Identification of basement rocks and their characteristics in the Naruto area, Northeast Chiba, Japan, based on newly found Triassic radiolarian fossils

Abstract

Quaternary sediments are about 2,000 m thick in the area of Sanmu City, northern part of Chiba Prefecture, on the Kanto Plain. Although the pre-Neogene basement is considered to be a Jurassic to Early Cretaceous accretionary complex, there are no direct constraints on the age of the complex. In this area, we obtained Triassic radiolarian fossils from bedded chert in drillcore recovered from 1,656 m below mean sea level at the Naruto FA-68 well drilled by INPEX CORPORATION. Because Triassic chert in Japan is limited to Jurassic to Early Cretaceous accretionary complexes, this rock type makes up the pre-Neogene basement in the area. The depth of the surface of pre-Neogene basement in surrounding boreholes is variable due to differential erosion.