66. Geology of the Kakuto Basin, Southern Kyushu, and the Earthquake Sivarm from February, 1968

Abstract

The immediate basement of the Kakuto Basin, on the boundary between Kagoshima and Miyazaki Prefectures, consists of Miocene (?) to Pliocene (?) volcanic rocks mainly of andesitic composition. The lower members are hydrothermally altered (Masaki propylite) and form a reservoir of active hot springs now widely exploited over the western part of the basin. The upper members (Kakuto volcanics) mainly consist of thick lava flows which form the continuous ranges on the northern, western, and eastern margin of the basin. The southern margin is composed of partly destroyed bodies of the older Kirishima Volcano group (Kurino and Shiratori andesites). In the later Quaternary, a large amount of dacitic magma was erupted mainly in the form of large-scale pyroclastic flows. The welded deposits are now found in a wide area surrounding the Kakuto Basin. A collapse caldera of the Crater Lake-type was formed thus clearly defining the depression topography mentioned above. The basin was quickly filled by tuffaceous sediments of lacustrine nature (Kakuto group). About a few tens of thousands of years ago, two major pyroclastic flows entered the basin and the pumiceous materials were dispersed in the waters of a shallow lake occupying the entire floor of the Kakuto Basin at that time. Each time a layer of lake sediments ranging from pumice lump aggregates through pumiceous fine sand to banded clay was formed which at pressent shows remarkable resemblance to the whitish unconsolidated pyroclastic flow deposit locally called ""Shirasu"" that is wide-spread all over southern Kyushu. Immediately after this, about 1 km3 of andesitic magma was erupted at the southern margin of the Kakuto caldera in the form of thick lava flows. The northern part of the flow reached the above-mentioned lake sediments and partly overflowed them. The weight of the thick lava flow caused a plastic deformation of the unconsolidated layers of the two pumiceous sediments (the lower Ikemure formation and the upper Kyomachi formation) and parallel anticlinal and synclinal structure was formed peripheral to the flow front of the lava. A symmetrical cone about 200 m high with a small top crater was formed at the vent (Iimori-yama). The paleo-Kauto lake was drained and the lake sediments were partly eroded to form terrace along the margin of the Kakuto Basion