Studies of Japanese Triassic foraminifers

本研究首次阐明了日本三叠纪有孔虫与与日本前白垩纪构造密切相关的石灰岩相的总体面貌。他们的研究结果是重要的基础数据，可用于特提斯和环太平洋地区三叠纪有孔虫动物群对比、古生物地理和构造发育的讨论。黑河和麦祖鲁带的三叠纪灰岩呈透镜状夹在具有许多三叠纪浅海动物群的碎屑架中。黑河史密斯灰岩逐渐变成周围的黑色泥岩，密密麻麻地堆积着含有稀有有孔虫的小型薄壳双壳贝类(Onquina石灰岩)。黑河有孔虫动物群主要分布在泥粒砾岩和生物碎屑砾岩中，含有大量的碎屑石英颗粒和岩石碎屑。它们包括致密球体螺旋体、南方锥体螺旋体、淀粉芽孢杆菌等，它们是欧洲Anisian的索引物种。三叠纪石灰岩在南赤部带分布最广，与黑河动物群相似。在其南部(桑博山带)，大量巨大的三叠纪石灰岩区块与奇异的玄武岩和硅质岩区块共生。桑博山带的远洋石灰岩产有许多晚三叠世的有孔虫，可归属于角盘类和其他渐开线类，巨齿形双壳类是上三叠统特提斯地区的典型。在南奇奇布带的北部，下(史密斯期和斯巴西亚期)、中(安尼西期)和上三叠世(诺里安)灰岩彼此不整合。科奎纳灰岩贯穿始终，上部以放射虫石灰岩为主。有孔虫动物群似乎与其他日本同时代的动物群在从属渐开虫类方面有很大的不同。

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.