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 southwestward 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 comer 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 comer of the Eastern Belt is coeval with the Sonoma Orogeny and the Middle-Late Triassic orogenesis of the Central Belt represents the Tahltanian Orogeny.Dane cinq l a l i t k s itudites dans le centre sud de la Colomhie-Britannique. on observe ou on i n h e que les roches du Trias sup6rieur recouvrent en discordance les roches du PaliazoIque superieusou des mches plus anciennes. Les mches du hleozoique sous la d~scordance exhibent une dPformation i multiples phases et un mitamorphisme kgional de faible intensit6 qui son1 absents dans les roches qui les recouvrent. Les ohscrvations dans ces localitis et en d'autres endmils defmissent une discordance anguEaire datant de la fin du Permien ou du dibut du Triav sur les bordures ouest et sud du complexe metamorphique de Shuswap. Les roches permiennes et triassiaues conservent la maraue de processus structuraux, sedimentaires et metamorphiques qui *errnettent de distinguer dansies roches du Trias trois zones tectonostratigraphiques limitees par des failles. La zone est contient la transition de sedimentation miogtosynclinale au cours du Trias dans les montagnes Rocheuses Bun volcanisme d'arc insulaire a I'ouest a la fin du Tr...

The timing and provenance record of the Late Permian Klondike orogeny in northwestern Canada and arc‐continent collision along western North America

“…Read and Okulitch [1977] reviewed the evidence for a regionally extensive sub-Triassic angular unconformity on Quesnellia in south-central British Columbia. Beneath the unconformity, they reported that Paleozoic rocks of Quesnellia were deformed by at least two phases of folding and metamorphosed to greenschist facies, whereas Late Triassic strata above the unconformity lack this tectonism.…”

Overturned Nicola and Ashcroft strata and their relation to the Cache Creek Group, Southwestern Intermontane Belt, British Columbia

“…This time scale has become increasingly used in the Intermontane Zone (e.g. Read and Okulitch 1977;Tipper and Richards 1976 the London Geological Society ('Phanerozoic Time Scale') (Harland et al 1964). The major difference between the two time scales is the age of the Triassic-Jurassic boundary.…”

“…Mesozoic plutons as old as 200 Ma1 have intruded into the Nicola Group. These rocks contain a complex record of marine sedimentation, deformation, and calc-alkaline volcanism and plutonism (Duffel1 and McTaggart 1952;Douglas et al 1970;Wheeler et al 1972;Read and Okulitch 1977). Their history supplies critical information for models that attempt to explain transformation of marine sediment and adjacent igneous rock into the deformed mountain belts of the circum-Pacific region.…”

Cordilleran metamorphic core complexes: An overview

“…It is the largest of all the Cordilleran metamorphic core complexes, and in the original conception of the problem, it was the type example and the source of the name (see my "Introduction" to this volume). As outlined earlier, studies indicate that some of the metamorphism is as old as Paleozoic and much of it is at least as old as Jurassic (Okulitch and others, 1975;Read and Okulitch, 1977;Brown and Tippett, 1978;Wheeler and Gabrielse, 1972;Hyndman, 1968). There are dated Upper Jurassic to middle Cretaceous plutons that crosscut metamorphic rocks (Gabrielse and Reesor, 1974).…”

“…The northern complexes are found in a region that was probably affected by middle and late Paleozoic Antler-Sonoma deformation and, in the case of the Shuswap complex, some Paleozoic metamorphism (Okulitch and others, 1975;Read and Okulitch, 1977;Brown and Tippett, 1978). Southward in Nevada and Arizona, the complexes lie well east or south of any profound Paleozoic thermal or tectonic events.…”