Abstract
The Upper Miocene to Lower Pleistocene Shimajiri Group, composed of muddy sediments, and the Lower to Upper Pleistocene Ryukyu Group, consisting of reef-complex deposits, are widely distributed throughout the Ryukyu Islands. The Lower Pleistocene Chinen Formation, which has an intermediate lithofacies, is sporadically distributed between both groups in the central and southern parts of Okinawa-jima. The Chinen Formation was previously believed to be limited to Okinawa-jima, but correlated outcrops, aged between 1.71 and 1.39 Ma, have recently been discovered in Kikai-jima, Kagoshima Prefecture. The outcrops consist of calcareous mudstone and sandstone, or sandy limestone rich in bryozoan fragments, with clear contacts between the underlying and overlying groups. The Chinen Formation overlies the So-machi Formation of the Shimajiri Group across a clearly defined angular unconformity, indicating post-depositional tilting and erosion of the Shimajiri Group. In contrast, there are no structural differences to indicate tilting during the period between the deposition of the Chinen Formation and that of the overlying Ryukyu Group. Therefore, Kikai-jima emerged as a result of relative sea-level fall after the deposition of the Chinen Formation, following which the supply of terrigenous sediment decreased.
