LOWER CAMBRIAN ARCHAEOCYATH LIMESTONES OF NORTHERN BRITISH-COLUMBIA (CANADA) - PALEOGEOGRAPHIC IMPLICATIONS AND PRECISIONS ON THE AMERICANO-KORYAKIAN CONTINENT EXTENSION

Upper Precambrian and Cambrian sections of the northern British Columb ia (Cassiar, Omineca and Rocky Mountains) yield a rich material suitab le 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 de xtral movement of 400-450 km along the Trench, from Cretaceous to Eoce ne. Within the limits of each block, succesive facies are observed, on which paleogeographic reconstructions of Northern British Columbia an d adjacent areas at the end of the Precambrian are based. Red beds wit h proximal characteristics prevailed; they were replaced by a beach fa cies of ubiquitous orthoquartzites. At, or near, the base of the ortho quartzites is apparently situated the transition between Precambrian a nd Cambrian. At the very beginning of the Cambrian, environmental cond itions were generaly similar, but changed toward the South and the Wes t being replaced by deeper water characterized by the formation of pel itic dark shales and sandstones. Close to the RMT, an emersive tendanc y is observed. Hematitic red beds and coarse conglomerates appearing a long the mountains from the Cassiars to the Cariboos indicating a mari ne regression. The end of the Fallotaspis zone and the beginning of th e Nevadella zone (or, in term of archaeocyaths, the interval between t he Ethmophyllum whitneyi/Sekwicyathus nahanniensis zones and the Claru scoscinus 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 transgressi on occured at the beginning of the Bonnia-Olenellus zone (the Upper Cl aruscoscinus fritzi/Metacyathellus caribouensis and Pycnoidocoscinus? serratus/Tabulaconus kordeae archaeocyath zones) and was accompanied b y a considerable terrigenous input. During the second half of the Bonn ia-Olenellus zone (i.e Archaeocyathus atlanticus and Tegerocyathus gre enlandensis/Pycnoidocyathus pearylandicus archaeocyath zones), a migra tion of the different facies to the West is observed. The coeval archa eocyath assemblages could have been slightly modified perpendicularly to the middle carbonate belt, but this problem needs further investiga tions. Presently, it is noteworthy that the biohermal archaeocyath ass emblages from the proximal part of the outer detrital belt (Gataga Riv er) resembles the eastern Alaska and the Mackenzie Mts assemblages whi le the coeval assemblage from the middle carbonate belt (Kechika Mts) contains elements recorded in South British Columbia and Washington St ate. On the whole, the North American archaeocyath assemblages are rem arkable for their uniformity over the whole craton periphery, from Ala ska and the Mackenzie Mountains to the Great Basin and Sonora (Mexico) ; later throughout Appalachians they reach western Newfoundland and L abrador. The whole region, as well as some terranes of Koryakia (North sibirian Far East), appears to be a single American-Koryakian archaeo cyath Province, characterized by a common zonal scale as defined above from the oldest to the more recent one.