Abstract
The Edosaki observation well was located at Edosaki Town, Ibaraki Prefecture, and reached at the depth of 1234.0 m. The basement rocks of the northeastern part of the Kanto Plain have been known as the Ryoke belt on the northern side of the Median Tectonic Line. The stratigraphy at this well is the Shimosa Group, Kazusa Group and the pre-Neogene clastics in descending order. The cores were recovered from three intervals; 843.40~845.15 m (pebbly mudstone), 1037.00~1039.70 m (fine-grained sandstone), and 1205.30~1207.70 m (black mudstone) . Among them, the sandstones from the middle interval were examined by modal composition and chemistry of detrital garnet. The sandstones were also collected from several localities in order to compare with the pre-Neogene, that is, sandstones from the Choshi, Nakaminato Groups, Sanchu Cretaceous, and Yorii and Atokura Formations. Our study proves that the sandstone of the Edosaki core is mainly classified into feldspathic wacke in QFR diagram and Dissected to Transitional Arc in QmFLt diagram, and the sandstone corresponds probably to the Atokura Formation. The detrital garnets seem to be less different from those of the Atokura Formation. The Atokura Formation lies on the Sambagawa metamorphic rocks as a Klippe in the Kanto Mountains. Therefore the Edosaki observation well is situated not above the Ryoke belt but above the Sambagawa belt.
