Studies of Japanese Triassic foraminifers

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The present study has first clarified the general aspect of the Japanese Triassic foraminifers, in relation to limestone faces deeply concerned with the Japanese pre-Cretaceous tectonics. Their results are important as the basic data and available for the discussion on the Triassic foraminiferal faunal comparison, paleo-biogeography, and tectonic development of Tethyan and Circum-Pacific regions.Triassic limestones in the Kurosegawa and Maizuru Belts are lenticularly intercalated within clastic racks having many Triassic neritic faunas. The Kurosegawa Smithian limestone, gradually changing into the surrounding black mudstone, is packed densely with small thin-shelled bivalves (onquina limestone) containing rare foraminifers. Most of the Kurosegawa foraminiferal faunas are found in ooid grainstone and bioclastic grainstone containing large amount of detrital quartz grains and rock fragments. They consists of Glomospira densa, Meandrospira dinarica, Arenovidalina amylovoluta, and many others, index species of the European Anisian. Similar faunas to those of the Kurosegawa are also obtained from the Maizuru.Triassic limestones are most widely distributed in the South Chichibu Belt. In its southern part (Sambosan Belt), numerous huge Triassic limestone blocks are prevailing in association with exotic blocks of basaltic rocks and chert. Pelagic limestone of the Sambosan Belt yields many late Triassic foraminifers assignable to Angulodiscus and other involutinids, and megalodontid bivalves typical to the Upper Triassic Tethyan Realm.In the northern part of the South Chichibu Belt, Lower (Smithian and Spathian), Middle (Anisian), and Upper Triassic (Norian) limestones are unconformable in each other. The coquina limestone is throughout and radiolarian limestone is dominant in their upper part. Foraminiferal faunas seem to be rather dissimilar to other Japanese contemporaneous ones with respect to subordinate involutinids.