3) a contractional event of pre-Mississippian and perhaps Devono-Mississippian age is recorded in the Kootenay Arc: (4) despite faulted oontacts, the eugeoclinal strata are parautochthonous and derived from adjacent portions of North America; and (5) structures and stratigraphy in the Kootenay Arc are broadly correlative with those in the Roberts Mountains allochthon in central Nevada. Two important implications of this study, requiring revisions to theories regarding the more general problem of Cordilleran accretion are that: (1) through a series of stratigraphic linkages it can be demonstrated that the Quesnel terrane, a Mesozoic arc-related assemblage often regarded on the basis of faunal evidence to be "exotic", is parautochthonous; and (2) the Antler Orogeny, often regarded as a localized disturbance, affected at least 1200 km of the Cordilleran margin, and perhaps the entire Cordilleran margin.