Geology and Paleontology of the Takakurayama-Yaguki Area, Yotsukura-cho, Fukushima Prefecture

Geological and paleontological studies were made of the structurally complex and paleontologically little known Takakura-yama--Yaguki area situated in the hilly region west of Yotsukura-cho, Iwaki City, Fukushima Prefecture. The terrain, made geologically complex by the many major reverse and thrust faults accompanied with normal ones and intimately associated with large intrusive rock bodies has been classified into two groups and six formations and one member. Of the two groups, the lower or Niidagawa (Devonian-Carboniferous) chiefly consists of metamorphic rocks whereas the upper or Takakura-yama (Early to Middle Permian) is of normal sedimentaries. Paleontological evidences (mainly fusulinids, brachiopods and molluscs) for geological age determination are found in the Takakura-yama Group but not in the Niidagawa, which age is judged based upon correlation with formations in the northern part of the Abukuma Massif (adjoining the present area) and the southern part of the Kitakami Massif of similar geostructure, lithological resemblance and stratigraphical relationship with subjacent and superjacent units. The metamorphic rock series (Niidagawa Group) seems to have originally been of marine neritic origin, but it is also assumed that the original sedimentary environment was not favorable to shallow water marine organisms. The sedimentary environment of the Takakura-yama Group was not an excellent one for the growth and flourishing of marine organisms, judged from the abundant dwarf forms. From the occurrence of many planktonic organisms (fusulinids, ammonites, nautiloids) collected from the muddy sediments, it is judged that the sedimentary basin had free connection with the open sea