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.

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Bed-by-bed correlation of sheet-like turbidite sandstone beds reveals variations in stacking patterns of beds and bed-sets in a 20-45 m-thick sheet-like turbidite package within the Miocene-Pliocene submarine-fan succession of the Kiyosumi Formation, Boso Peninsula, central Japan. Migration distance of the thickness-weighted depocenter (TWD) of each sheet-like turbidite sandstone bed relative to its underlying bed show increase-and-decrease cycles. This is interpreted to document the repetition of infilling and flattening out of topographic lows, and the development of younger lows as a result of stacking of sheet-like turbidite sandstone beds with convex-up geometry in a submarine-fan system. Furthermore, the sequential changes of bed stacking permit subdivision of the studied package into five bed-sets. The bed-set stacking shows migration distance of TWD smaller than those of component beds and represents less variable distribution in the stacking processes.

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A new method to reconstruct the sediment transport pathways, which is based on the sediment grain-size distribution and uses principal component analysis (P-GSTA), is proposed here. In this method, a linearly-combined value of six grain-size parameters (mean, coefficient of variance, skewness, kurtosis and log-ratios of mud and gravel contents) with different weighting factors is used as a proxy of sediment transport processes. The combination of this method and cluster analysis is very effective to reconstruct sediment transport patterns in the depositional environments where compositive depositional processes exist.

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This study examined depositional processes of muddy deposits intercalated in coarse-grained sandridge deposits of the Middle Pleistocene Mandano and Ichijiku formations on the Boso Peninsula, central Japan. We identified these intercalated muddy deposits as fluid-mud deposits on the basis of the combination of lithofacies, geometry, bioturbation, and clay fabric. The fluid-mud deposits are interpreted to have formed in response to storm events. Thus, muddy intercalations, which may have formed as fluid-mud deposits, are interpreted to be more common than those having been identified in coastal and shallow-marine sandy successions.

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The Pleistocene Ryukyu Group composed mainly of shallow-marine carbonates was deposited with hig-amplitude and short-termed glacioeustatic sea-level changes in the Middle Pleistocene age. We carried out core observation, carbon and oxygen isotopic analyses and XRD analyses to recognize subaerially exposure surfaces (SES), and to make clear the precise sea-level changes in the Middle Pleistocene age. For the purpose, six boring cores in the southern part of Miyako Island, southwestern Japan, were examined. As a result, the followings are revealed; (1) the Ryukyu Group of the study area is divided into 6 units, (2) each unit can be subdivided into two to four subunits based on SES, (3) each subunit was deposited during the interstadial period such as 5a, 5c and 5e during the Marine Oxygen Isotope Stage 5.

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