Miocene Offshore Tractive Current-Worked Conglomerates -Tsubutegaura, Chita Peninsula, Central Japan- (Chapter 4: SLOPE TO DEEP-WATER FACIES)

Abstract

The Tsubutegaura Conglomerates occur in the middle part of the Miocene Morozaki Group, developed in southwestern Chita Peninsula, central Japan. Most of the group is characterized by storm-induced deposits from the uppermost bathyal environment (200~400 m deep). The Tsubutegaura Conglomerates, consisting of several hemilenticular or bunch-shaped bodies 10 to 40 meters across and a few meters high, were strictly distributed at one horizon in the storm sequence. Some of the hemilenticular bodies consist of a few layers. The framework gravels of the layers are angular to subangular and form clast-supported fabrics; a mud matrix is absent. The gravel size is variable, from pebbles to giant boulders. The finer gravels are characteristically imbricated in many places; the larger ones, i.e. boulders, often form gravel clusters as seen in alluvial channels. Most of the gravels in the Tsubutegaura Conglomerates are of Ryoke gneissose rocks, which are assumed to underlie the Morozaki Group. Very undurable soft siltstone and carbonized woods are minor constituents. No simple mass transportation model can synthetically explain all these sedimentary features; only submarine tractive current-worked deposits after some mass movements seem to be possible. Synsedimentary fault movements, previously clarified by other authors, are consistent with the idea of a submarine avalanche and a subsequent wash by offshore tractive currents induced by a tsunami.