Abstract
Geological profiling and tephra analysis of muddy sediments has clarified subsurface geology in the eastern part of the Nohbi Plain:
(1) The upper pumiceous sand member of the Late Pleistocene Atsuta Formation (D3U) contains two intercalated muddy horizons, and it is rare to find the pumice in the basal section.
(2) The lower marine clay member of the Atsuta Formation (D3L) contains an intercalated ash fall bed of the Nagashima tephra (B75-2).
(3) The intercalated Nagashima tephra lies at the upper horizon of the D3L member in the study area (eastern Nohbi Plain), whereas this tephra lies at the lower horizon in the western Nohbi Plain. The implications of this difference in the tephra horizon are unclear and needs further investigation.
(4) The D3L member in the eastern Nohbi Plain has a thickness equal to the same member in the western Nohbi Plain.
(5) In the eastern Nohbi Plain, the middle Pleistocene Ama-Yatomi Formation contains an intercalated thick marine clay bed (Dmc-m) in which the Ata-Th tephra (240 ka) lies at the middle section.
(6) The westward-tilting Nohbi Plain appears to have caused smaller amounts of subsidence in the eastern part compared to the western part; however, the marine D3L and Dmc-m members in the east have thicknesses comparable to the same units in the west; it is as yet unclear why this is the case.
