1999 Volume 53 Issue 5 Pages 340-358
The middle to upper Pleistocene formations distributed around Lake Kamo, Sado Island, are divided into the Ryotsu First, Ryotsu Second. Ryotsu Third, and Ryotsu Forth Terrace Deposits, and Ryotsu Younger Fan Deposit in ascending order. The Ryotsu Third Terrace Deposit called the Katabata Formation, intercalates the Katabata volcanic ash and the drifted pumices which are correlated with Kitsugi pumice fall deposit (K3) and Sambe-Kitsugi tephra (SK), respectively. The Ryotsu Forth Terrace Deposit is called the Agata Formation, and contains the Agata volcanic ash which may be correlated with the On-Pm1 or Pm-1A tephra. Brown-colored eolian soil deposits cover the terrace deposits, and are divided into the A to D Members in ascending order. Volcanic ash layers in these members are correlated with the marker tephras as follows: Akasaka B1 ash to On-Kt, Akasaka B2 ash to Aso-4, Akasaka B3 ash to DKP, Akasaka C ash to AT, Akasaka D ash to As-K. On the basis of relationship between the terrace deposits and these marker tephras, the Katabata and Agata Formations are correlated with the Shimosueyoshi and Obaradai Formations in the Kanto Plains, respectively. Depositional sequence of upper part of the Katabata Formation shows a typical upward-coarsening succession, and was formed by progradation of fan delta system. The altitude of foreshore sediments tend to become lower northward. This suggests that they are deposited at the regressive stage. The lower sequence of the Agata Formation shows a typical upward-finning succession. The upper sequence of the Agata Formation shows upward-coarsening succession in the northern part, while the sequence in the southern part includes nearshore and coastal plain facies, lacking foreshore facies. Depositional facies change of the Agata Formation shows a transgressive-regressive cycle. The formation was in barrier-island and estuary or lagoon systems in the transgressive stage, and in coastal plain systems in the following regressive stage.
