A HOT MIOCENE SUBAQUEOUS SCORIA-FLOW DEPOSIT IN THE SHIMANE PENINSULA, SW JAPAN

A subaqueous scoria-flow deposit occurs in the Miocene Takashibiyama F ormation in SW Japan. Faunal assemblages in associated sediments sugge st an outer shelf to upper bathyal environment, with water depth likel y between 200 and 1500 m. The deposit, 5-7 m thick, is similar to suba erial scoria flow deposits, containing mainly non- to moderately vesic ulated hyalopilitic basaltic andesite lapilli and coarse ash. The esse ntial basaltic andesite clasts are variably vesiculated even within si ngle clasts, and are commonly cracked with curviplanar surfaces, where as the inner parts are cracked in a pattern similar to perlitic cracks . The clasts are blocky with curviplanar surfaces if non- to poorly ve siculated, and are fibrous with irregular shapes and flattened by comp action if more vesiculated. At one proximal location, fines-depleted, rootless pipes 5-90 cm wide and 1-3 m long occur, and spherical to amo ebic vesicles 0.01-0.3 mm in diameter and filled with stilbite, quartz or greenish-brown clay are common in the fine-ash matrix and in silts tone clasts contained in the deposit. The vesicles represent open spac es once supported by gas, and the rootless pipes are interpreted as ga s-escape pipes. Magnetization vectors of essential clasts are disperse d not at random but around a single direction, suggesting that the cla sts gained thermoremanent magnetism mainly during cooling in the flow or during post-emplacement cooling and then rotated to some extent by differential movement of the flow or by gas-escape in the deposit. The vesicularity, shape and texture of essential clasts suggest a phreato magmatic eruption, involving magmatic vesiculation and dynamic interac tion of magma and ambient water. The perlitic cracks in the inside of the essential clasts and the paleomagnetic vectors of the essential cl asts indicate that the flow was initially hot and that the deposit coo led slowly. Internal depositional features change rapidly over only 50 0 m. Rootless pipes and vesicles in the fine-ash matrix and siltstone clasts are therefore absent at distal locations, indicating that an in itially cohesionless debris flow quickly became sufficiently turbulent to mix with ambient seawater.