Interlayered sill-sediment structure at the base of a Miocene basaltic andesite sheet intrusion, Rebun Island, Hokkaido, Japan.

Abstract

A unique, interlayered sill-sediment structure occurs along the lower contact of a Miocene basaltic andesite sheet intrusion on Rebun Island, northern Hokkaido, Japan. The intrusion is a sill or laccolith 1,200×800 m across and more than 150 m thick, which has intruded alternating diatomaceous mudstone and siltstone. The interlayered sill-sediment structure is 25 m high and 70 m wide in cross section and comprises alternating fluidally shaped basaltic andesite sills and sediment layers. Each basaltic andesite sill forms a thin wedge-like projection, 10-20 m long and 30-60 cm thick, which has quenched glassy margins. The sediment layers are 10-25 m long and 30-100 cm thick, and consist of massive diatomaceous mudstone. Contacts between the basaltic andesite sills and sediment layers are mostly sharp. In places, the margins of the basaltic andesite sills are brecciated, forming peperite zones 50×100 cm wide. The peperite consists of polyhedral clasts of basaltic andesite 5-20 cm across, separated by massive, indurated mudstone. Some large clasts show a jigsaw-fit texture. The interlayered sill sediment structure is inferred to have formed by parallel, sheet-like injections of basaltic andesite magma along bedding planes of wet and poorly consolidated sediment in an upper bathyal marine environment (several hundred metres deep). During parallel injections of the basaltic andesite magma into the wet sediment, a large amount of water vapour was generated in the wet sediment, and thin water vapour films were maintained continuously at the magma-sediment interface, insulating the magma from the host sediment. Following injections of the magma into the wet sediment, the temperature of the magma declined, so that the water-vapour films on the magma gradually disappeared. The breakdown of the insulating vapour films at the magma-sediment interface resulted in quench fragmentation of the magma, generating small zones of peperite around the basaltic andesite sills.