Abstract
Sado Island, situated in the Japan Sea about 35 km off the central Honshu, is surrounded by a series of well-developed Pleistocene and Holocene marine terraces. The height distribution of the Holocene marine terrace was closely investigated in order to estimate the pattern of vertical deformation in the Holocene and discuss sequential changes of rate and pattern of vertical deformation in the late Quaternary by comparing with the height distribution of the Pleistocene terraces.
The Holocene terrace, separated from higher terraces by high cliffs and topographically, well-defined is subdivided into two levels, L 1 and L2. L1 is almost continuous around Sado Island. L1 is usually composed of beach deposits about 2_??_4m thick, consisting of sorted round gravels and sands. In Kuninaka Plain, however, L1 is composed of thick marine deposits of more than 50m. Thus, it is suggested that L 1 was formed at the time of the Postglacial transgression dated about 6, 000 y. B. P.
The height of the former shoreline of L 1 is 3_??_9m in Osado and 2.5_??_6m in Kosado. L2 is limited in distribution and is less than 6 m in height. As a whole, it is higher in Osado than in Kosado. An average rate of the maximum uplift is 1.5 m per 1, 000 years in Osado and 0.9 m per 1, 000 years in Kosado. Moreover, in Osado and Kosado, there is a tendency for former shorelines to be higher on the northwest coast than the southeast coast, suggesting southeastward tilting. The rate of tilting of L 1 is 0.06/1, 000 per 1, 000 years in Osado and 0.021, 000 per 1, 000 years in Kosado.
Pleistocene terraces have been deformed in the same pattern as with the Holocene ones but they are more deformed in the former in the latter. Thus, it is probable that south eastward tilting has been continuing throughout the late Quaternary in Osado and Kosado. However, localized northward tilting can be seen in the southern tip of Kosado.
