Abstract

Nine stromatolite localities in the Back River volcanic complex occur at the boundary between 2692 Ma felsic dome-flow complexes, marking the latest eruptions of this stratovolcano, and overlying turbiditic sedimentary rocks of the Beechy Lake Group, Yellowknife Supergroup. Stromatolites form lenses isolated within coarse volcanic breccia at margins of felsic dome-flow complexes, and 2 m thick bioherms that extend laterally for hundreds of metres. Thin units contain wavy laminae and open-spaced, linked mounds, which form thin encrustations on breccia blocks, or clusters of mounds with low synoptic relief. Thick successions comprise undulatory, flat laminated dolomite that contains wrinkled wavy laminae, pseudocolumnar forms, and locally elongate, low-relief mounds. These units typically contain millimetre-scale layers of fine volcanic ash at regular intervals, testifying periodic explosive eruptions during deposition of microbial mats. The stromatolites, which are identified by gross morphology and distinctive laminae, are all stratiform types. Carbonate units all occur on the seaward side of the volcanic dome-flow complexes that straddled the shoreline around the volcano. The stromatolites probably represent isolated microbial communities that may have developed around areas of fumarolic (or hydrothermal) activity associated with these domes. Stratigraphy seaward from the domes comprises carbonate-cemented dome-flanking breccia, stromatolitic and oolitic carbonate, pebbly rhyolite volcarenite, carbonaceous mudstones, banded iron formation, and turbidites. Thus the stromatolites mark a local environment where life flourished in an Archean sea that lapped onto active volcanic domes along the shallow flanks of an emergent stratovolcano.

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