Abstract
Several widespread tephras have been identified in core samples recovered from the coastal area of Nohbi Plain. They are K-Ah (Kikai-Ah tephra) and U-Oki (Ulreung-Oki tephra) in the Holocene Nan'yo Formation ; BT8 (a tephra layer intercalated in bottom sediments of Lake Biwa) and AT (Aira-Tn tephra) in the latest Pleistocene Nohbi Formation ; and BT34 and BT36 (tephra layers intercalated in bottom sediments of Lake Biwa) in the lower marine clay member of late Pleistocene Atsuta Formation. The following conclusions were reached, based on ages and horizons of the identified widespread tephras.(1) Many researchers have agreed that the basal gravel of Alluvium (BG) coincides with the Last Glacial Maximum (LGM, about 18 ka), middle stage of the Marine Oxygen Isotope Stage 2 (MIS-2). However, it was found that BG in the Nohbi Plain (First Gravel Formation) does not coincide with LGM but precedes it, because BG in the Nohbi Plain underlies the AT (about 25 ka) horizon of the Nohbi Formation. The age of BG is 35± to 30± ka (late MIS-3).(2) BG in the coast area of Nohbi Plain formed when fluvial gravel advanced downstream during the sea-level falling stage before LGM.(3) The deposition of BG is divided into two stages in central and western Japan. The earlier one precedes LGM, and the later one is concurrent with LGM. It is inferred that the distance from the coastline at LGM determined the formational stage of BG ; BG was formed later in places where the distance from the coastline was shorter.(4) Based on the widespread tephra layers intercalated in the lower marine clay member of the Atsuta Formation, it can be concluded that the deposition of the lower marine clay member started and ceased at approximately 130 ka and 110 ka, respectively.
