POSTGLACIAL SEA LEVEL CHANGE ALONG SAN'IN DISTRICT, JAPAN

Abstract

The coast of San'in seems to have two advantages at least for examining the recent sea level changes. Its tidal range is the smallest in Japan so that we can study levels of shore features more accurately on the coast. Moreover San'in district has many prehistoric and historic remains in the alluvium ranging above and below present sea level so that we can well estimate the date of the deposits and correlative former sea levels. It is roughly agreed that the rising sea approached to its present level during the Holocene in Naka-no-umi area (Mizuno et al., 1972) . However the detail is far from clear. Recently the author proposed small oscillations of sea level from the evidences of the Holocene coastal deposits and of sand dune formations (Toyoshima, 1975). In this paper the author attempted to examine the evidences of sea level change along the coast and to propose the curve of sea level change analizing more wide and full data. On rocky coast, there are many evidences about change of sea level. For instance, three well preserved low level platforms are recognized at A, B, C in Fig. 1. They stand 2, 5m (higher platform), 0.7m (middle platform) and 0.4m (lower platform) above mean sea level. These evidences bear witness to recent change in the relative levels of land and sea. Comparison of these common and similar profiles of A, B and C where their localities are fairly remote each other could suggest that the sea level change clearly exist along the San'in coast. The higher platform, dissected and vegetated, is thought to be developed during Jomon transgression the mddle platform is emerged slightly after the middle and late stages of _??_omon period. The lower platform is developing at present time. It is possible to note the sea level change on sand beach ridges and on shingle beach ridges in a same way. A, B, C and D in Fig. 2 show the profiles of shingle beach ridges. They stand over 6.0m (higher), 4.0_??_5.5m (middle) and 1.5_??_4.0m (lower) above mean sea level. The higher ridge is covered by thick black soil (Kuroboku), the middle ridge has no black soil but its is vegetated. Both ridges consist of weakly weathered shingle but the lower ridge consists of fresh beach shingles and is thought to be the present beach. From these fact, the higher and middle beach ridges are considered to be emerged ridges. Profiles of E and F also show the evidence of the relative change of sea level. These sandy beach ridges have significant high elevations with remains of AD S00 1, 000 which were discovered in black soil under the beach ridge formation. These facts would suggest that a very recent transgression (possibly Heian transgression) existed on the coast. Fig. 3 is a series of alluvium section with human site. These alluvium successions also might suggest the fluctuation of relative sea level. Based upon the leveling of some coastal features (bench, notch, cave, submarine notch, beach ridge and etc), many small evidences of sea level change were collected along the San'in coast. They are classified into 8 levels ( I_??_VIII) as shown in Fig. 4. A proposed curve of sea level change is also shown in Fig. 5.