TSUNAMI-INDUCED CONGLOMERATES IN MIOCENE UPPER BATHYAL DEPOSITS, CHITA PENINSULA, CENTRAL JAPAN

The Tsubutegaura conglomeratic tsunamiites occur in the middle section of the Miocene storm-related sand-silt alternation system in the Chit a Peninsula, central Japan. Deposition of the system in the upper bath yal environment in a bay and synsedimentary seismic activity has been elucidated by palaeontological, palaeogeographical and sedimentologica l studies. Two coupled units, each built up by a conglomerate layer wi th an overlying sandstone layer and the alignment of lenticular sedime ntary bodies are exceptional in this sequence. Some typical lenticular bodies, confined within sedimentary troughs, consist of a boulder bea ring conglomerate layer and calcareous sandstone layer of the lower co uple, and another conglomerate layer of the upper couple. The tuffaceo us sandstone layer of the upper couple is distributed more widely than the other lithologies. The framework gravels in the coupled conglomer ates form a clast-supported fabric and are quite notably monomictic. T he clasts are angular and imbricated partly, forming peculiar gravel c lusters due to traction current transportation. Conspicuous lamination s develop in the coupled sandstone layers. Antidunes with chute and po ol structure in the upper couple sandstone layer indicate deposition f rom an upper flow regime current. On the other hand, the calcareous co mposition of the lower couple sandstone layer reveals shallow-water pr ovenance. Thixotropic deformations and diastasis cracks in the siltsto ne bed immediately beneath the layers of the couples show that a sever e earthquake tremor and a rapid change of water pressure occurred just before the deposition of the tsunamiites. Submarine debris flows due to collapse of a fault scarp in a shallow bank and the ensuing wash by two pulses of the tsunami-induced ebb current comprise the scenario f or the formation of the Tsubutegaura conglomeratic tsunamiites.