Significance of “500m Deep Island Shelf” Surrounding the Southern Ryukyu Island Arc for its Quaternary Geological History

Abstract

Most recently (UJIIÉ, OKI and HIGASHIKAWA, 1979), a remarkable island shelf ca. 50 to 70km wide and ca. 360km long has been recognized at the apprroximately 500m water depth on the southern Ryukyu Island Arc by an examination of 1/200, 000 bathymetric charts prepared by the Hydrographic Office of the Maritime Safety Agency for publication (Figure 1). Meanwhile the southern two thirds of the shelf was figured out in details by a number of seismic profiling surveys around the area carried out by the Hydrographic Office (HAMAMOTO, SAKURAI and NAGANO, 1979). In this paper I suspect the shelf as to be covered with a submerged reefal limestone according to the reasons mentioned below. 1) A petroleum exploration well located on the shelf (“WELL” in Figure 1) revealed the presence of 64m thick reefal limestone at the top. 2) Smith-McIntyre-type snapper and Phleger-type gravity corer failed to acquire enough amount of sediment samples from the shelf in the October 1978 cruise aboard on the R/V Kagoshimamaru. Besides, great damage upon the core edges at every time of operation suggests that the samplers always hit on the hard rock. 3) Grain-size composition of the sediments from the shelf rather resembles to that of some sand patches observed on the littoral reef off Okinawa-jima (Figure 3). 4) Benthonic foraminiferal assemblages from the same samples show generic compositions characteristic to the inner neritic environment, differently from those expected from such a depth as 500m. The discrepancy may be ascribed to the present environment similar to coral reef, although Recent littoral reef is characterized by common occurrence of Soritidae, Amphisteginidae, Calcarinidae, and so on, which are seldom on the island shelf. 5) Geomorphology of the island shelf provided with fairly flat surface and sharp marginal edge suggests that the surface is inevitably capped with such hard rocks as reefal limestone resistant against the erosion by water turbulence, particularly by the Kuroshio Current flowed over. 6) Depending upon the same reason, the “Southern Boundary Fault of the Central Ryukyu Island Arc” could well keep its sharp scarpment in the shelf area, whereas it is unable to trace the scarpment on the shelf slope which is composed of the semiconsolidated muddy facies of the Shimajiri Group (Figure 4).
If the limestone-capped island shelf around 500m water depth is accepted as a new hypothesis, some significant events will be supposed for the Quaternary geological history of the Ryukyu Island Arc system. Several blocks shallower than 200m, including all island masses and banks, abruptly stand up on the island shelf bounded by many faults running in perpendicular to the Arc trend (Figure 2). Almost all the blocks are also covered with 60 to 70m thick reefal limstone, called the Ryukyu Group, in a narrow sense. Therefore, it is able to regard that these blocks represent relatively upheaved parts of the island shelf. The hereby deduced block-movement corresponds to the “Uruma Crustal Movement” of KIZAKI and TAKAYASU (1976) who proposed this by geological survey on land. By the movement, moreover, the “South Boundary Fault of the Central Ryukyu Island Arc” named in this paper may have been formed in accompanying about 1500m or more down-throw of the northern side, in another word, of the southern end of the Central Ryukyu Island Arc. The Fault must replace the Miyako Depression, which has currently been utilized since KONISHI (1965) for the discussion in structural geology and biogeography; the Depression has ever implied no precise definition in geography.
Beneath the reefal limstone spread over the whole of the island shelf region, gently folded or inclined strata of the Shimajiri Group are distributed throughout.