九州・琉球地方の下部および中部更新統の層序

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The Early and Middle Pleistocene sediments are distributed in several districts in Kyushu such as the Kuchinotsu Group of the Shimabara Peninsula, the Kuroki Formation of the Kuroki basin, the Oita Group of the hilly land area of the Oita Plain, the Kokubu Group of the coastal area north of Kagoshima bay, south Kyushu. They are mainly composed of silt, sand, gravel and pyroclastics of the late Pliocene and the early Pleistocene volcanic activities and some times yield the mammalian fossils and abundant plant fossils called the Metasequoia flora. The Kuchinotsu Group, one of the most important mammal-bearing strata in Kyushu, is composed of fluvio-deltaic sediments in the lower part and shallow marine sediments in the upper part with intercalations of lava or pyroclastics of andesite and basalt. Abundant mammalian fossils showing the faunal resemblance to those from the Early Pleistocene formations in Japan and North China are recorded from this group. The Oita Group has been regarded to represent one of the standard succession of Early to Middle Pleistocene age in Kyushu. This group overlies the Pliocene Sekinan Group and is composed mainly of clastic sediments deposited under the non-marine environment and occationary intercalated with lava and pyroclastics of andesite. The Kuroki Formation, one of the early Pliocene formation in Kyushu, is composed of fluvio-deltaic sediments with intercalation of lignite beds. From this formation, a molar of archetypal Stegodon has been yielded. The Kokubu Group, typically distributed in the hill side areas north of Kagoshima bay, is made up of silt, sand and gravel of marine origin with the intercalations of thick layers of pyroclastic flow deposits and yields the flora characterized by abundant occurrence of Fagus. The occurrence of rhinocerid's molar is also noticeable. For the purpose of biostratigraphic zoning, the Kuchinotsu Group and its correlative formations have been divided into following four units. These subdivisions are based on the geologic range of vertebrate remains yielded, especially of fossil proboscidean species. 1) Stegodon cf. bombifons Zone - Kuroki Formation, 2) Stegodon cf. insignis Zone - Oya Formation (Kuchinotsu Group), 3) Stegodon shodoe. sis akashiensis Zone - Oya Formation (Kuchinotsu Group), 4) Mammuthus armeniacus proximus Zone - Kita-arima Formation (Kuchinotsu Group), 5) Stegodon aurorae Zone - Kita-arima Formation (Kuchinotsu Group), 6) Stegodon orientalis Zone - Tsurusaki Formation of Oita Group. Among these mammalian zones in Kyushu, zones from 1) to 3) are correlated to the Stegodon cf. elephantoides Zone in the Pliocene Tokai Group, the Stegodon insignis sugiyamai and the Stegodon shodoensis akashiensis Zones in the Osaka Group in Kinki district, respeetively. The zone 5) are correlated to the Mammuthus protomammonteus Zone and the Mammuthus armeniacus proximus Zone in the Osaka Group. The Zone 6) is regarded to be contemporaneous with the early Middle Pleistocene Stegodon orientalis Zone in the Osaka and the Kobiwako Groups. On the Ryukyu Islands, the Shimajiri Group erops out on Kikai-jima, Okinawa-jima, Kume-jima, Miyako-jima and Hateruma-jima. This group is made up largely of marly siltstone and sandstone and contains intercalated dacitic tuff. By the paleomagnetism and the paleontological evidences around the Plio-Pleistocene boundary in the Cenozoic strata in the Ryukyu Islands, the uppermost part of the Shimajiri Group and the Ryukyu Group are tentatively correlated to the Kuchinotsu Group.
日本地質学会の論文
1988-04-25

九州・琉球地方の下部および中部更新統の層序

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