関東平野の地下地質と地層流体  [in Japanese] Underground geology and formation fluids of the Kanto Plain, Japan  [in Japanese]

The basement rocks distributed in Central Honshu are divited from the Sea or Japan to the Pacific side into the Hida, Hida marginal Joetsu, Mino-Ashio, Ryoke, Sambagawa, Chichibu, and Shimanto Belts. The Median Tectonic Line forms the boundary between the Ryoke and Sambagawa Belts. The Hida and the Mino-Ashio Belts are characterized by some Helium-rich natural gascs. The Kashiwazaki-Choshi Tectonic Line trends NW-SE from Kashiwazaki in the Sea of Japan side to the southern part of the Sea of Kashima-nada in the Pacific side. The line is confirmed on the underground geology known from many deep wells and some geophysical data. Although there are three tectonic belts, that is, the Sambagawa, Chiochibu, and Shimanto Belts on the southern side of the Kashiwazaki-Choshi Tectonic Line, the Mino-Ashioand Ryoke Belts are recognized on the nirthern side of the line. At present the front of the north American plate, to which Northeast Japan belongs, is situated at the Kashiwazaki-Choshi Tectonic Line, on which the nest of earthquakes in the southwestern part of Ibaraki Prefecture rests. The upper Cenozoic formations cover the basement rocks dcscribed above with an ungular unconformity in and around the Kanto Plain. The formations are divided into three parts, that is, the pre-Kazusa group, Kazusa Group, and post-Kazusa group. The typical section of the pre-Kazusa group is found in the Tomioka-Takasaki area, Gunma Prefecture. The section represents a continuous cycle of sedimentation. The lower, middle, and upper parts of the section are characterized by the Nephrolepidina-Miogypsina fauna, Kadonosawa molluscan fauna, and the coarser sediments of regressive facies. The whole or partial deposits of the section are found in the hilly area at the eastern margin of the Kanto mountains. The upper Miocene-lower Pliocene deposits are not found in the northwestern Kanto Region including the typical section of the pre-Kazusa group of early and middle Miocene ages. The hot spring of Utsyunomiya is originated in the basal pyroclastic rocks of the Miocene deposits underlain by the granitic rocks belonging in the Mino-Ashio Belt. The principal ions found in the hot spring water of 42.0℃ in temperature and 9.5 in pH are as follows: Na^+ (370.1 mg/kg), Cl^- (178.3 mg/kg), and SO_4^<2-> (497.9 mg/kg). The water accompanies some gas bubbles in the gas-water ratio of 0.001. The composition of the gasis as follows: He (0.415 vol.%), H_2 (1.108 vol.%), O_2 (0.21 vol.%)., N_2 (97.47 vol.%), Ar (0.79 vol.%), and CO_2 (0.00 vol.%). The formation water derived from the upper coarser sediments of regression facies has been known from the hot sprong ownered by the religious body "Gedatsukai", Kitamoto City, Saitama Prefecture. The principal ions detected in the water of 38.4℃ in temperature and 8.6 in pH are as follows: Na^+ (3,100 mg/kg), Cl^- (5,648 mg/kg), and HCO_3^- (168.9 mg/kg). The center of the strainer of the hotspring well is 965 m in depth, and then the geothermal gradient may be over 2.52℃/100 m. The upper pre-Kazusa group of late Miocene to early Pliocene is widely distributed in the southeastern part of the Kanto Plain including the Oyama-Yuki area. The group covers unconformably on the basement rocks or the middle and lower Mioccne rocks. There are some informations on the formation fluids derived from the group. The geochemical features of the gases from three wells, that is, Soka R1, Kawasaki GS1, and Nagareyama NP1, are as follows: 1) CO_2 contents are lower than those of the gases derived from the Kazusa Group. 2) He is rich in the N_2-rich gas. The regression line on the Br^-I^- diagram of the underground brines derived from the group and the upper part of the Miyazaki Group: Br^-=83.65+0.4714×I^- (c.c. 0.8110). Both Br^- and I^- in the Br^-I^- diagram have been recalculated so that the chlorosity is 19,864 mg/l that corresponds to the chlorosity or the normal seawater. The Kazusa Group is also widely distributed in the southern par