Reconstructed oceanic plate stratigraphy of the Ino Formation in the Ino district, Kochi prefecture, central Shikoku, Japan

Abstract

The Ino Formation, which has been regarded as a member of the Kurosegawa terrane, is distributed in the Ino district, central Shikoku, Japan. It is composed of metamorphosed and unmetamorphosed rocks. Radiolarians in the Late Triassic to Early Jurassic strata are found in the unmetamorphosed components. The Late Triassic radiolarians were collected from chert and siliceous mudstone. The Early Jurassic radiolarians are discriminated from the siliceous mudstone and mudstone samples. Based on chronological data and findings from previous studies, the Oceanic Plate Stratigraphy (OPS) of the unmetamorphosed part of the Ino Formation is reconstructed as follows: Late Carboniferous to Late Triassic (or earliest Jurassic) bedded chert, Late Triassic (late Carnian) to Early Jurassic siliceous mudstone and Early Jurassic mudstone in ascending order. Based on this observation, it is confirmed that the unmetamorphosed part of the Ino Formation is an Early Jurassic accretionary complex. Its origin can be attributed to the Chichibu terrane. The reconstructed OPS of the unmetamorphosed part of the Ino Formation is possibly correlated to the Yusugawa, Sumaizuku and Kamiyoshida units of the Northern Chichibu terrane or the unknown oldest unit of the Southern Chichibu terrane.