Abstract
The Miocene Koura Formation is distributed mainly in the eastern part of the Shimane Peninsula, southwest Japan. The lower formation was accumulated in a shallow fresh water lake or a fluvial environment. In contrast, the middle formation was accumulated in a lake shallowed over time. The upper formation consists of shallow blackish lake deposits. The appearance of hummocky cross-stratification in the upper formation reveals that the lake became wider in this phase. A predominance of coarse sediments with slumped deposits in the upper formation implies that it was of a fan delta origin. The overlying Josoji Formation is of a marine origin, comprising the deposits of the climax phase of the opening of the Japan Sea.
On the bases of the middle and upper formations, a conglomerate and sandstone interval of up to 10 m overlies the underlying terrestrial sediments and grades upward into a lake deposit. The widespread distribution of these two sedimentary intervals implies that each interval was deposited from a basin-wide flow event during a rising lake level. Each interval was probably deposited from a lake outburst that resulted from a connection of basins, which commonly occurs in the initial phase of rifting. The connection of this basin with a marine environment probably occurred in the phase of the middle-upper formation boundary prior to the climax phase of the opening of the Japan Sea.
