砂質干潟におけるｆｌｕｉｄ ｍｕｄ堆積物の時空分布：東京湾小櫃川河口盤州干潟

Abstract

Fluid mud is hig-concentrated near bed suspension, and form relatively thick muddy deposits with rapid sedimentation rate even in coastal and shallow-marine environments that are influenced by wave and/or tidal currents. However, the development of fluid-mud deposits in sandy coastal and shallow-marine environments have not yet clearly been understood, compared with that in muddy coastal and shallow-marine environments. This study investigated lithofacies and distribution patterns of possible fluid-mud deposits in the Banzu sandy flat, Tokyo Bay, central Japan. About 1-m-thick subsurface sandy deposits in the Banzu sandy flat locally contain muddy deposits of 0.5-7 cm thick, and these muddy deposits are interpreted to be fluid-mud deposits on the basis of their distinctive lithofacies and clay fabric. Furthermore, wood and plant fragments and freshwater diatoms in the muddy deposits suggest that these deposits formed from flood-discharge-induced suspensions from an adjacent river called the Obitsu River. Random occurrence and lenticular geometry of the muddy deposits within about 1-m-thick subsurface sandy deposits are also interpreted to indicate that the muddy deposits formed by episodic depositional processes and were preserved as drapes over topographic lows of sandy bars. Furthermore, the maximum thickness of the muddy deposits is documented in a location about 920 m north from the main channel of the Obitsu River. This spatial variation in thickness of the muddy deposits is interpreted that fluid mud, which had once formed at the river mouth during flood discharges, was subsequently transported along the coast. Thus, these spatial and temporal distribution patterns of the muddy deposits are interpreted to be one of the distinctive features of possible fluid-mud deposits, and can be used for the identification of fluid-mud deposits in ancient sandy coastal and shallow-marine successions in conjunction with distinctive features of lithofacies and clay fabric.