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Abstract

This paper presents the most complete results yet published of geological surveys in Malaita, north of latitude 9°05′S between 1990 and 1995. The geology of Malaita reflects its position as an obducted part of the Alaska-size Ontong Java Plateau (OJP). The geology comprises a monolithological Cretaceous basalt basement sequence up to 3–4 km thick, termed the Malaita Volcanic Group (MVG), conformably overlain by a 1–2 km-thick Cretaceous–Pliocene pelagic sedimentary cover sequence. Cretaceous–Pliocene pelagic sedimentation was punctuated by alkaline basalt volcanism during the Eocene and by intrusion of alnöites during the Oligocene. Basement and cover sequences were both deformed by an intense, but short, middle Pliocene event. A number of localized, Upper Pliocene–Pleistocene, shallow-marine–subaerial, predominantly clastic formations overlie the middle Pliocene unconformity surface. The MVG comprises a monotonous sequence of pillowed and non-pillowed tholeiitic basalt lavas and sills with a predominant clinopyroxene–plagioclase–glass–opaques ± olivine mineralogy. The basaltic plateau morphology of the MVG is reflected in the presence of trap-like topographic features exposed in numerous river sections. Remarkably little sediment is present between basalt flows (most interlava contacts are basalt–basalt), indicating high to very high effusion rates. When present, inter-lava sediment is laminated pelagic chert or limestone, millimetres to centimetres thick, reflecting emplacement of the basalt in deep water (near or below the calcite compensation depth). Gabbro intrusions, dolerite dykes and an unusual spherulitic dolerite facies are locally present. The deep-water eruptive environment of the MVG probably was defined by the accumulation of voluminous eruptions from a multi-centred, submarine, possibly fissure-fed, volcanic source. The Malaitan cover sequence largely comprises a series of foraminifera-rich, pelagic calcilutites and calcisiltites with chert and, in the younger formations, arc-derived mudstone interbeds at various stratigraphic levels.

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