Large-scale stratiform and intrusive sedimentary breccias of the lower Proterozoic Goulburn Group, Bathurst Inlet, N.W.T.

Abstract

Two types of sedimentary breccia have been identified in the lower Proterozoic (1.7 to 2.5 Ga) Goulburn Group, Bathurst Inlet, N.W.T. One type is a single unit of strata-bound breccia consisting predominantly of sedimentary clasts surrounding large (up to 500 m in length) sheets of chaotically folded and brecciated allochthonous carbonates. This breccia, the Omingmaktook member of the Brown Sound Fm., is an olistostrome interpreted to have formed by slumping of carbonate sediments during differential basin subsidence and (or) evaporite solution. The other type of breccia occurs as intrusive dykes and large cylindrical bodies (up to 1 km in diameter) that cut through more than 2000 m of Goulburn strata. The intrusive breccias also cross-cut and rework the olistostrome. These intrusive breccias: are of entirely sedimentary composition, and restricted to the upper Goulburn Group; are generally unsorted and exhibit both massive and flowage-layered textures; and contain quartz, dolomite, and minor tourmaline crystals. The intrusive breccias are interpreted as exhumed conduits through which trapped intrastratal fluids migrated to a post-Goulburn paleosurface. Intrusion of the breccia is interpreted to have been produced by the accumulation of pools of intrastratal fluids in structural traps, which under pressure intruded and stoped into overlying strata, eventually reaching a Goulburn paleosurface where they extruded mud flows and formed sedimentary volcanoes.