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.
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.