Volcaniclastic gravity flow sedimentation on a frontal arc platform: The Miocene of Tonga

Abstract

Marine volcaniclastic gravity flow deposits of Miocene age are described from island exposures on the Tongan frontal arc platform (southwest Pacific Ocean). Background sedimentary rocks between gravity flow beds include non‐calcareous brown mudstone, calcareous pebbly sandstone, and chalk. Depositional environments inferred from microfaunas, macrofaunas, trace fossils, and sedimentary structures range from shallow (shelf) to deep water (c. 1500 m). The depth range of the deposits is considered deeper than continental shelf, and shallower than typical non‐volcanic large‐scale depositional gravity flow environments such as submarine fans. Six lithofacies are distinguished. They embrace a wide range of gravity flow deposits, but within each lithofacies/environment there is one dominant association. The lithofacies contain varying proportions of mafic and silicic volcanic clasts. Some are solely mafic, some contain interleaved mafic and silicic intervals, and some contain mixed mafic and silicic clasts in the same beds. Clast size ranges from silt (<1/16 mm) to boulders (>64 cm). Accretionary lapilli are present in three lithofacies. The dominant gravity flow mechanisms were turbidity currents and debris flows. Derivation from underwater eruptions is likely in some lithofacies, while others are likely to be from subaerial eruptions. It is rarely possible to make the distinction from the clasts themselves. On Mango Island, bouldery debris flow material was transferred directly from a probable subaerial volcano to the basin. In all other cases a marked upper limit of clast size suggests that eruption process(es), or processes in the transfer of sediment before generation of gravity flows, effectively removed the largest clasts (>5 cm). The overall control on deposition is considered to be eruption‐controlled sediment supply.