Developmental history of a coral reef complex controlled by the sea-level change and terrigenous influx: the Pleistocene Ryukyu Group of Nakijin Village, Okinawa Prefecture.

JIJU Katsutoshi

ORITA Seiko

Journal of science of the Hiroshima University. Series C, Earth and planetary sciences

The Pleistocene coral reef complex, the Ryukyu Group, is widely distributed in Nakijin Village, Okinawa Prefecture. The geology and stratigraphy of the group was investigated on the basis of sedimentological and topographic characteristics. Because the Quaternary carbonate was deposited under a strong control of the sea-level change, information on paleodepth was as much as extracted by use of the recent knowledge on the living subtropical reef biota.
We subdivided the Ryukyu Group into three units. Unit 1 consists of alternating deposits of different facies in lower and thick coral limestone in upper. It is highly terrigenous in the northeastern Motobu Peninsula due to a strong influx of silisiclastics. The alternation seen in localities along the northern coast of the Motobu Peninsula indicates several cyclic change in paleodepth. Unit 2 covers a large area and exhibits a zoning in distribution of different facies parallel to the present altitude. From the coast to the high-lying area, the facies generally changes from rhodolith limestone, through clastic limestone, and to coral limestone. This distribution pattern is consistent with that of the present coral reef complex around the Ryukyu Islands. The unit 3 mainly consists of coral limestone and restricts its distribution in the lowermost terrace along the coast.
Kouri Island shows much better development of the coral limestone than the northern Motobu Peninsula. Moat facies developed on the tops of units 1 and 2 in the island is obscure in the peninsula. This regional difference in reef development is associated with the difference in depositional settings; especially intensity of the terrigenous influx. The basement rocks are widely exposed in the middle-southern Motobu Peninsula, and the Ryukyu Group in the northern peninsula contains a large amount of silisiclastics which had probably been disturbing the growth of the reef-forming corals. In Kouri Island, where is separated from the peninsula by a relatively deep strait, the terrigenous influx did not sufficiently affect the carbonate deposition.
The cyclic facies change found in unit 1 is hardly seen in units 2 and 3. The columnar sections of unit 1 indicate a short-term and small amplitude of sea-level change, which may correspond to the climatic characteristic of the Early Pleistocene (mostly before 0.7-0.5 Ma). From the Middle Pleistocene, both the interval and amplitude of sea-level increased. The amplitude was too large to record the cyclic facies change, and the younger units (units 2 and 3) were deposited mainly during sea-level highstands.

英語

Hiroshima University

[ISSN] 0075-4374

[NCID] AA11095875