Abstract
Relative timing of the maximum flooding surface (MFS) varies in response to spatial variation in the rate of a relative sea-level rise. This variation is interpreted to be controlled by an interaction between the rate of rising of eustasy and that of vertical tectonic movement of a sedimentary basin. Where the rate of subsidence is large, the relative sea-level rise is accelerated and the relative timing of the MFS is delayed compared to areas in which the rate of subsidence is smaller. In this paper, this kind of spatial variation in the relative timing of the MFS was examined based on correlation of volcanic ash beds of the middle Pleistocene Yabu Formation in the Boso Peninsula, Japan.In the southwestern area of the Boso Peninsula, the Yabu Formation is 40 to 60 m thick and the stratigraphic horizon of the MFS is above the Yb 3 volcanic ash bed. In contrast, in the northeastern area of the Boso Peninsula, the Yabu Formation is only 15 m thick, and the MFS is below the Yb 2 volcanic ash bed, which is older than the Yb 3 volcanic ash bed. This spatial variation in the relative timing of the MFS is interpreted to have been controlled by the interaction between glacioeustatic sea-level rise during the oxygen isotope stages 10 through 9 and local variation in tectonic movement along the eastern margin of the paleo-Tokyo Bay. This tectonically active area is named the Kashima-Boso Uplift Zone. In the northeastern area of the Boso Peninsula, tectonic uplift along the Kashima-Boso Uplift Zone decelerated the rate of a relative sea-level rise. This is interpreted to have resulted in the development of the MFS earlier than the southwestern area, in which the paleo-Tokyo Bay appears to have continued to subside actively and the rate of a relative sea-level rise was accelerated.
