Mixed Siliciclastic-Skeletal Carbonate Lagoon Sediments from a High Volcanic Island, Viti Levu, Fiji, Southwest Pacific

Abstract

Modem sedimentation in the Navua-Suva Lagoon, southeastern Viti Levu, Fiji, derives from both allochthonous siliciclastics and autochthonous marine carbonates. Sediments are characterized by a high insoluble load, small grain size, a wide range of textures, and a high degree of mixing. The distribution of the two facies (skeletal-dominated muddy sandy gravel and skeletalbearing very fine sand to mud) is controlled by both the shallow-marine carbonate sediment productivity and sediment supply and dispersal processes from siliciclastic point sources across a narrow lagoon. Mollusks and Halimeda dominate the gravel fraction of the skeletal grains. Sediment budget estimates indicate that 97% of the siliciclastic supply bypasses the lagoon. Some 0.2 Mt/yr is accumulating in the lagoon, not yet enough to inhibit potential carbonate production (~0.1 Mt/yr) by a interreefal benthos that is at least somewhat sediment-tolerant. Contemporary allochthonous siliciclastic and autochthonous skeletal carbonate sedimentation in the lagoon results in true syndepositional (in situ) mixing. The central high volcanic island mass in a tropical setting produces the geomorphological (high topographic relief, narrow shelf), environmental (high rainfall), and ecological (shallow benthic area) conditions that lead to carbonate-siliciclastic mixing in lagoons along adjacent, mostly carbonate, coasts of oceanic islands, a high volcanic island mass effect. We propose that tropical in situ mixing of carbonate and siliciclastic sediments is more common in high volcanic island settings than previously appreciated. Such islands are thus excellent testing grounds for the study of carbonate-siliciclastic interactions. Their special characteristics highlight the need for better understanding of coastal physical processes of tropical Pacific high volcanic islands.