Southren Kyushu has been the region of intense volcanism at least since Pliocene time. One of the most characteristic features is the prevalence of the large-scale pyroclastic flow eruptions which originated from such gigantic calderas as Aira, Ata, Kikai and Kakuto. There exist a considerable number of literature on the stratigraphic sequence and distributions of the pyroclastic flow deposits in South Kyushu. However, relatively small number of reports are available on air-fall tephra deposits, which are useful for establishing Quaternary chronology both of source volcanoes and of marine or fluvial sediments in the coastal regions such as the Miyazaki Plain. In this study, each bed of maker-tephras which erupted during the time from ca. 100, 000 to 25, 000y.B.P., is precisely discriminated and described in the northern part of the Osumi Peninsula, Kagoshima Prefecture first. And then each tephra is traced northeastward along the main axis of distributions to the Miyazaki Plain. Of many tephras, the following four well-dated tephras are used as fundamental timemakers because of their widespread occurence; Ata pyroclastic flows, originated from Ata caldera in 95, 000-90, 000y.B.P. ; Kikai-Tozurahara ash falls, originated from Kikai caldera in 75, 000y.B.P. ; Aso-4 pyroclastic flows, originated from Aso caldera in 70, 000y.B.P.; Ito pyroclastic flows and AT ash, originated from Aira caldera in 22, 000-21, 000y.B.P. Several air-fall tephras from the Aira and Kirishima volcanic centers are identified in detail and roughly dated from their stratigraphic positions between these fundamental maker-beds. About 75, 000-70, 000y.B.P., explosive activity of Aira caldera occurred resulting in the formation of plinian pumice fall deposit, Fukuyama pumice falls, which is found from the Osumi Peninsula to the Miyazaki Plain. During ca. 60, 000-25, 000y.B.P., intermittent eruptions occurred forming five sheets of tephras, of which the Iwato eruption was greatest in producing pumice falls, pyroclastic surges and pyroclastic flows. Iwato pumice falls mantle extensive area from the Osumi Peninsula to the Miyazaki Plain. Cataclysmic eruption occurred from Aira caldera, producing Osumi pumice falls, Tsurnaya and Ito pyroclastic flows and AT ash 22, 000-21, 000y.B.P. Most of these eruptions were accompanied with phreatomagmatic ones. Eruptive history of Kirishima volcano is divided into two stages deduced from the tephra sequence. At ca. 40, 000 y.B.P., older stage of activity started with ejection of relatively felsic pumice falls, Iwaokoshi pumice fall, and graded to more mafic and frequent eruptions, Awaokoshi scoria fall. Younger stage began with the plinian eruption of Kobayashi pumice fall at ca. 15, 000y.B.P. Of many terraces in Miyazaki Plain, Sanzaibaru terrace is the most extensive one and is accompanied with transgressive marine deposits. Stratigraphic relation with tephra sequence shows that Sanzaibaru terrace was emerged before the Ata pyroclastic flow eruption, ca. 95, 000y.B.P., probably indicating the Last Interglacial Stage. Most of terraces younger than Sanzaibaru are of fluvial origin, except for Nyutabaru II and probably III terraces which are partly of marine origin, and are largely devided into two groups, older and younger. Older terraces, Nyutabaru terrace group, formed during the time from the Ata eruption to the Aso-4 eruption, were chracterized by the profiles with more gentle gradient. Younger ones which were chracterized by the profiles with steeper gradient, were formed after the Aso-4 eruption and before the Kobayashi pumice fall. The difference of their profiles reflects the sea level after the maximum stage in the Last Interglacial Age.
Slope stability and its change play important roles in geomorphic development both of mountains and coastal plaines. Changes in slope stability is expected to have been recorded in geological sections at depositional areas close to slopes and/or in mountain areas. Changes in slope stability were investigated on the basis of stratigraphical analysis, especially analysis of variation of inorganic content in peat deposits at moors and a grassland of Yakushima Island (30°20'N, 130°30'E), which is famous as one of the most rainy regions in Japan. Three localities on an alluvial cone, a flood plain and a gentle slope on a ridge were selected as the study areas (Fig. 1). Using the panning method, the inorganic content is calculated as the ratio of inorganic content against dry bulk weight of each horizon and expressed by percent. The change in inorganic content was concordant with those in facies and water content (Fig. 2, 3), and showed good agreement with that of ignition loss of thick humic soil (Fig. 7). From the sequential changes in degree of plant decomposition and the inorganic content, peat and thick humic soil profiles can be divided into four layers, namely, the Lowest layer, the Lower one, the Middle one, and the Upper one in ascending order. The change in the composition of materials from the Lower to the Middle layer is the most conspicuous. The changes in the inorganic content suggest that the yield of detritus was the most intensive at the Lower layer and was reduced towards the surface. These inorganic materials on the alluvial cone and the flood plain were transported by mud flows and floods. Therefore, the high content of inorganic materials can be considered to have been caused by heavy rainfall. Consequently these changes in inorganic content roughly suggest the changes in heavy rainfall occurrences. Initiation of the deposition of the Lower layer is presumed to be after 2000BP from the 14C date (2600-2700BP) of the base of peat layer at Hananoego moor and the thickness of the Lowest layer. Therefore, the slope condition on Yakushima Island is considered to become less stable after 2000BP.
This article comprises fifteen chapters on Foreword (1), From orography to comparative stratigraphy (2), Older rocks in the southwestern part of the Soviet Priomrie and its adjacence (3) and Older rocks in the east Manchurian mountainous land, South Manchuria and the Korean peninsula (4) in Part I, the Triassic and Jurassic systems in Southwestern Primorie (5), the Akiyoshi mountains and Tsushima basin (6), the Cretaceous system of the Sikhote Alin and the Sakawa orogeny (7) and the Amur subgeosyncline and the Sungari basin (8) in part II and the Cenozoic systems on the northwestern side of the Japan sea (9), from the destruction of the Tsushima basin to the construction of the shore line of the Japan sea (10), the Akiyoshi and Sakawa folded mountains (11), the PeriTunghai geotectonic zone (12), the Fossa Magna and Uetsu subgeosyncline (13), sundry notes concerning the Geology of the Japan sea (14) and the summary and retrospect (15) in this part, HI to which a comprehensive bibliography (16) is appended. It is well known that the Chichibu geosyncline which was confluent with the Mongolian one at the Touman-Suifung and adjacent areas, turned out the orogenic zone from the continental side by the early Mesozoic and later orogenies. As detailed here, the Triassic Akiyoshi folded mountains were located on the same line with the contemporaneous Songnim mountains in North Korea and not very far removed from the Touman area and the Sikhote Alins. The Akiyoshi mountains were later deformed and the late Jurassic inland sea to the north of the Eo-Nippon cordillera became land by the Oga orogeny causing a long shifting of the folded mountains to the south as shown in fig. 7. It is still a question how far W est Japan was displaced to the south by the Sakawa orogenies in the Middle Cretaceous-Palaeocene age. It is, however, certain that Northeast Japan has advanced much more than Southwest Japan toward the oceanic side by these orogenies in addition to the Tertiary disturbances. Likewise, the Ryukyu islands advanced further toward the Pacific basin than West Japan forming a distinct flexure at Southwest Kyushu.