SILURO-DEVONIAN TECTONOSTRATIGRAPHIC RELATIONSHIPS IN THE PORTAGE BROOK AREA, NORTHERN NEW-BRUNSWICK - IMPLICATIONS FOR TIMING OF DEFORMATIONAL EVENTS IN THE BATHURST MINING CAMP

In the Portage Brook area, Siluro-Devonian rocks of the Chaleurs Bay S ynclinorium are juxtaposed against polydeformed Cambro-Ordovician rock s of the Bathurst Mining Camp along the northerly trending Portage Bro ok Fault. Rocks of the Chaleurs Group have been identified for the fir st time west of the Portage Brook Fault, and are subdivided into four lithological units: conglomerate, sandstone and siltstone of the Simps ons Field Formation; mafic volcanic rocks of the Bryant Point Formatio n; 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-depositional cleavages in Ordovician pebbles in the middle Wenlockian to late Ludlovian Simpsons Field Formation i ndicatezs that D-2 deformation in the Bathurst Mining Camp started pri or to the Late Silurian, i.e., earlier than considered by some workers . This observation is supported by the truncation of at least two stro ng 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, elastic 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 l imit.