Calcaires à Archéocyathes du Cambrien inférieur du Nord de la Colombie britannique (Canada). Implications paléogéographiques et précisions sur l'extension du continent Américano-Koryakien

Abstract

Upper Precambrian and Cambrian sections of the northern British Columbia (Cassiar, Omineca and RockyMountains) yield a rich material suitable for regional paleogeography and Lower Cambrian archaeocyath studies. Research on archaeocyath and coralomorphs assemblages distributed on both sides of the Rocky Mountain Trench (RMT) result in defining a dextral movement of 400-450 km along the Trench, from Cretaceous to Eocene. Within the limits of each block, succesive facies are observed, on which paleogeographic reconstructions of Northern British Columbia and adjacent areas at the end of the Precambrian are based. Red beds with proximal characteristics prevailed; they were replaced by a beach facies of ubiquitous orthoquartzites. At, or near, the base of the orthoquartzites is apparently situated the transition between Precambrian and Cambrian. At the very beginning of the Cambrian, environmental conditions were generaly similar, but changed toward the South and the West being replaced by deeper water characterized by the formation of pelitic dark shales and sandstones. Close to the RMT, an emersive tendancy is observed. Hematitic red beds and coarse conglomerates appearing along the mountains from the Cassiars to the Cariboos indicating a marine regression. The end of the Fallotaspis zone and the beginning of the Nevadella zone (or, in term of archaeocyaths, the interval between the Ethmophyllum whitneyi/Sekwicyathus nahanniensis zones and the Claruscoscinus fritzi/Metacyathellus caribouensis zone), partly correspond to the initial formation of the three distinctive facies belts (outer and inner detrital, middle carbonate). A rapid small scale transgression occured at the beginning of the Bonnia-Olenellus zone (the Upper Claruscoscinus fritzi/Metacyathellus caribouensis and Pycnoidocoscinus? serratus/Tabulaconus kordeae-archaeocyath zones) and was accompanied by a considerable terrigenous input. During the second half of the Bonnia-Olenellus zone (i.e. Archaeocyathus atlanticus and Tegerocyathus greenlandensis/Pycnoidocyathus pearylandicus archaeocyath zones), a migration of the different facies to the West is observed. The coeval archaeocyath assemblages could have been slightly modified perpendicularly to the middle carbonate belt, but this problem needs further investigations. Presently, it is noteworthy that the biohermal archaeocyath assemblages from the proximal part of the outer detrital belt (Gataga River) resembles the eastern Alaska and the Mackenzie Mts assemblages while the coeval assemblage from the middle carbonate belt (Kechika Mts) contains elements recorded in South British Columbia and Washington State. On the whole, the North American archaeocyath assemblages are remarkable for their uniformity over the whole craton periphery, from Alaska and the Mackenzie Mountains to the Great Basin and Sonora (Mexico); later throughout Appalachians they reach western Newfoundland and Labrador. The whole region, as well as some terranes of Koryakia (North sibirian Far East), appears to be a single American-Koryakian archaeocyath Province, characterized by a common zonal scale as defined above from the oldest to the more recent one. © 1993.