Reconstruction of Paleo Sea-level and Landform Changes since the Marine Isotope Stage 5a, Using Buried Terraces in the Lower Sagami Plain, Central Japan

Abstract

The Sagami River mouth faces the Sagami Trough without a distinct continental shelf, and the lower Sagami plain has well-developed marine and fluvial terraces. This area is also favorable for interpreting the tephrochronology of the Last Glacial Cycle using tephras from Fuji and Hakone volcanoes. The subaerial terraces and their constituent deposits in the lower Sagami plain correspond to the Marine Isotope Stages (MIS) 5c-5a (S1-S3 terraces), 3 (N and T) and 2 (M), respectively.
The buried terraces were examined by bore-hole logs and correlated with subaerial ones in the upper reaches using morphological continuity and tephra sequence lying on the terraces. These buried terraces and deposits correspond to the MISs 5a, 4, 3, and 2, based on tephrostratigraphy.
The landform development of the lower Sagami plain since the MIS 5a is as follows: A broad fan-deltaic plain was formed in the culmination of the MIS 5a. A deep and narrow valley incised the plain during the MIS 4, because of the sea-level drop. During the MIS 3, this deep valley was filled, and a relatively wide plain of compound fans developed. A deep and narrow valley was formed again in the MIS 2. Sagami Bay has intruded into the lower Sagami plain in the MIS 1.
Relative sea-level changes are shown by subtracting tectonic components from the observed heights of buried terraces near the present river mouth. The tectonic component (average uplift rate) is estimated to be 0.07m/ky near the present river mouth. The calculated sea-level height at the culmination of each stage was as follows: -30 to -40m in the MIS 5a, ; -95 to 110m in the MIS 4; -80 to -90m in the MIS 3; and -100 to -110m in the MIS 2. These results suggest a low sea level for the MIS 4 and that a relatively low sea-level remained throughout the MIS 3.