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The North Kitakami Belt of Northeast Japan, which consists of a Jurassic accretionary complex, is divided by the Iwaizumi Tectonic Line into two sub-belts, the Kuzumaki–Kamaishi and Akka–Tanohata sub-belts, which differ in terms of the dominant feldspar type in sandstone and the presence or absence of Paleozoic oceanic sedimentary rocks. We carried out a detailed study of the geology of the boundary area between these two sub-belts in the west Akka area in Iwaizumi Town, Iwate Prefecture. Our survey resulted in the identification of three tectonostratigraphic units. From lower to upper (east to west), these are the Takayashiki (mixed facies), Seki (coherent facies of chert–clastic sequences associated with thick sandstone layers), and Otori (coherent facies of chert–clastic sequences and dominant mudstone) units. Paleozoic fossils occur only in the Seki and Otori units, supporting the criterion proposed previously for identifying the location of the Iwaizumi Tectonic Line. However, the dominant feldspar composition of sandstone changes gradually from plagioclase-dominant in the Otori Unit, through equal amounts of both plagioclase and Kfeldspar in the Seki Unit, to K-feldspar-dominant in the Takayashiki Unit. Thus, the feldspar compositions from west to east are not consistent with the proposed location of the tectonic line. The tectonostratigraphic units and sandstone compositions of the North Kitakami Belt are well correlated with those of both the Oshima and South Chichibu belts, which are the northern and southern tectonic extensions of the North Kitakami Belt, respectively. However, the oldest age of each unit is different between these three geologic belts. Paleozoic fossils have not been reported from the Shimonosawa Unit of the Oshima Belt, which corresponds to the Seki Unit, whereas the Sambosan Unit of the South Chichibu Belt, which can be correlated with the Akka–Tanohata sub-belt, yields Paleozoic fossils.

Subdivisional scheme of the North Kitakami Belt, Northeast Japan and its tectonostratigraphic correlation to the Oshima and South Chichibu belts: