琉球列島南部における完新世の海岸地形および海水準

Abstract

1. Marine terraces, marine notches, beachrock, in situ coral reef and emerged shellbeds are used as indicators of Holocene sea-levels in Kohama, Kuro and Hateruma Islands, the South Ryukyus. A sea-level slightly higher than the present one, about 1 m AMSL is found from ca. 3, 300 to 800yBP in Kohama and Hateruma Islands and about 1.5m AMSL at about 4, 200yBP in Kuro Island (Table 2). It is, however, difficult to distinguish local differences in these three islands, owing to the limited number of observations and some uncertainty in the value of some coastal features as indicators of former sea-level. Late Holocene emergence is very small. 2. Shorelines which records the culmination of Postglacial (Jomon) transgression at about 6, 000yBP have been found along many parts of the Japanese coastline. Such evidences of shorelines, however, have not been observed in these three islands. The buried notch in Kuro Island, covered by beachrock at ca. 4, 200yBP, may correspond to this early transgression. 3. The relative sea-level drop causing the emergence of marine features in several Ryukyu islands is a recent phenomenon. Slight emergence dated at ca. 800 to 600yBP occurs not only on Hateruma Island, but also on Yoron Island (DELIBRIAS and PIRAzz0LI, 1983), on central Okinawa Island (KAWANA and PIRAZZOLI, 1983) and on Tarama Island (PIRAZZOLI et al., 1984). 4. Deposits found on the southeast coast of Hateruma Island, can be interpreted as the result of a tsunami, which accompanied the great earthquake of 1771.