THE SEDIMENTARY STRUCTURES AND THE FACIESMODEL OF THE LATE PLEISTOCENE EOLIANBIOCALCARENITES IN SHIDAO ISLAND OF

Shidao Island, the highest one among the Xisha Archipelago, is some 15 metersabove sea level.The sequence of the biocalcarenites occurred on the island is cha-racterized by the alternation of that with cross-beddings and parallel beddings.O wing to the misinterpretation of the sedimentary structures, it has caused so m econ troversaries. T he materials of the biocalcarenites are obviously from the destruction of thecontemporaneous reef complex. Most of the grain components range in medium tofine sands with perfect roundness. T hree kinds of boundary surfaces have been found in the sequence.First-orderboundaries are marked with the occurence of paleosols which is nearly horizontal.T he paleosols, w hick indicate a non deposition period and a change of clim ate, arecharacterized by the dece mentation and phosphorization of the rocks, and the occu-rence of continental snail fossils.Wide distributed rhitholiths demonstrate thegrowth of plant during the stage of pedogenesis. The second-order boundaries are those between dune deposits and interdunedeposits.They are formed by the climbing of the dunes and interdunes under theaction of NE-trade wind.The boundaries sli,;htly dip at an angle less than 10.aga-inst the wind direction and comparable with the boundaries found in modern eoliandeposits. T'he third-order boundaries are the boundaries of costs，which inclinc exclusi-vely windward. Based on the principles of comparative se;dimenlology，she first一order and third-order boundaries are isochronous, but the second一order boundories only of facies. At least two first-order boundaries occur in the sequence of biocalcarenites onthe island.The lower one might be traced on the whole island and is 20-30cm inthickness. The upper one is coincident with the modern topographical surface. The dune deposits, usually 3-7m in thickness, have well一preserved cross-beddin-gs on great scale, including trough cross一beddings, convex cross-beddings and planarcross-beddings. The dip argle of the cross-beddings may reach 35 or so. The interdunedeposits show roughly paralled bedding,but in fact, they are all cross-beddings withvery small dip angle. In conclusion, the coexistence of the three boundary surfaces and the dune andinterdune deposits marks the feature of the eolian biocalcarenits. A facies modelshowingin figure 15 is proposed, which may be used elsewhere, Basedon the data of C¹⁴ dating collected from the previous works, the authorsbelieve that the biocalcarenites on Shidao Island were formed some 3200 years agowhen the sea level stood at a low level，then the island passed a transgression per-iod with alternation of dry and wet weather. All the controversaries might besolved by the model mentioned in the paper.