Tsunami-induced conglomerates in Miocene upper bathyal deposits, Chita Peninsula, central Japan

Shiki, T.; Yamazaki, T.

Sedimentary Geology 104(1-4): 175-188


ISSN/ISBN: 0037-0738
DOI: 10.1016/0037-0738(95)00127-1
Accession: 020524604

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The Tsubutegaura conglomeratic tsunamiites occur in the middle section of the Miocene storm-related sand-silt alternation system in the Chita Peninsula, central Japan. Deposition of the system in the upper bathyal environment in a bay and synsedimentary seismic activity has been elucidated by palaeontological, palaeogeographical and sedimentological studies. Two coupled units, each built up by a conglomerate layer with an overlying sandstone layer and the alignment of lenticular sedimentary bodies are exceptional in this sequence. Some typical lenticular bodies, confined within sedimentary troughs, consist of a boulder bearing conglomerate layer and calcareous sandstone layer of the lower couple, and another conglomerate layer of the upper couple. The tuffaceous sandstone layer of the upper couple is distributed more widely than the other lithologies. The framework gravels in the coupled conglomerates form a clast-supported fabric and are quite notably monomictic. The clasts are angular and imbricated partly, forming peculiar gravel clusters due to traction current transportation. Conspicuous laminations develop in the coupled sandstone layers. Antidunes with chute and pool structure in the upper couple sandstone layer indicate deposition from an upper flow regime current. On the other hand, the calcareous composition of the lower couple sandstone layer reveals shallow-water provenance. Thixotropic deformations and diastasis cracks in the siltstone bed immediately beneath the layers of the couples show that a severe 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 for the formation of the Tsubutegaura conglomeratic tsunamiites.