Reexamination of the Sagara Fauna: Middle Miocene molluscan assemblage from the Sugegaya Formation Sagara Group, Shizuoka Prefecture, central Japan

Volume: 1Start Page: 110End Page: 12

関東山地南東部多摩川流域の中・古生層の層序

The Tamagawa area in the southeastern part of the Kawanto Mountains geotectonically occupies the eastern extension of the Chichibu Terrane of the Outer Zone of Southwest Japan. Fossiliferous Upper Paleozoic and Mesozoic formations are distributed in several subparallel belts striking NW-SE which are separated by thrust faults. The older strata such as the Lower and Upper Carboniferous crop out along some of these thrust faults. On the other hand the Upper Permian and Upper Triassic rocks are found in synclinal parts (Figs. 3-4). Strstigraphic successions of these formations are summarized in Fig. 2. The Takamizuyama Formation, the oldest unit, attains 500 m in thickness and is characterized by chert, siliceous shale, sandstone and basic pyroclastic rocks (Fig. 6). In association with basic lava and tuff, a bedded dolomitic limestone of about 15 m thick is intercalated in chert beds of the upper part. Late Visean to early Namurian conodonts and corals cocur from the limestone. The Kitaosoki Formation, about 200 m thick, is divided into the lower and the upper subformation by a thin (1 to 2 m thick) limestone conglomerate between the two (Fig. 7). The lower subformation, about 150 m thick, consists chiefly of black mudstone and fine to medium grained sandstone, and is accompanied with pale gray lenticular limestones an.d basic pyroclastic rocks at various horizons. The limestones carry rich and varied fusulinacean foraminifers including Profusulinella, Fusulinella, Beedeina, Fusulzna, Protriticites and Triticites. The limestone conglomerate at the basal part of the upper subformation seems to record evidence of erosion of a part of the lower subformation. The Permian-Carboniferous boundary in this area can be drawn between the two subformations. It is known that the upper half of the so-called Fusulinella-Fusulina zone and the Triticites zone are lacking in many Paleozoic areas (Akiyoshi, Atetsu and Taishaku limestones) in the Inner Zone of Southwest Japan. Strata of these ages in the Japanese Islands are, however, represented by the Yayamadake limestone in the Kuma Mountains (KANMERA, 1952) and also by the Kitaosoki Formation of this area. The basal limestone conglomerate of the upper subformation is succeeded by medium to coarse grained sandstone and mudstone, in which a considerable number of lenticular limestones are intercalated. The limestones are characterized by the occurrence of schwagerinid fusulininans such as Triticites, Pseudoschwagerina, Paraschwagerina, Pseudofusulina and Schwagerina. The Raidenyama Formation, about, 400 m thick, conformably ovelies the Kitaosoki and is characterized by medium to coarse grained massive sandstone and intraformational conglomerate of 5 to 50 m thick at several horizons. In the southern part of the studied area, it consists of sandstone, basic tuff and limestone (Fig. 8). Several species of Misellina, Neoschwagerina and Cancellina occur from the limestone of this formatiton. The Nariki Formation is widely distributed in the area along the Nariki and Naguri rivers, forming gentle folds of about 1 km half-wave length. It consists mainly of coarse grained calcareous and feldspathic sandstone, calcareous shale and subordinately of pale gray limestone, basic pyroclastic rocks and chert (Fig. 9). Neoschwagerinid fusulininans such as Neoschwagerzrina craticulifera, N. margaritae and Yabezina globosa occur from the limestones of this formation. The Futamatao Formation, a representative of the Upper Permian Kuma Series, is narrowly exposed along the fault zone limiting the southern border of the Futamatao belt. It consists mainly of sandstone, mudstone and conglomerate, with a Lepidolinamultiseptata-bearing limestone at the middle part (Fig. 10). The conglomerate intercalated in the lower part (Fig. 11) contains cobbles of various kinds of igneous rocks. The Carnian Tachigawa Formation which ontains Oxytoma mojsisovicsi" bed (OZAWA and HAYAMI, 1969) and the Norian Ishigami Formation are narrowly distributed along thrust faults in the eastern part of this area. These strata consist chiefly of sandstone and conglomerate. With a distinct lithological contrast to these formations, chert-rich Triassic strata, here called the Sawai Formation, are distributed in the central part of the mapped area. They are characterized by chert, silicious claystone, sandstone and alternating beds of sandstone and shale. Anisic to Noric condonts occur from some chert beds. The Takamizuyama, Kitaosoki, Raidenyama, Nariki and Futamatao Formations in the Tamagawa area provide us a good reference section of the Upper Paleozoic in the Japanese Islands. The fusulininan foraminifers obtatined from the Carboniferous and Permian formations are systematically listed with notes on collecting localities and stratigraphic horizons. Some comments are also given with respect to the geologic age and correlation of the Paleozoic formations based on the foraminifers. The geolgic structures in this area are also outlined