Stratigraphy, sedimentology and engineering aspects of Holocene organo-calcareous silts, Suva Harbour, Fiji |
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| Authors: | Graham G Shorten |
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| Institution: | School of Geology, Queensland University of Technology, 2 George St., Brisbane, Old. 4000, Australia |
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| Abstract: | A deposit of post-glacial organo-calcareous silt 25–40 m thick blankets the floor of Suva Harbour and its marginal inlets, including Draunibota Bay, on the southeast coast of Viti Levu, Fiji. This deposit is notable for its involvement in a large number of foundation failures in the area. The organo-calcareous silt is the latest sedimentary infill over a late Pleistocene erosional surface which is dissected characteristically down to around 50 m below current mean sea level at the harbour margins, and to at least 90 m at the seaward end of the study area. The gross distribution of Pleistocene and Holocene sedimentary units is strongly influenced by Pliocene graben formation in the Mio-Pliocene bedrock. The silt deposit forms part of a transgressive and highstand systems tract related to the latest glacio-eustatic sea-level rise, and includes a series of lobate delta deposits currently prograding at the mouths of streams entering the harbour. The silt was deposited in a lagoonal/estuarine environment of restricted circulation in a marginal/back-barrier reef situation and received a variety of marine carbonate and terrigenous sediment contributions, as well as organic substances from both autochthonous and allochthonous sources. Undisturbed sampling indicates the silt to be homogeneous, non-layered and thoroughly bioturbated, but a distinct layering with a spacing in the order of 1–5 m, probably related to periodic influxes of terrigenous material, is obvious on continuous seismic profiles. Acoustic turbidity due to biogenic gas in thick accumulations over palaeo-channels may well represent zones of high pore fluid pressure, and indicate lower effective soil strength. There is also evidence that the passage of the overpressured gas front results in hydraulic fracturing and consequent degassing of the silt. The silt deposit displays a slight upward coarsening in response to shoaling conditions, accompanied by an upward increase in both organic carbon and calcium carbonate proportions, and a decreasing terrigenous contribution. The increase in organic carbon is closely mirrored by increased diagenetic development of bacterially-deposited agglomerates of iron sulfide crystals in the pore spaces. The resulting pore-space microstructure appears to exert a direct control on the in-situ void ratio, and hence directly affects the consolidation and strength characteristics of the sediment. Radiocarbon dates from the silt deposit indicate a consistent rate of deposition in the Draunibota palaeo-channel of 4.3 m/1000 yr since the marine transgressive front passed the site some 9600 yr B.P. The dates extend the reliable local sea-level data for southeast Viti Levu back some further 4000 years, and support an earlier contention that sea level reached its current position here at around 4000–4500 yr B.P., about 2000 years later than on the coast of southeast Australia. Water depth at the Cement Works Bridge site reached a maximum of 20 m some 5000 yr B.P. and, subsequently, has gradually shoaled to its present position just below mean sea level. |
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