Thirteen boreholes were drilled on and around Irabu Island in the southern Ryukyus, from which limestones of the Pleistocene Ryukyu Group (50-110m) were recovered. The seven lithologies in the Group fall into the coral limestone facies and the rhodolith limestone facies. The former is composed mainly of coral floatstone/framestone and the latter is characterized by rhodolith-large foraminifera floatstone/rudstone. By comparing the bio-sedimentary facies of the core samples with those of present sediments around Irabu Is., the coral limestone facies typically represents a reefal-shelf area of 0-60m water depth, and the rhodolith limestone facies an open shelf of 60-150m deep.
The Ryukyu Group is widely distributed on Irabu Is., gently dipping west. It can be divided into three Members, and each separated from its neighbours by a solution unconformity. The Lower Member comprises coral framestone/floatstone and overlies the Shimajiri Group with an angular unconformity, which gradually dips westward. The Middle Member is characterized by five cyclic sequences of alternating coral limestone facies (Unit C-1, C-2, C-3, C-4, C-5) and rhodolith limestone facies (Unit R-1, R-2, R-3, R-4). However, the coral limestone facies dominates in the eastern hilly area but abruptly decreases in thickness and is increasingly replaced by the rhodolith limestone facies towards the western lowland. The Upper Member is a large foraminifera grainstone/packstone. Boreholes can be correlated on the basis of lithostratigraphy and calcareous nannofossil biostratigraphy. Five nannofossil-datum levels show the Ryukyu Group to be 1.36-0.39Ma.
The Ryukyu Group accumulated in a reefal to shelf environment where the water depth was increasing toward the west. At low sea level stand, the coral limestone facies extended for a distance westward and at high sea level stand, the rhodolith limestone facies was distributed widely in the area.
The unconformity between the Lower and Middle Members falls between 1, 10Ma and 0.89Ma. A number of unconformities, regressive sequences, and cooling episodes are recorded at this time in Japan. According to nannofossil-datum, the lowest rhodolith limestone facies of the Middle Member may be correlated with the oxygen isotope stages 25-23 of Williams et al. (1988), and the coral-rhodolith sedimentary cycles in the Middle Member are considered to be formed in response to glacial eustacy (oxygen isotope stages 26-16).
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