Abstract
The Kokubu Group, distributed in the northern coastal area of the Kagoshima Bay, South Kyushu, is composed of shallow marine to lacustrine deposits which formed under the condition of strong volcanic activities in the Kajiki Sedimentary Basin. The Kokubu Group is considered to be the northern extension of the Kagoshima Graben and is regarded as one of the standard sedimentary successions of the middle Pleistocene series in Southwest Japan. The Kokubu Group rests unconformably on the Pliocene to early Pleistocene volcanic basement rocks and is overlain by the middle to late Pleistocene pyroclastic-flow deposits. Based on lithologic differences, the Kokubu Group can be divided into seven formations : i.e. the Kajiki, the Nabekura, the Kamo, the Obama, the Asahi, the Oda, the Hayato and the Fumoto Formation in the ascending order. Among these, three formations, namely the Nabekura, the Obama and the Oda, are considered to have formed from the volcano-tectonic event deposits under similar geologic processes. Such processes can be ascribed to the repeated strong eruptions of pyroclastic flows followed by subaqueous abrasion of bottom sediments by a large amount of gravel and sand that flow into the sedimentary basin. The layers of these coarse clastic sediments are accompanied by mega-mud clasts and are associated with the development of a large-scale, low-angle cross-stratifications which are suggestive of tsunami deposits formed under violent flow conditions. The results of analysis of the conjugate faults in the Kokubu Group imply that these volcano-tectonic events might had occured under the tension stress condition of the basement rocks of the Kajiki Sedimentary Basin which is believed to have resulted from the geotectonic evolution processes of the Kagoshima Graben.
