Abstract
The Jurassic accretionary complex of the Ashio Terrane, distributed in the Kuromatagawa region of Niigata Prefecture, central Japan, is divided into two tectonostratigraphic units; the Kuromatagawa and the Oshirakawa Complexes, based on lithology and geologic age.
The Kuromatagawa Complex is composed mainly of coherent sequences of interbedded sandstone and shale, and slabs of basaltic rocks and chert ranging from 300 to 500 m and over in thickness. The Oshirakawa Complex consists of melange-type rocks and slabs of basaltic rocks, limestone, chert and sandstone ranging from 50 to 300 m in thickness. It is characterized by melange-type rocks including basaltic rocks and sandstone less than 50 m in thickness within pelitic matrix. According to radiolarian fossils obtained from shale, the accretionary age of the Kuromatagawa Complex ranges from middle Middle to early Late Jurassic (late Bathonian - Oxfordian), and the Oshirakawa Complex was accreted in middle Early Jurassic (late Sinemurian - early Pliensbachian).
Considering the lithology and geologic age, the Kuromatagawa and the Oshirakawa Complexes are correlated with tectonostratigraphic units of the Tamba-Mino Terrane in the Inner Zone of Southwest Japan. The Oshirakawa Complex is one of the early Jurassic complexes which is the oldest in the Tamba-Mino-Ashio Terrane, and occupies the structurally uppermost unit in the Tamba-Mino-Ashio accretionary wedge.
