Landform Development of Kimotsuki Plain before Deposition of Osumi Pumice Fall, Kyushu, Japan: Formation of Buried Ata Welded Ignimbrite Plateau beneath Ito Ignimbrite

Abstract

― ― Abstract Ata welded ignimbrite （ 110 ka ） lies beneath Ito non-welded ignimbrite and Osumi pumice fall deposit （ 30 ka ） in the Kimotsuki Plain, southern Kyushu. Previous geomorphological stud-ies of the Kimotsuki Plain focused on landform development after deposition of Ito ignimbrite. Landform development of Kimotsuki Plain since the last interglacial, especially until just before deposition of the Osumi pumice fall, is reconstructed using geological data collected from outcrop observations and borehole records. The basal-surface of the Osumi pumice fall deposit obtained shows that Ata welded ignimbrite had been dissected by the Kimotsuki River and its tributaries in response to the last glacial sea-level drop before the Osumi pumice fall was deposited. Longitudinal profiles along the Kushira River in 110 ka and 30 ka indicate recession of the Tanida waterfall, which formed at the edge of the Ata welded ignimbrite plateau. These profile changes imply that the Tanida waterfall retreated 2.4 ­ 6.0 km upstream between 110 ka and 30 ka. The Kushira formation, Marine oxygen Isotope Stages （ MIS ） 5e marine deposits under Ata welded ignimbrite, was found below the present sea-level at multiple locations in the Kimotsuki Plain. This vertical distribution of the Kushira formation indicates that the Kimotsuki Plain has been in a tectonically stable or subsidence area since the MIS 5e, in contrast with the Onejime and Natsui areas, which have been tectonically uplifting. The depositions of the two ignimbrites had significant impacts on filling the Paleo-Shibushi Bay （ Sea ） and the development of the Kimotsuki Plain under sea-level lowering during the last glacial period. The top-surface of basement rocks is more than - 120 m below sea level at the floors of paleo valleys, even though it is adjacent to mountains composed of the basement. Further investigation of lower alluvium and its basal-surface is required for an understanding of valley incision and delta evolution of the Kimotsuki Lowland after deposition of the Ito pyroclastic flow.