The geologic unit comprising the Jurassic accretionary complex in the Kaminokuni area in the Southwestern Hokkaido is newly defined as the Kaminokuni Terrane. It is composed of the Matsumae Group which is characterized by overlying and underlying tectonic units forming the two-storied structure. The Matsumae Group of the overlying unit (unit 2) consists of the a, b and c formations in ascending order, while the underlying unit (unit 1) is composed mainly of the b formation. These three formations are tectonically repeated within both units.
The a formation is composed of the oceanic greenstone and bedded chert of Late Carboniferous through Permian age and the lower part of bedded chert includes bedded manganiferous iron oxide and manganese ore deposits, in ascending order. The nature and age of formation of these deposits are camparable to those of the Outer Zone of Southwest Japan but are dissimilar to those of the Inner Zone. The b formation consists of Middle to early Late Jurassic mudstone including olistoliths of Permian greenstone, bedded chert reworked from the a formation, and Triassic to Early Jurassic bedded cherts derived from the strata of the neighboring Kamiiso or Toi-Esan Terrane. The c formation is composed mainly of early Late Jurassic sandstone of which the provenance is regarded to be the continent or island arc.
The two-storied structure of the Kaminokuni Terrane was formed during the underplating through the duplex accretion to the South Kitakami massif in post Late Jurassic time.