Distribution and Age of the Marine Terraces and their Deposits in the Reef-capped Ryukyu Islands, Japan

Abstract

The Ryukyu island arc is divided into three regions by Tokara channel and Miyako depression (Figure 6), between which a region is called the Central Ryukyus. The region south of the Miyako depression is named the South Ryukyus. Throughout the two regions the Pleistocene coral reef sediments are distributed and united into the Ryukyu Group (MacNeil, 1960), in which the Upper and Lower Formations are classified (Table 7).
The Upper Formation is thought to have been deposited in the last interglacial stage and the following sub-interstadials, according to the dating by ²³⁰Th and ²³¹Pa growth method (Konishi et al., 1974) and the nannofossil biostratigraphy. Emiliania huxleyi, a nannofossil species, commonly occurs in the lowermost part of the Upper Formation in Ishigaki-jima (Figure 2) and such occurrence has been considered to show the δ¹⁸O stages from the 6th on by GARTNER (1972) and others.
The age of the Lower Formation may be the Middle Pleistocene, probably δ¹⁸O stages 15 to 13 (Table 7), because its lowermost part contains rare Pseudoemiliania lacunosa and abundant Gephyrocapsa oceanica (NISHIDA, 1977; MINOURA, 1979).
The Upper Formation forms the Upper Middle, Lower Middle, Low and Lowest terraces, while the Lower Formation sometimes makes the High terrace.
Depending upon the characters of the basement beneath the Ryukyu Group, the Ryukyu Islands are also classified into four areas. As the backbone of the Ryukyu island arc was made up in some period of the Late Miocene (KONISHI and SUDO, 1972), the distribution of the Ryukyu Group and of the accompanied marine terraces must be related to the tectonic movement after the Late Miocene. The above-mentioned four areas are the Late Miocene to Early Pleistocene Shimajiri Group area, pre-Late Miocene areas of the Central and South Ryukyus, and Late Miocene to Holocene volcanic inner arc area (Table 7). The Lower Formation of the Ryukyu Group is not distributed in the pre-Late Miocene area of the South Ryukyus. In the Shimajiri Group area, however, the two formations of the Ryukyu Group are distributed although the High terrace is not recognizable. The Lower Middle terrace keeps its reef barrier topography in many islands. The Low terrace consists of “Awaishi limestone”, foraminiferous calcarenite with uniform texture, sometimes.
The distribution of Pleistocene marine terraces is outlined in Figure 3. While the Late Pleisotocene terraces are ubiquitously distributed, that of the Middle Pleistocene ones is limited into the Central Ryukyus. This fact may mean that the period both the Central and South Ryukyus commenced to be upheaved is the latest Middle Pleistocene.
Although KONISHI and SUDO (1972) pointed out the tilting toward the Asiatic continent of the whole Ryukyu arc, it is not recognized concerning the South Ryukyus. From the facts that the Central Ryukyus tilts toward the continent, that Hateruma-jima in the South Ryukyus does westwards, and that the height of the last interglacial terrace in the Shimajiri Group area decreases southwestwards (Figure 5), a hypothesis that the Philippine Sea plate is being subducted under the Asian continental plate from the southeast or southeast-east can be supported.
The distribution of the Late or Middle Pleistocene maximum sea-level height (Figures 4 and 5) shows that the uplifting rate of the pre-Late Miocene area in the Central Ryukyus is faster in the middle part and slower near the Tokara channel and Miyako depression. In the volcanic inner arc area behind the above-mentioned area the rate is rather faster near the two tectonic depressions. These phenomena may be related to the movement of such a left strike-slip fault as predicted for this case by KONISHI (1965).