Measurements of river bed gravel samples from the Sabeto River, Fiji

Published: 01-07-2021| Version 1 | DOI: 10.17632/gwnfv4c6b5.1


Bedload gravels were collected from seven stations along the length of the Sabeto River of western Viti Levu, Fiji. The Sabeto River is an active, bedload transporting, gravel-bed system (Gale et al., 2019). Sampling was confined to clasts of the distinctive and isotropic Navilawa Monzonite, an intrusive rock that crops out only along the upper reaches of the river (Rao, 1983; Hathway, 1993). The river has entrenched the Navilawa Monzonite outcrop resulting in the direct supply of monzonite gravels to the fluvial system. Bedload gravels were collected along transects at seven stations (1 to 7) over a distance of 9.7 km. One of the sampling sites was located above the downstream boundary of the monzonite outcrop, the rest below. In addition, samples of fresh Navilawa Monzonite undergoing active physical breakdown were collected from alongside the bedrock channel at Station 0. The shape of these clasts provides us with the opportunity of observing the nature of the precursor particles as they are liberated from the rock and before they experience fluvial modification. The density of the fresh Navilawa Monzonite was estimated. A total of 883 particles, ranging in diameter (b-axis) from 12 to 337 mm, was collected. The long (a), intermediate (b) and short (c) axis of each particle was measured, along with the diameter of the sharpest corner of the maximum projection outline (Di) and the diameter of the maximum inscribed circle (Dk). At six of the stations, the mass of each particle was recorded. The dataset includes calculations of the Modified Wentworth Roundness, Maximum Projection Sphericity and Oblate–Prolate Index of each clast. At six of the stations the volume of each particle was estimated using measurements of particle mass and the density of the fresh Navilawa Monzonite. Gale (2021) cites this dataset. Two files are provided: one is a KMZ file (containing information on the environmental setting of the river and the sampling locations) and the second is an MS Excel file (containing the bedload gravel measurements). ACKNOWLEDGEMENTS We thank the Turaga ni Koro (Village Heads) of the villages of Korobebe and Navilawa for permission to conduct field work along the Sabeto River. REFERENCES Gale, S.J., 2021. Gale, S.J., Ibrahim, Z.Z., Lal, J., and Sicinilawa, U.B.T, 2019. Hathway, B., 1993. Rao, B., 1983. Geology of Lautoka Area. Viti Levu Sheet 4. 1:50,000 scale geological sheet, Mineral Resources Department, Suva.


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Sampling was undertaken at seven stations (1 to 7) along the length of the river. Distances between stations were determined from the digital topographic map (Fiji Department of Lands and Surveys, 1985). At each station, a transect was established normal to the river and across the entire width of the active channel, defined as comprising the unvegetated part of the cross-section. Every gravel of Navilawa Monzonite ≳ 10 mm lying on the stream bed beneath the transect line was sampled. If a minimum of 99 clasts was not obtained, the transect was moved 1 m downstream and sampling continued along the entire length of the new transect. At Station 0, the entire volume of physically loosened particles was collected from the outcrop for measurement. Particle length and mass were determined following the procedures of Gale and Hoare (2011; 118–122). The long (a), intermediate (b) and short (c) axis of each clast was measured, along with the diameter of the sharpest corner of the maximum projection outline (Di) and the diameter of the maximum inscribed circle (Dk). Our measurement of b-axis length followed the protocol of Krumbein (1941, 65–66), in which the b-axis represents the longest axis orthogonal to the longest (a) axis. Using this information, the Modified Wentworth Roundness (Dobkins and Folk, 1970), Maximum Projection Sphericity (Folk, 1955; Sneed and Folk, 1958) and Oblate–Prolate Index (Dobkins and Folk, 1970) of each particle were calculated. Five rectilinear blocks of fresh Navilawa Monzonite of varying sizes were sawn and their volume and mass determined in order to calculate the density of the monzonite. REFERENCES Dobkins, J.E., and Folk, R.L., 1970. Fiji Department of Lands and Surveys, 1985. Fiji map series 31 1:50 000 topographic maps, Lands and Survey Department, Suva. Folk, R.L., 1955. Gale, S.J., and Hoare, P.G., 2011. Quaternary Sediments: Petrographic Methods for the Study of Unlithified Rocks, 2nd ed., Blackburn Press, Caldwell, New Jersey. Krumbein, W.C., 1941. Sneed, E.D., and Folk, R.L., 1958.