Abstract
Pleistocene marine terraces distributed in Kikai Island generally have three steps. They consist of mainly thick Middle Pleistocene and thin Upper Pleistocene sediments. The higher two terraces are composed of coral limestone and the lowest terrace, extensively developed southwest of this island, overlies carbonate sediments composed of well-preserved biogenic remains. We applied α-spectrometric Useries dating to three genera of solitary corals collected from the lowest terrace sediments. They provided reliable dates ranging from 96.6±2.8 to 56.7±2.2ky (2σ statistical error), corresponding to MIS 5b to 3. These can be divided into four age groups. The lithofacies and dates suggest that this sediment was deposited on the insular shelf during both relatively high and low sea stands. We can recognize the correlation between the age and elevation of samples; the oldest sample occurred at ca. 40m, the youngest at ca. 18m above present sea level (apsl). In addition, corals obtained from the proximal location of this terrace tend to show older ages, while corals from the distal site showed younger ones. Coral limestone is also sporadically distributed on this terrace. The youngest date obtained from them was 40.2± 1.2ky from a sample collected at the southwest end of the terrace, about 25m apsl. Dates of three other samples from the same site are comparable within the margin of error; the average was calculated to be 41.2±0.8ky. We infer that this terrace was formed in different depositional environments between MIS 5b and 3 with the fall of relative sea level: from the insular shelf where fore-reef sediments were deposited to the shallow water environment where coral reefs were formed. Thus, no Pleistocene carbonate sediments, younger than about 41ky (MIS 3), are exposed on the surface of Kikai Island.
