2006 Volume 115 Issue 1 Pages 41-50
Three-dimensional structures of the Latest Pleistocene to Holocene shallow marine and fluvial sequence at the Nobi Plain, Central Japan, are reconstructed from 510 boring logs using a 1 km × 1 km grid system on GIS. The sequence unconformably covers the upper Pleistocene Atsuta formation, and consists of 5 depositional units - basal gravel bed, Lower Sand (LS), Middle Mud (MM), Upper Sand (US), and Top Mud and Sand (TM/TS), in ascending order. The basal gravel bed is the fluvial gravel bed, part of which fills a valley cutting the depositional surface of the Atsuta formation. The depositional surface of the gravel bed is slightly undulating, and is dissected in the north-south direction by incised valleys, which were formed during the Last Glacial Maximum. Unit LS, mainly the fluvial deposit, fills the incised valleys on the basal gravel bed. The upper boundary of LS is fiat and tilts toward the Yoro active fault, which is the western boundary of the Nobi Plain. Unit MM is mainly composed of prodelta mud. There is westward tilting and thickening of the unit MM. The Unit US is submarine deltafront sand, and is thickly distributed along grooves in the Unit MM. The thick part of the Unit US shows the main axis of the delta progradation or the main river channels. Unit TM/TS is a fluvial deposit covering the upper surface of the unit US. In this study, we confirm the effectiveness of the method using many boring logs and GIS to reconstruct the three-dimensional pattern of valley fill.