The stratigraphy and sedimentology of the Waindalithi Conglomerate of northeast Viti Levu, Fiji: hyperconcentrated flows and Pliocene environmental history

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Exposures of the Early Pliocene Waindalithi Conglomerate are confined to no more than a dozen scattered outcrops in the northeast part of Viti Levu, the main island of the Fijian archipelago. The Waindalithi Conglomerate is waterlain, but its sedimentology is unlike that expected either in debris flow deposits or in sediments entrained by conventional fluvial processes. Instead, the unit is likely to represent deposition from hyperconcentrated flows. Deposits of this sort have probably been under-identified in the rock record. In part, this is because no single characteristic is diagnostic of hyperconcentrated flow deposition. A combination of gravel lithology, sedimentary fabric, particle-size distribution and the sedimentology of individual beds proved valuable for the recognition of these deposits in this work. The deposition of the Waindalithi Conglomerate appears to have been immediately predated by a significant episode of volcanicity along the Nakauvadra and Nakorotubu ranges to the north of the region. Several eruptive centres may be recognised in the ranges, including the 25 km-diameter Rakiraki Volcano and the newly identified Nadelanadro, Namarai, Tova and Naimasi volcanoes. Fan-like masses of volcanic conglomerate drape the southern flanks of the ranges. Southerly drainage down these fans was diverted east along the gutter developed where the fans abutted the ridge-and-vale topography of the Wainibuka and Matailombau hills. Downstream, the drainage resumed its southerly course via the topographic outlet of the Wainibuka gap and onto the coastal lowlands of the Early Pliocene sea. The Waindalithi Conglomerate forms a fan-like sediment body at the southern end of the gap. While the basal components of the Waindalithi Conglomerate may have been laid down under terrestrial conditions, the upper elements may have been deposited in shallow marine conditions, with the environment of deposition changing over time as local sea-levels rose.