The Upper Jurassic-Lower Cretaceous carbonate-terrigenous succession and the development of a carbonate mound in western Shikoku, Japan

The stratigraphy and sedimentology of the Upper Jurassic-Lower Cretaceous Torinosu Group were studied in western Shikoku (western Kochi Prefecture). The Torinosu Group is interpreted as a transgressive succession deposited in a foreland basin. The group can be divided into a lower coarse siliciclastic unit (the Tsukadani Formation) and an upper carbonate and fine siliciclastic unit (the Yatsuji Formation). The Tsukadani Formation is characterized by conglomerates with chert gravel and probably represents submarine fan deposits. The gravels were derived from an exposed accretionary wedge containing pelagic siliceous rocks. The Yatsuji Formation includes a limestone unit up to 150 m in thickness and a few kilometres in lateral extension, which comformably rests on siliciclastic rocks and marlstone. The limestone unit is interpreted as a carbonate mound and laterally shows considerable variation in thickness. The conglomerate with limestone gravels which infills the channel and flanks of the mounds supports a topographic relief associated with the carbonate build-ups. The facies of the limestone unit changes vertically from marlstone to stromatoporoid facies. Change in fossil components associated with a decrease of terrigenous components indicates that the initial community was replaced by the reef-forming community which is interpreted as a response to the decrease of terrigenous influx. Periodical input of terrigenous sediment originating from movement of thrust mass interrupted the carbonate deposition.