The Shibata-Koide tectonic line has been proposed to occur along the boundary between the preTertiary basement region and the Niigata Oil Fields, although its nature has not been clarified. The authors carried out a gravity exploration in order to make its character clear in the eastern margin of the Niigata Plain, where the line is topographically distinct, and a detail geological survey was performed by the Collaborative Research Group for the Sasagami Hills (1980). The following results were obtained through the present investigation. 1. The Shibata-Koide Tectonic Line which has been assumed through geological survey was certified to occur as a large normal fault. The total throw is estimated to be about 1,500 m vertical in the southern part of the investigated region, and about 4,000 m in the northern part. 2. This line, however, does not consist of a single fault, but of four to five sets of normal faults, namely a step-fault system with westerly throwdown sence. The cross section drawn from the gravitational exploration is consistent with those presumed by the Collabarative Research Group for the Sasagami Hills (1980). 3. On the west side of the line, the depression zone parallel to the line was found in the basement underlaid with the Tertiary sediments. This depression zone may have been existed at the stage of sedimentation of the Nishiyama Formation and is probably related to the Shibata-Koide Tectonic Line. 4. The Shibata-Koide Tectonic Line is thought to be an significant tectonic boundary between the pre-Tertiary basement region and the Tertiary to Quaternary sedimentary basin as proposed by YAMASHITA (1970).

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