Oceanography and sedimentology of a tropical lagoon, southeast Viti Levu, Fiji.

Author: Gussmann, Oliver
Year: 1996
Type: MSc thesis

A literature review on oceanography and sedimentology along southeast Viti Levu, Fiji, identifies gaps in scientific knowledge on physical coastal processes. The relatively pristine Navua-Suva Lagoon, between the capital Suva and the township of Navua, is investigated to obtain data on oceanography and sedimentology. The lagoon is characterised by significant terrigenous input via the Navua River from the southwest, which influences its oceanography and sedimentology.

Terrigenous influence is limited to be southwest of the lagoon, observable in bathymetry (suggesting possible infilling of the basin), reduced water opacity near fluvial inputs, local surface salinity depression, surface sediment texture, calcium carbonate content, and skeletal associations. The northeast of the lagoon is little affected by Navua River, and other, siliciclastic input.

Lagoonal water structure is homogenous at least during the dry season according to temperature and salinity profiles. Daily temperature stratification develops due to solar heating at the surface, and is related to depth of the photic zone. Cooling and mixing occurs at night, homogenising the water column. Temperature stratification, and subsequent cooling and mixing, depend on time and strength of wind stress on the surface layer. Oscillation of the heated surface layer suggests the presence of an internal wave in Veivatuloa Passage. Spatial salinity and temperature trends are less easily explained, but probably involve a combination of local low-volume freshwater input via Veivatuloa River, and oceanic inflow through reef passages.

The Navua-Suva Lagoon, classified as a restricted lagoon, is characterised by low-energy conditions, reflected by siliciclastic deposition, in situ carbonate deposition, surface currents, and absence of significant density gradients during the dry season. Surface drogue speeds in two of the reef passages measured between 0.03 and 0.11 m s-1. Flow is directed out of the lagoon. No reversal of flow occurred with progressing flood tide.

A transitional sedimentary environment from dominantly river-born volcaniclastics to biogenic carboante detritus, along the lagoon axis, characterises the lagoon. In situ mixing occurs between the two sediment types, mainly related to provenance. Depositional zones identified are 1) a fluvial-influenced, near-deltaic siliciclastic zone, 2) a lagoonal mixed carbonate-siliciclastic zone, 3) a near-shore terrigenous-sand zone, 4) a lagoonal sandy carbonate zone, and 5) a reef-associated coarse-carbonate zone. A terrigenous-tolerant foramol association displaces the chlorozoan assemblage typical for tropical unmixed carbonate systems in the region of terrigenous influence.

The depositional geometry of the Navua-Suva Lagoon developed during Holocene sea-level rise and fall, and Navua River delta progradation, superimposed on tectonic land movements, and is maintained by climate, tectonic setting, and reef build-up. Mixed terrigenous-carbonate sediments are more common than previously appreciated, particularly within sedimentary settings characteristic of high, volcanic islands of the South Pacific.