Volcanism and related slope to shallow-marine volcaniclastic sedimentation: an Archean example near Chibougamau, Quebec, Canada

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Volcaniclastic sedimentary rocks and K-rich lava flows of the Archean Hauy Formation are discontinuously exposed in a 850 m thick sequence in the Waconichi synclinorium northwest of Chibougamau, Quebec. The interstratified sedimentary rocks comprise two major sedimentary facies associations: the Graded Bedded Facies Association (WBFA), indicative of deposition by sediment gravity flows, and the Wavy Bedded Facies Association (WBFA), defined by wave-induced structures. The GBFA is near the base of the stratigraphic section and is interstratified up-section with the WBFA. In the GBFA the abundance of synsedimentary folding, truncated slump folds, load casts, and low-angle erosional surfaces, as well as sediments deposited by turbidity currents and debris flows, support designation as a volcanic slope deposit. In the WBFA trough crossbeds, wavy bedded gravels, and plane-bedded volcaniclastic sandstones are indicative of a nearshore setting. The abundance of angular to subangular lithic volcanic fragments, and broken and euhedral pyroxene crystals in the sediments is suggestive of a pyroclastic origin. Some beds may represent primary pyroclastic material, but unequivocal petrographic recognition of pumice or glass shards is not possible because of metamorphic overprint. Composition suggests an initial magmatic or phreatomagmatic eruption, subsequently reworked along the shoreline and transported downslope. Lava flows comprising massive to lobate andesites and massive to blocky basalts constitute most of the stratigraphic sequence; pillowed flows are strikingly absent from the section. Both compositional types have an increase in vesicularity and change in flow form up-section from massive to lobate or blocky, suggesting progressively shallower water depths. The composite character represented by Hauy lava flows and associated volcaniclastic sediments in characteristic of stratovolcanoes. The studied section is an integral part of an evolved arc with an adjacent backarc basin. The volcano-sedimentary facies indicate the development of a volcanic island in one of these basins.