Sediments of the North Fiji Basin

Abstract

Sediments of the North Fiji Basin have been studied as part of a geological and geophysical survey of the central and western parts of the basin. The study is based on a detailed examination of five piston cores that have sampled a continuous sedimentary record of the last 0.8 m.y. Three cores are from the sediment-covered, abyssal hill terrain in the central part of the Basin and two are from an archipelagic apron (New Hebrides apron), which extends east from Vanuatu into the western part of the North Fiji Basin. In the central part of the basin, calcareous pelagic oozes (nannofossil oozes) are the predominant sediment. Volcanic glass is also common, occurring in a single ash bed in each core, in ashy intervals where glass is less than 50%, and in pelagic ooze where it is persistently present as a few percent of the total sediment. From its physical and chemical characteristics the glass appears to be mostly airfall ash, derived from the central chain volcanoes of the New Hebrides arc to the west. A westerly origin is supported by the existence of strong tropospheric westerlies which dominate the structure of the atmosphere. Bioturbation is moderate to intense throughout all the central basin cores, the upper 5-20 cm commonly being continually churned over. Only the thickest ashes (greater than 2 cm) have survived this biological mixing moderately intact. Thinner ashes have either been partially mixed with ooze to form an ashy interval, or completely mixed and lost as recognizable layers. Three stratigraphic units are recognised in the central basin cores. The middle unit, approximately 0.5-0.25 Ma in age, has significantly more ash than the other two units and represents a period of increased volcanic activity in Vanuatu. Sediment accumulation rates are as low as 9 m/m.y. for the ooze and as high as 20 m/m.y. for the middle unit in one core where, in addition to ooze and ash, there has also been some reworking of sediment from an adjacent topographic high. In the western part of the North Fiji Basin, sediments are predominantly alternating ashy pelagic ooze and volcaniclastic graded beds of the New Hebrides Apron. Sediment in the graded beds was derived from shallow water along the eastern side of the New Hebrides arc and transported by turbidity currents eastward into the western part of the North Fiji Basin.