Geology of the Omoto-Tanohata District, Outer Kitakami Belt, Northeast Japan

Stratigraphical and sedimentological studies in the Omoto-Tanohata district in the northeastern part of Iwate Prefecture, Northeast Japan, resulted in the recognition of a large scale overturned structure with isoclinal folding. The new order of the stratigraphic sequence of the Omoto-Tanohata district was established and the previously accepted wide distribution of the rocks of Cretaceous age should be substantiated for the Upper Jurassic System. This revised procedure has resulted in the classification of problems related with correlation, lithological similarity and age relationships within the Mesozoic terrain of the northern Kitakami massif. The Mesozic and the rocks of unknown age distributed in the district studied are classified on the basis of the lithology as shown in Fig. 3, and divided into four structural zones (I-IV) as shown in Fig. 2. The Older Formation of unknown age (I) is distributed in the narrow area of the southwestern part of the district. It trends N-S to NNW-SSE and is separated from the Pre-Miyakoan Mesozoic formations (II-III) by the "Taro tectonic line" of nearly NNW-SSE trend of Onuki (1963) and Kano (1950). Granodioritic intrusions occur along this thrust zone. The Pre-Miyakoan Mesozoic rocks are widely distributed in the central part of the district and most of them consist of the overturned eastern wing of the Shimizuno anticline with the folding axis trending NNW-SSE in the southern part and NNW-SSW in the northern part and plunging towards the south. These structures were confirmed chiefly by the graded-bedding and other sedimentary structures, as shown in Plates 1, 2. In the Shimizuno area, the Magisawa Formation, the lowest of the Pre-Miyakoan Mesozoic stratigraphic units is distributed in the V-shaped area owing to the plunging of the anticlinal axis and other Mesozoic Systems, namely, the Koshimeguri, Omoto and Harachiyama formations are developed in ascending order with conformity along the southern outerside of the Magisawa Formation. This successive deposition of the strata suggests the continuous development of the sedimentary basin in the Omoto-Tanohata district, during the Late Jurassic to the Early Cretaceous. The Upper Jurassic so-called "Torinosu-Type" Hexacorals and Stromatoporoids were collected from the Koshimeguri Formation (=Iwaizumi Formation of Hase) by Hase (1956). And the Omoto Formation yielded molluscan and plant fossils correlating to the Early Cretaceous Kochian, in age. Therefore the geologic age of the Magisawa Formation (=Matsumaezawa Formation of Suzuki, 1957), the lowest of the Mesozoic stratigraphic units is not Early Cretaceous as has been shown by Suzuki (1967) and Onuki et al. (1960), but Late Jurassic. The rock facies and the fossils of the Omoto Formation represent a brackish to shallow water marine environment. The upper part of the formation is intercalated with andesitic pyroclastics and this means the begining of the igneous activities. The Harachiyama Formation represents the maximum phase of submarine volcanisms. The formations mentioned above, dipping mostly towards the west, are isoclinally folded, intruded by granodiorites and are overlain with the Miyako Group (IV), dipping gently towards the east, with clino-unconformity. This conspicuous contrast of the geologic structures and rock facies between the Pre-Miyakoan Mesozoic System and the Lower Cretaceous Miyako Group is the most striking feature of the geology of the district. The Miyako Group is composed of basal conglomerate conformably overlain with fossiliferous calcareous sandstone, which show one unit of a sedimentary cycle and a nearshore environment. As shown in Fig. 12, the harmonious trends of the granodioritic intrusions and folded Mesozoic structures seem to suggest the close relationship between them. On the basis of the noteworthy wide distribution of the Upper Jurassic System recognized in the Omoto-Tanohata district, the expectation of the Jurassic formations among the unclassified rocks widely distributed in the northern outerside of the area studied is probable