南関東ガス田のいくつかの坑井堆積物を用いた地球化学的指標成分の研究

抄録

The depositional environment of the Kazusa Group Plio-Pleistocene epoch in the southeastern part of Kanto Plain is discussed, based on chemical and mineralogical analyses of 51 mudstone samples taken from Ichihara, Kami-ota and Oami bore holes.
Total sulfur, form of sulfur compound such as monosulfide, disulfide and sulfate sulfur, organic carbon, and total nitrogen are analyzed for the above samples.
The authors discriminate two kinds of fromboidal pyrite, namely, the pyrite filling the foraminiferal cells and that covering the cells. The former type is found in the upper horizons such as the Kakinokidai, Chonan, and Kasamori Formations, and the latter one occurs in the lower horizons as the Kiwada, Otadai and Umegase Formations. The former one might be formed under the aerobic condition in the depositional area, and the anaerobic condition prepared locally in the cells of foraminifers due to the decomposition of organic matter. On the other hand, the latter one is formed under the anaerobic condition due to the higher rate of deposition and the closed circumstance of the basin.
The ratio of pyrite in sulfur compound is lower in the samples of bore holes compared with that taken from the outcrops. This is due to the oxigenic condition near the surface of the earth.
The ratio of total carbon to total nitrogen reflects the contribution of higer plants to plankton in the sea. It is higher in the lower part of Umegase Formation, the Otadai and the Kiwada Formations. This might be due to the narrow basin at those stages.