琉球石灰岩の堆積相

The Ryukyu Limestone is the main constituent of the Pleistocene Ryukyu Group developed in the Ryukyu arc, and is important to elucidate the depositional process of Quaternary coral reefs and reef limestones on the northern margin of the western Pacific. In this paper, the stratigraphy and sedimentary facies of the Ryukyu Limestone distributed in three islands of Kikai-jima, Okinoerabu-jima and Yoron-jima are describid, with discussions on sedimentary environment and history.
The basement rocks of Okinoerabu-jima and YOron-jima, both of which belong geotectonically to the Motobu Belt, are composed of Palaeozoic strata and intrusive rocks, whereas these of Kikai-jima situated in the Shimajiri Belt consist of Pliocene mudstone. The Ryukyu Group is mainly composed of calcareous rocks (Ryukyu Limestone), accompanied with terrigenous conglomerate at several horizons. Two facies, the reef facies and the off-reef floor facies, can be recognized in the Ryukyu Limestone. Stratigraphically, the Ryukyu Group is divided into two formations of the Lower and the Upper. The Lower Formation is deposits of transgressive stage, whereas the Upper Formation is those of regressive stage, forming terrace surfaces.
In Kikai-jima, the Lower Formation of Ryukyu Group is subdivided into the Hyakunodai Member below and the Nakanishi Member above. The Hyakunodai Member overlies unconformably the basement rocks without basal conglomerate, and is mainly composed of off-reef floor facies characterized by algal pisolite and bio-micrite ～-micrudite. Algal-coralline biolithite of reef facies is locally found in patches. The Nakanishi Member is mainly composed of intra-bio-sparite ～ -sparrudite at the type locality, and of algal-coralline biolithite and bio-micrite ～-micrudite elese-where. The Upper Formation is subdivided into the Wan Member below and the Araki Member above. The Wan Member consists of intra-bio-sparite～ -sparrudite with cave deposits, and the Araki Member of intra-bio-sparite --sparrudite accompanied with algal-coralline biolithite.
In Yoron-jima, the Lower Formation is subdivided into the Maebama Member below and the Gusuku Member above. The Maebama Member is mainly composed of algal-coralline biolithite and terrigenous conglomerate. The Gusuku Member represents a reef complex comprising reef facies and off-reef floor facies. The Upper Formation is subdivided into the I and the II Member in ascending order. The I Member consists of foraminiferal biosparite and terrigenous conglomerate, and the II Member of algal-coralline biolithite, foraminiferal biosparite and terrigenous conglomerate.
The Lower Formation of Okinoerabu-jima abuts on the basement rocks, and represents a typical reef complex composed of reef facies and off-reef floor facies. It is subdivided into four members of A, B, C and D in ascending order, by intercalation of terrigenous conglomerate as key beds. The Upper Formation is thin and accessory, and is subdivided into the I and the II Member on the basis of its geomorphological distribution. The I Member consists of algal-coralline biolithite and foraminiferal bio-sparite～ -sparrudite, and the II Member of intrasparite, foraminiferal bio-sparite ～- sparrudite and terrigenous conglomerate.
As stated above, the Ryukyu Limestone is mainly made up of autochthonous biolithite and bioclastic and intraclastic calcarenite～calcirudite, accompanied with algal pisolite. Main constituents of the biolithite are corals and algae. Most of the calcarenite ～calcirudite are composed of forami-niferal tests, fragments of alge and intraclasts, with either sparry or micritic matrix. Based on the field observation and microscopic examination in thin sections, the following seven types of litho-facies are recognized in the Ryukyu Group:
(1)Terrigenous conglomerate～mudstone type
(2)Algal pisolite type
(3)Algal-coralline biolithite type (comprising two subtypes of algal biolithite and coralline biolithite)
(4)Foraminiferal bio-sparite～-sparrudite type
(5)Algal biosparrudite type
(6)Bio-micrite～ -micrudite type (comprising three subtypes of larger foraminiferal biomicrudite, bryozoan biomicrudite and planktonic foraminiferal biomicrite)
(7)Intrasparite type (comprising intra-bio-sparite～-sparrudite subtype).
Among them, (3), (4) and (5) constitute the reef facies, whereas (2) and (6) the off-reef floor facies.
Four foraminiferal assemblages of A, B, C and D are recognized in the Ryukyu Limestone. Assemblage A is characterized by the abundance of Amphistegina, Cycloclypeus and Operculina, whereas assemblage B by Amphistegina and Calcarina. Assemblage C is quite similar to that of Recent beach sands, being characterized by the predominance of Baculogypsina and Calcarina, accompanied with Amphistegina. In assemblage D occur a large number of planktonic foraminifers. The distribution of these assemblages is fairly concordant with the litho-facies. The assemblage A is found in the algal pisolite type and larger foraminiferal biomicrudite subtype, and the assemblage D in the planktonic foraminiferal biomicrite subtype.
Comparing the distribution and characteristics of litho- and bio- facies of the Lower Formation of Ryukyu Group in three islands, we can recognize a difference among islands, especially between Kikai and Yoron-Okinoerabu. It is considered that this difference is concerned with characters of the basement rocks. The basement rocks of Yoron-jima and Okinoerabu-jima are composed of Palaeozoic hard rocks as well as intrusive rocks, and might have formed isolated islands during the deposition of the Lower Formation. Fringing the islands, the reef facies were developed, in front of which the off-reef floor facies were deposited. Thus, an organic reef complex was built up in each of Yoron and Okinoerabu, although such depositional environments were interrupted a few times by an increase in supply of terrigenous material. The zonal distribution of litho-facies is recognized in the Lower Formation. The arrangement is as follows from near-shore to off-shore; algal-coralline biolithite type (accompanied with algal biosparrudite and foraminiferal bio-sparite ～-sparrudite types) , algal pisolite type and bio-micrite ～-micrudite type (larger foraminiferal biomicrudite sub-type — bryozoan biomicrudite subtype — planktonic foraminiferal biomicite subtype). We can also recognize the zonal distribution of foraminiferal assemblages; C, B, A, and. D from near-shore to off-shore.
The basement rocks of Kikai-jima are composed of loosely consolidated Pliocene mudstone accompanied with sandstone. They were eroded above and under the sea level. When the even sea-bottom was subsided 50m or more under the sea level, the deposition of the Hyakunodai Member of Lower Formation began with the algal pisolite and bio-micrite ～-micrudite types of off-reef floor facies. When the sea level fell, the algal-coralline biolithite type of reef facies was deposited as stratigraphic reefs here and there on shallower mounds, surrounded by the off-reef floor facies.
The characteristics of litho- and bio-facies of the Upper Formation in three islands of Kikai, Yoron and Okinoerabu are similar with one another. The middle and lower terrace-surfaces were formed at the time of the deposition of Upper Formation.