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| Abstract |
The Onuma Formation, which is a basal one of the Kumano Group in the northern district of the Kumano basin, has a maximum thickness of 800-900 m, and is lithologically divided into the Komatsu, Kohno-...ue and Kawabata members in ascending order. The Komatsu Member consists of basal conglomerate and sandstone, and is characterized by a considerable variation of thickness. The Kohno-ue Member is composed mainly of massive to thick-bedded dark grey siltstone, of which silicified and hardened rock is known as 'Nachi-guro' stone. The Kohno-ue Member is estimated to be of offshore sediments under the continental shelf or more deeper environments judging from fossil habitat. The Kawabata Member is made up largely of rhythmically alternating beds of sandstone and mudstone, and contains a channel-fill conglomerate lens in the lower part and pebbly mudstone closely associated with slump beds in the middle and upper parts. This member is interpreted as a turbidite clastic wedge which was deposited in a continental slope basin having some submarine canyons, which inclined toward the south or southwest. The clastic wedge thins away to the southward. A detailed field-research has con6ned that the Onuma Formation is correlative with the lower part of the Koguchi Formation in the southern district and that the Kawabata Member of the Onuma Formation and a part of the Shikiya Member of the Koguchi Formation in the southern district are heteropic.show more
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