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In the Portage Brook area, Siluro-Devonian rocks of the Chalcurs Bay Synclinorium are juxtaposed against polydeformed Cambro-Ordovician rocks of the Bathurst Mining Camp along the northerly trending Portage Brook Fault. Rocks of the Chaleurs Group have been identified for the first time west of the Portage Brook Fault, and are subdivided into four lithological units: conglomerate, sandstone and siltstone of the Simpsons Field Formation; mafic volcanic rocks of the Bryant Point Formation; felsic volcanic rocks of the Benjamin Formation; and thickly bedded, brownish grey to reddish grey sandstone of the Greys Gulch Formation. The presence of two pre-deposi-tional cleavages in Ordovician pebbles in the middle Wenlockian to late Ludlovian Simpsons Field Formation indicates that D 2 deformation in the Bathurst Mining Camp started prior to the Late Silurian, i.e., earlier than considered by some workers. This observation is supported by the truncation of at least two strong fabrics in Cambro-Ordovician rocks by Late Silurian/Early Devonian mafic intrusions. An alkali granite (radiometrically dated at 414 +11/-1 Ma) clearly intruded, and altered to hornfels, clastic rocks of the Simpsons Field Formation. The granite appears to grade laterally into and to be coeval with volcanic rocks of the Benjamin Formation, thus suggesting that the age of the intrusion is close to the lower error limit.   RESUME  Dans le secteur du ruisseau Portage, des roches siluro-devoniennes du synclinorium de la baie des Chaleurs sont juxtaposes contre des roches polydeformees cambro-ordoviciennes du Camp minier de Bathurst le long de la ligne d'orientation vers le nord de la faille du ruisseau Portage. On a pour la premiere fois repere des roches du groupe de Chaleurs a l'ouest de la faille du ruisseau Portage. Celles-ci se subdivisent en quatre unites lithologiques : le conglomerat, le gres et le siltstone de la Formation de Simpsons Field; les roches volcanomafiques de la Formation de Bryant Point et les roches volcanofelsiques de la Formation de Benjamin; ainsi que le gres allant du gris brunâtre au gris rougeâtre en couches epaisses de la Formation de Greys Gulch. La presence de deux schistosites anterieures e la sedimentation dans des galets de l’Ordovicien de la Formation du Wenlock moyen au Ludlovien tardif de Simpsons Field revele que la deformation D 2 a I'interieur du Camp minier de Bathurst a debute avant le Silurien superieur, c.- a-d. plus tot que le pensaient certains chercheurs. La truncature d'au moins deux structures dans des roches cambro-ordoviciennes d'intrusions mafiques du Silurien superieur au Devonien inferieur appuie cette observation. Du granite alcalin (situe a 414 +11/-1 Ma par datation radiometrique) a nettement circonscrit et transforme en hornfels des roches clastiques de la Formation de Simpsons Field. Le granite semble se fondre lateralement dans des roches volcaniques de la Formation de Benjamin auxquelles il semble contemporain, ce qui laisse supposer que Pintrusion est d'un âge proche de la limite d'erreur inferieure.  [Traduit par la redaction]

atlantic geology

Siluro-Devonian tectonostratigraphic relationships in the Portage Brook area, northern New Brunswick: implications for timing of deformational events in the Bathurst Mining Camp