The Triassic unconformity of south-central British Columbia

Abstract

At five localities investigated in south-central British Columbia, Upper Triassic rocks are observed or inferred to unconformably overlie upper Paleozoic and older rocks. Paleozoic rocks beneath the unconformity show polyphase deformation and low-grade regional metamorphism which are absent in overlying rocks. Data from these and other localities define a regional angular unconformity of Late Permian or Early Triassic age on the western and southern margins of the Shuswap Metamorphic Complex. Permian and Triassic rocks preserve evidence of structural, sedimentary, and metamorphic events which permits separation of Triassic rocks into three fault-bounded tectonostratigraphic belts. The Eastern Belt contains the transition from miogeoclinal sedimentation throughout Triassic time in the Canadian Rockies to island arc volcanism in the Late Triassic to the west. Basal beds of the Triassic sequence become younger southwest-ward from the axis of the Early to Middle Triassic depocentre lying west of the Rockies. Rocks preserving Early Triassic deformation and metamorphism are restricted to the southwest corner of the belt and are truncated by the Pasayten Fault. The Central Belt, dominated by the products of Late Triassic volcanism in northern and central British Columbia, consists mainly of Middle (?) and Upper Triassic sediments in the south. Meagre evidence indicates that widespread deformation and low-grade regional metamorphism occurred just prior to the Late Triassic. Evidence for these events is not found beyond the faulted margins of the Central Belt. In the Western Belt, an Upper Triassic sequence of tholeiitic basalt and overlying calcareous sediments disconformably overlies Permian rocks. In the western Cordillera, low-grade regional metamorphism and minor plutonism characterize Triassic orogenies. Early Triassic orogenesis in the southwestern corner of the Eastern Belt is coeval with the Sonoma Orogeny and the Middle–Late Triassic orogenesis of the Central Belt represents the Tahltanian Orogeny.