稲敷台地南部の下総層群

上岩橋層と木下層の堆積相,層序,古環境(その1)

抄録

The Middle to Late Pleistocene Shimosa Group distributed in the southern Inashiki Upland, Kanto Plain, is mainly composed of sand and mud which were deposited under shallow marine conditions. On the basis of sedimentary facies of the Group, seven sedimentary environments were recognized, that is, transgressive zone (foreshore-shoreface), inner shelf, lower shoreface, upper shoreface, shoreface, foreshore and estuarine channel. The authors have made it clear that the Shimosa Group in this area consists of two cycles of sedimentation, each of them was formed by the sea-level change. The lower one is the Kamiiwahashi Formation and the upper, the Kioroshi Fomation. There are transgressive zones (foreshore-shoreface) at the base of both Formation, and the sedimentary facies successions above inner shelf faceis on the transgressive zones (foreshore-shoreface) are considered the progradational succession. The depth of the sea which has formed the Kamiiwahashi Formation is 9 meters and that of the Kioroshi Formation is 6 meters. These values minus the thickness of inner shelf facies, which is 1 meter, leaves the depth of the mud line. If this depth compared with that of the mud line of Japan's present bays, the Kamiiwahashi Formation can be considered to have been deposited on an open bay like Ishikari Bay; on the other hand, the Kioroshi Formation on an inner bay like Tokyo Bay or Osaka Bay. The Kamiiwahashi Formation was formed under the rapid transgression and following the progradation of the strand plain system along an open bay, while the Kioroshi Formation was formed also under the rapid transgression but following the progradation of the depositional system of coastal plain (estuarine depositional system) along an inner bay.