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New 1:20 000 scale mapping within the pericratonic rocks of the Eagle Bay Assemblage near Vavenby, southcentral British Columbia, has revealed a stratigraphic transition from continental slope siliciclastic rocks of Latest Neoproterozoic or Early Cambrian age to a sequence dominated by thick successions of Early Cambrian mafic volcanic and volcaniclastic rocks with an associated archaeocyathid-bearing carbonate platform. These rocks are overturned and structurally emplaced above the Devonian and Mississippian portion of the Eagle Bay Assemblage. Overall, this section is interpreted as recording the transition from deeper water, turbiditic and pelagic 2001-A9 N.D. Hughes et al. 1 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 1 Contribution to the Ancient Pacific Margin NATMAP Project 2 Department of Geological Sciences, University of Montana, Missoula, MT, 59812 3 Department of Geology, Washington State University, Pullman, WA 99164-2812 sedimentation to a period of tectonism and creation of a shallow-water to subaerial, mafic volcanic edifice. These Latest (?)Neoproterozoic and Early Cambrian pericratonic rocks within the Eagle Bay Assemblage record a period of Early Cambrian tectonism and mafic volcanism, supporting recent models of continental separation in this region at that time. Résumé La nouvelle cartographie géologique à l’échelle de 1:20 000 des roches péricratoniques de l’Assemblage d’Eagle Bay près de Vavenby, dans le centre sud de la Colombie-Britannique, a révélé le passage stratigraphique des roches silicoclastiques (Néoprotérozoïque terminal ou Cambrien précoce) de talus continental à une séquence dominée par des successions épaisses de roches volcaniques mafiques et de roches volcanoclastiques (Cambrien précoce) associées à une plate-forme carbonatée à archéocyathidées. Ces roches sont renversées et mises en place structurellement au-dessus de la portion devonienne et mississippienne de l’Assemblage d’Eagle Bay. Dans l’ensemble, on interprète cette section comme témoignant du passage d’un milieu d’eau profonde à sédimentation turbiditique et pélagique à une période de tectonisme et de construction d’un édifice volcanique mafique dans un milieu d’eau peu profonde ou subaérien. Ces roches péricratoniques du (?)Néoprotérozoïque terminal et du Cambrien précoce de l’Assemblage d’Eagle Bay témoignent d’une période de tectonisme et de volcanisme mafique au Cambrien précoce, ce qui corrobore des modèles récents d’une ouverture du continent dans cette région à cette époque. INTRODUCTION T Vavenby map area straddles the North Thompson River near a prominent westerly bend in the river at the town of Vavenby, approximately 20 km east of Clearwater in south-central British Columbia (Fig. 1). It lies approximately between latitudes 51°31 ́ and 51°39 ́N and between longitudes 119°48 ́ and 119°38 ́W (NTS map sheets 82 M-052 and 82 M-062). Bedrock in the area consists of CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 2001-A9 N.D. Hughes et al. 2 metasedimentary and metavolcanic rocks of the Eagle Bay Assemblage. The area is characterized by an Early Cambrian (and possibly older) succession tectonically emplaced (and overturned) over a Devonian–Mississippian succession (Schiarizza and Preto, 1987). Most of the rocks within the study area are highly deformed and have undergone lower greenschist facies metamorphism, except within contact aureoles of late granitic intrusions where higher metamorphic grades are present. The main goal of this study is to place the rocks of the Vavenby area in the proper tectonostratigraphic and paleogeographic context using traditional mapping techniques, geochronology, and geochemical characterization. Fieldwork in the Vavenby map area (Fig. 2) was conducted during an eight-week period in the summer of 2000 as the M.Sc. thesis project of the first author (Noah Hughes). This work was carried out in conjunction with the Geological Survey of Canada as part of the Ancient Pacific Margin NATMAP Project, a comprehensive analysis of the pericratonic terranes of the Canadian Cordillera. This paper presents preliminary results and interpretations of the work conducted thus far. Future work will focus on the geochemical characterization of mafic metavolcanic units, thin-section analysis of all units to better understand their nature and origin, and radiometric dating (U-Pb) of any zircon-bearing rocks. The Vavenby map area is characterized by steep topography, dense vegetation, and thick deposits of glacial till and river gravels. Bedrock exposures can be found along logging roads (mostly inactive), clearcuts, mountain tops, cliffs, and creek canyons. 2001-A9 N.D. Hughes et al. 3 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter

Lithology, tectonostratigraphy, ans paleogeography of the Vavenby area, Eagle Bay Assemblage, south-central British Columbia, a possible constraint for the timing of the rifting of Laurentia