Geological study of the Neogene basins in the Horonobe-cho

This report presents the regional and detailed geological study and shallow drilling programs of the Neogene basins in the Horonobe-cho for the Horonobe Underground Research Program at Horonobe-cho, Hokkaido by the Japan Nuclear Cycle Development Institute. The data obtained from the deep drill-holes near the site of the Underground Research Laboratory have not been clearly correlated with the surface geological data from the regional area and even the area around the site of the Underground Research Center. Therefore, the purpose of this study is to confirm the regional geology and detailed local geology confined the site of Underground Research Laboratory, and to understand the geological characteristics of the Omagari Fault that passes through the west part of Horonobe-cho. The study area in underlain chiefly by sedimentary rocks from the middle Miocene to the Pleistocene periods. The Neogene to Pleistocene rocks are divided into Soya coal bearing, Masuporo, Wakkanai, Koetoi, Yuchi, and Sarabetsu Formations in ascending order. The Neogene to Pleistocene rocks are cut by several north-south running faults, Omagari Fault and Nukanan Faults, in the Horonobe-cho area. The Omagari Fault is thought to be reverse and dip east. Region field mapping including detailed field mapping confined around the site of Underground Research Laboratory was conducted along four east-west traverse routes that was set to intersect the direction of regional geological structures. More than 300 samples were collected and conducted various tests such as hardness, color and magnetic susceptibility measurements, mineral identification by X-ray diffraction, density and porosity measurement, and the observation of thin sections. In addition, diatom fossils and dinoflagellate cyst fossils were identified and determined their assemblage and biostratigraphic zone. These data were compiled and correlated between lithological columns of each traverse routes. The shallow drilling programs were carried out on two traverse lines intersected the interred surface positions of the Omagari Fault, the area around the site of Underground Research Laboratory and the Kamihoronobe area. All 14 shallow drill-holes around the site of Underground Research Laboratory were observed diatomaceous mudstones of the Koetoi Formation. The H15-1-12 drill-hole located middle of the traverse line was confirmed more than ten meters zone of weathered and brecciated diatomaceous mudstones. Seven shallow drillings were carried out at the Kamihoronobe area. The west two drill-holes were observed the sandstones of the Yuchi Formation, while the east holes were diatomaceous mudstones of the Koetoi Formation. A small-scale of fracture zone was found at the easternmost H15-2-1 drill-hole, and it is inferred that the main fracture zone of Omagari Fault may pass through to the east of these drill-holes. (author)