Ryukyu Limestone" of Okinawa-Jima, South Japan : A Stratigraphical and Sedimentological Study

Abstract

The "Ryukyu Limestone", the host of the Quaternary geology of the Ryukyu Islands, is divided fundamentally into the main limestones and the accessory limestones. This division is based on the differences in lithological characters, fossil assemblages, distribution and thickness of beds, age of sedimentation, etc. The main limestones composing the major part of the "Ryukyu Limestone" constitute the Ryukyu Group together with the contemporaneous noncalcareous deposits, and they are the basement of the accessory limestones. The Ryukyu Group is divided stratigraphically into two formations; the Lower and the Upper. The Lower Formation is the sediments which accumulated in the basins of the tectonic origin or in the valleys of the eroded basement and is represented by various lithofacies. On the other hand, the Upper Formation is more widely distributed than the Lower Formation, and its stratigraphical sequence of the lithofacies is rather constant everywhere. The Upper Formation is subdivided into the A, B and C Members based on characteristic fossil assemblages. Within these Members, the A Member is represented by some foraminiferal and molluscan fossils showing deeper water than a coral reef environment. Furthermore, the Cycloclypeus-Operculina bed of this Member, being fairly continuous, is a good horizon marker for the correlation of the Ryukyu Group not only in Okinawa-jima but also other islands. The biolithite facies, though poor in the Ryukyu Group as a whole, is recognized in the C Member of the Upper Formation and a part of the Lower Formation. In order to analyse the sedimentary environments of the Ryukyu Group, the petrography and the fossil analysis under the optical microscope were made in connection to the field observation. As a result, the sedimentary environments of the Ryukyu Group can be generally inferred to have been shifted from the land prevailing condition disqualified for coral reef formation to the favourable condition for reef building, though the process of reef forming of the latter was interrupted for a short time by the intervention of deep sea environment as represented by the A Member in the Upper Formation. As for the age of the Ryukyu Group, it is regarded to be Early and early Middle Pleistocene from the paleontological and stratigraphical aspects. The geologic structure of the Ryukyu Group is composed of many tilting blocks brought in by faulting after the completion of sedimentation. These tectonic movements of the maximum stage brought about the original configuration of the present islands. Since then, to the fringing areas of these emerged blocks the accessory limestones represented by the Minatogawa Limestone Formation were attached. However, it is also evident that mainly the tectonic movements affected the mode of the Ryukyu Group sedimentation such as local unconformities between the Lower and the Upper Formation and accelerated the change to shallow water of the sedimentary environment. Therefore, the Ryukyu Group is the product of transgression due to tectonism, especially shown in the later stage. Contrasting with the main limestone deposition, the accessory limestones were the deposits formed under the influence of the eustatic sea level changes. Lastly, the Quaternary geohistory of the Ryukyu Islands is reviewed from the viewpoint of the "Ryukyu limestone" stratigraphy.