The area investigated around Mt. Nikoro-yama is made up of the Mesozoic sedimentary and volcanic rocks which are divided into the Yubetsu Group, Nikoro Group, and Saroma Group. The Nikoro Group is composed mainly of greenstones such as basaltic and trachytic pillow lavas,, hyaloclastites, dolerite and trachyte dykes, ultramafic cumulates, and a small amount of chert and limestone. Volcaniclastic conglomerate, sandstone, siltstone, and shale are associated with the greenstones and are found in the Lower Part of the Nikoro Group. The volcanic piles of the Nikoro Group are considered as remnants of Jurassic seamounts from the following reasons; (1) explosive volcanism producing a large amount of hyaloclastite and strongly vesiculated pillow lavas, (2) alkali basalt magmatism from alkali olivine basalt to trachyte, (3) presence of volcaniclastic sedimentary rocks, and (4) lack of terrigenous sediments. Boulder and cobble conglomerate observed at the base of the Saroma Group irregularly covers greenstones of the Nikoro Group. No evidence of submarine basaltic volcanism is recognized in the Saroma Group composed entirely of clastic sedimentary rocks. The rapid change of lithofacies from the Nikoro to Saroma Groups as well as the distribution and occurrence of the basal conglomerate might indicate the presence of unconformity The basal conglomerate contains large numbers of well-rounded boulders and cobbles; 58〜78 modal % pyroxene andesite, 0〜10% hornblende andesite, 0〜2% rhyolite, 0〜4% monzonite, 0〜16% porphyritic monzonite, 0〜2% monzogabbro, 0〜2% slate and sandstone, 2〜4% trachyte, 5〜18% basalt and hyaloclastite. The abundance of calc-alkaline andesites and alkaline plutonics suggests that the materials have been derived mainly from a continental margin or an island arc. In this connection, it is noticed that the similar rock types are known in the Cretaceous formations of the Nemuro Group. Accordingly, the boulders and cobbles are considered to be originated in the Paleo-Kurile Arc in late Cretaceous time. At the time forming the unconformity, seamounts of the Nikoro Group might have been emplaced to the western margin of the Paleo-Kurile Arc, where a large amount of terrigenous elastics were supplied.

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