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The Tertiary sediments in the Tsuyazaki area rest on the Cretaceous granodiorites with nonconformity and is covered with olivine basalt flow. This Tertiary sequence, which attains about 430 to 640m in... thickness, is divided into two formations, the Tsuyazaki Formation and the Yamaga Formation in ascending order. The former is of non-marine deposits which are characterized with striking volcanic sediment cemented usually with stilbite and are intercalated with red siltstone and coaly shale, bearing not uncommonly silicified woods, and the latter comprises conglomerate, glauconitic sandstone and sandy siltstone, which all contain numerous remains of sea-shells characteristic of the "Upper Oligocene Ashiya fauna". The Tsuyazaki Formation is correlated from palynological evidence to the Onga Formation of the Otsuji Group (Oligocene) in the Chikuho coal-field of north Kyushu, and the Yamaga Formation is regarded as an extension of the basal part of the type Yamaga Formation of the Ashiya Group (Upper Oligocene) in the same area. These Tertiary strata form a syncline plunging north-northwe~t, being modified with some major faults, especially the "Chikuho type" fault trending northwest. Some remarks have been given to the significance of the acid to intermediate volcanism during the period of sedimentation.show more
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