Abstract

Oceanic tholeiitic basalt flows of probable Early Cretaceous age form the basement rocks exposed in the southern half of Malaita and also on adjacent Small Malaita in the Solomon Islands. In the northern part of the area, these are conformably overlain by as much as 270 m of siliceous mudstone. This appears to wedge out farther south, where Upper Cretaceous to Eocene pelagic limestone containing chert horizons is found conformably overlying the basement tholeiites. Similar limestones with chert horizons also conformably succeed the siliceous mudstone in the north.

In this Upper Cretaceous to Eocene limestone in the south, thick horizons of oceanic alkalic basalt are found that differ from the basement tholeiite in micropetrography as well as in having higher TiO2, P2O5, and total alkali percentages. Oligocene to Pliocene pelagic limestones conformably overlie the older limestone throughout the whole area.

Folding and uplift occurred during Pliocene time, and Pleistocene reef limestones and conglomerates were formed in places during the final emergence of the islands. The pre-Pleistocene succession shows close similarities with those discovered in nearby ocean-floor boreholes.

It is evident that Malaita is a part of Pacific Ocean floor that has a very similar stratigraphic succession to the Ontong Java Plateau to the north and probably originated somewhat to the east of its present position. Subduction of the Pacific plate along a southwest-dipping Benioff zone on the north side of the Solomons may possibly have occurred in pre-Miocene times, until it was blocked by the thick oceanic crust of the Ontong Java Plateau. Further subduction may then have occurred on the south side of the Solomons, along a steep northeast-dipping seismic zone, which is, at least in part, still active today.

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