ABSTRACT The volcaniclastic, Lower Sarava Formation of north‐eastern Maewo was deposited in a Late Oligocene‐Early Miocene deep sea fan environment. Eight lithofacies can be recognized from a westward‐trending, deep‐sea fan which prograded at a depth of >4.25 km within an extinct interarc basin. These lithofacies include massive spilite‐rich rudite, graded coarse rudite‐arenite, graded fine rudite‐arenite, intraformational rudite, brown and grey siltstone, minor calcarenite and a lithofacies composed of turbidites with Bouma sequences. The sediment was essentially derived from the Vitiaz palaeoarc to the east. Non‐calcareous red and green siltstone and minor airfall tuff was deposited distal to the arenite and rudite, in areas, or during periods, of slower sedimentation. Slightly more than half of the lower part of the Sarava Formation is represented by massive rudite, graded coarse rudite‐arenite, and graded fine rudite‐arenite. These deposits, and some of the calcarenites and massive arenites. were deposited from decelerating high density turbidity currents. Some of the thicker rudite‐arenites may have been induced by tsunamis. Classical turbidites deposited, perhaps, from low density turbidity currents are rare. The petrography of igneous clasts indicates that Late Oligocene‐Early Miocene volcanism on the Vitiaz palaeoarc was largely basaltic‐andesitic but there were some dacitic eruptions.
The Solomon Islands comprise three main geologic provinces: The Volcanic province is composed of Pliocene to Holocene volcanic rocks (Coleman and Packham, 1976); the Pacific province is composed of pillow lava and limestone deposited on a segment of a westward-frifting Pacific plate during Late Cretaceous and most of Cenozoic time (Hughes and Turner, 1977); and a Central province is composed of metamorphic rock, lava, and much post-Oligocene sediment, present on many of the major islands (Coleman, 1966). Ultramafic rocks (ophiolite?) are present on Choiseul, Santa Isabel, Florida Group, Guadalcanal, and San Cristobal (Thompson, 1965), and all these ultramafites lie within the Central province of Coleman (1966). Diapiric masses. of ultramafic rocks have been described in the New Hebrides (Mallick and Neef, 1974) and in the Solomons (Stanton, 1961; Stanton and Ramsay, 1975).
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