Northern and Middle Chichibu Belts of the Eastern Part of the Kanto Mountains, Central Japan

Abstract

The stratigraphy, geologic structure and ages of the Mesozoic strata exposed in the eastern part of the Kanto Mountains, central Japan, are discussed based on recent fossil findings. The Jurassic formations trending WNW-ESE direction, are subdivided into the following six formations from north to south: the Kuroyama, Takahata, Kabasaka, Hanagiri, Nakato and Nitayama Formations. The Middle Jurassic Kuroyama Formation is composed of chaotic rocks consisting of exotic blocks of Permian to Early Jurassic chert, Carboniferous to Permian limestone and volcaniclastic rocks in a shaly matrix. The Takahata Formation consistsof Middle Jurassic shale and volcaniclastic rocks with Permian limestone blocks. The Kabasaka Formation is characterized by a chert-clastic sequence, which is made up of Triassic to Early Jurassic chert and overlying Middle Jurassic shale. The Early Jurassic Hanagiri Formation is composed of chaotic rocks consisting of exotic blocks of Permian to Early Jurassic chert, Permian limestone and volcaniclastic rocks in a clastic matrix. The late Early Jurassic Nakato Formation is composed mainly of chaotic rocks consisting of exotic blocks of Permian to Early Jurassic chert in a shaly matrix. The Nitayama Formation is characterized by chaotic rocks comprising of Permian to Early Jurassic chert and Permian limestone in a Middle Jurassic shaly matrix. The first three formations are in contact with vertical or steeply southward-dipping faults. On the contrary, the latter three are in contact with northward-dipping reverse faults. The Koma Orbitolina Formation, which consists mainly of Early Cretaceous calcareous shale and sandstone unconformably overlies the Takahata Formation in the most eastern part of the studied area.
Based on stratigraphical and structural features and radiolarian dating, the Kuroyama, Takahata and Kabasaka Formations are correlative with the Northern Chichibu Belt (northern portion of the three folds Chichibu Belt) in the Outer Zone of Southwest Japan. The Kabasaka Formation is regarded as an accretionary wedge formed by offscrape-accretion mainly during Middle Jurassic time. The Hanagiri, Nakato and Nitayama Formations correspond to the Middle Chichibu Belt and are presumed to be products of the convergent complex of an oceanic plate during Early to Middle Jurassic times.