Grain-size characteristics of sandy deposits of an incised-valley fill : Upper Pleistocene in the Makinohara Upland, Shizuoka, Japan

Abstract

The Upper Pleistocene (the last interglacial period) of the Makinohara Upland, Shizuoka, Japan, is divided into four formations, and constructed from a drowned valley, estuary, beach-shoreface and an alluvial system. These comprise one depositional sequence. Grain-size characteristics of each facies are clarified in this study. The assemblages of each facies can be classified into two types (closely and scattered distributed groups) based on 3-D diagrams (arithmetic mean versus standard deviation versus mud content). Fluvial deposits of LST have poorly sorted fine sand, whereas fluvial deposits of HST have well sorted coarse sand. In the beach-shoreface system, grain-size gradually fines and mud content increases, from the beach to the lower shoreface. In the drowned valley- estuary system, estuary mouth bars have higher arithmetic mean values than the bay-head delta. Both environments have moderate standard deviation and mud content values. Central bay floor (central basin) and drowned valley have high arithmetic mean and mud content values, and low standard deviation values. Seaward barrier have intermediate arithmetic mean values, low standard deviation and mud content values. These facts indicate that 1) terrigenous coarse sediments were trapped in the bay-head delta and were not transported to the central basin, 2) seaward coarse sediments were not transported into the central basin (tidal inlet is an exception). The horizon of the MFS have high arithmetic mean and mud content values compared with other similar facies. Grain-size characteristics differ between the tsunami deposits in the relict barrier and drowned valley. The tsunami deposits in drowned valley include numerous suspended (from landward and/or seaward) load by oscillationary currents along the paleo-valleys, whereas the tsunami deposits in relict barriers consist of offshore sands and erosion of the barrier itself.