On Malekula Island pre-Miocene pelagic red mudstones are in tectonic contact with a thick marine succession of Early Miocene age, which is intruded by basaltic and andesitic dykes and sills with a calc-alkaline composition and by plutons. This succession is in faulted contact with Middle Miocene sedimentary rocks.

The Lower Miocene succession consists of quartz-free volcanoclastic rocks, detrital limestones and pelagic sediments, and rare lava flows. Fifteen sedimentary facies have been recognized, all of which accumulated below the zone of strong wave or current action. Vertical changes in facies are common; facies relationships and associations and internal sedimentary structures indicate that many facies were transported by mass gravity processes. Texture and composition of clasts in the volcanoclastic rocks show that some facies were derived from subaerially erupted volcanic rocks, and others from submarine volcanoes. Clastic limestones consist largely of algal and coral fragments and benthonic foraminifers.

The Lower Miocene palaeogeography resembled that in and around volcanic chains in present island arcs. Subaerial and submarine volcanoes fed by minor intrusions supplied epiclastic, pyroclastic and autoclastic debris which accumulated in deep water around the volcanic chain. Carbonate detritus was derived from reefs bordering the volcanic islands.

The Middle Miocene succession consists of volcanic and reef-derived turbid-ites, pelagic sediments and tuffs. The epiclastic rocks in this succession were derived from the uplifted rocks of Early Miocene age and from bordering reefs.

The presence of some ancient volcanic arcs, now located either within existing ares or within geosynclinal successions lying in or on the borders of continents, may be indicated only by quartz-poor turbidites containing abundant calc-alkaline volcanic detritus.