沖なわ国際海洋博・アクアポリス係留装置打設工事海域の海底地質――特に新期礁性たい積物について

Abstract

A submarine late Quaternary succession off the west coast of the Okinawa Island disclosed by anchor hole drilling and test boring is described (Figs. 1, 2). The late Quaternary sediments constituting the sea bottom have an average thickness of ca. 12m, being represented mostly by highly calcareous sands more or less rich in gravel-sized fragment of reef-building corals. Simplified biofacies analysis reveals that these sediments can be subdivided into several major facies (Figs. 4, 7) which in turn correspond in a rough approximation to the stratigraphic subdivision (Fig. 10, units A-C). The upper two units, A and B, are both assigned to Holocene on the basis of their stratigraphic position, high carbonate content and low L/H ratio (ratio of low-Mg calcite to high-Mg calcite) (Fig. 8; Tabs. 2, 3). The 14C dates prove that this part of the succession has resulted from the Flandrian (Jomon) transgression since early Holocene (Tab. 4, samples 1, 3-7). The lowest unit C can be tentatively assigned to latest Pleistocene, probably Bolling to Allerod interstades, as inferrid from the 14C date (Tab. 4, sample 8), the position of near-reef facies more distant from the present shoreline, the presence of finer, back-reef sediments, the low carbonate content and high L/H ratio (Fig. 8). No distinct unconformity has been found between the units B and C and this stratigraphic relation remains open to future studies. From a further discussion on the age of the buried topographic surfaces covered by the late Quaternary sediments, early Wurmian age is suggested for the lowest buried surface (-50 to -55m below sea level).