The Neogene Aoshima Formation of the Miyazaki Group, well exposed along the Nichinan coast, has been considered to be a turbidite succession deposited at a forearc basin, although the thickness-frequency distributions and the sedimentary facies of the formation are in disagreement with those of typical turbidite successions. In this study, we investigated the depositional system and the sedimentary environments of the formation, on the basis of the facies of sediment-gravity flow deposits and stratigraphic variations of bed-thickness distributions. From the results, we can classify the sediment-gravity flow deposits of the formation into the following types; Type G: sedimentary structures containing grading beds, whose wavelength is in the range of 0.5-5m; Type I: inverse grading structures composed of a finer lower unit with climbing-ripple cross lamination or HCS-mimics, and a coarser upper unit with massive sandstones; and Type M: massive structureless sandstones. Individual sediment-gravity flow deposits can be correlated over 10 km using key-tuff markers along palaeocurrent directions. Stratigraphic fluctuations of the hemipelagic-mudstone thickness and sediment-gravity flow thickness show that the formation along the Aoshima Island consists of a lower progradational system indicated by a thinning-upward trend of hemipelagic mudstones and an upper system dominated by lobate sand bodies characterized by the cyclic sedimentation of thick-sediment-gravity flow deposits. These results indicate that the depositional setting of the formation is a deep-offshore basin of a fandelta along the south-east coast of the Kyushu Island.

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Scour marks up to 1.5m deep with smooth wall, rich in fine-grained heavy grains are found in the massive division of pyroclastic-flow deposits of the Seiganji Tephra Formation in northern Kyushu. The scour marks are asymmetric with steeper slope on the upstream side, and filled with poorly sorted grains of a(p) a(i) fabric. Some massive turbidite sandstones of the Pleistocene Umegase Formation in Boso Peninsula also exhibit similar scour marks up to 0.7m deep. However, the scour marks are characterized by symmetric shape and high-angle imbrication grain-fabric. The results of open-channel flume experiments indicate that such scours can not be formed as flute marks or pot holes, but by scouring around obstacles. Mud clasts may be favorable for such obstacles in turbidites, and rip-up soil blocks or tree debris are for pyroclastic flows. Shallow flow comparable to the obstacle thickness is responsible for deep sours, especially for scours with secondary hollow and backset fillings. The estimated underflow thickness is 0.7m for turbidity current, and 1.5m for pyroclastic flow, respectively.

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Sedimentology had been developed in Japan, as in Europe, from the 1950s, though the term “taisekigaku” (sedimentology in English) was proposed in Japan by Prof. T. Yagi as early as in 1929 (Okada with Kenyon-Smith, 2005). For the actual development of sedimentology in Japan, Prof. T. Matsumoto, Professor Emeritus of Kyushu University and Member of the Japan Academy, has made a great contribution to the sedimentology, in addition to his works as a worldwide palaeontologist for Cretaceous ammonites.

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