FORAMINIFERAL FAUNA DISTRIBUTION IN REEF-FACIES BEDS SINCE LATE MIOCENE IN XISHA ISLANDS AND ITS SIGNIFICANCE
Han Chunrui Meng Xiangying (Institute of Marine Geology, Ministry of Geology and Mineral Resources, Qingdao. China)
Based on the analysis of foraminifera in two 400-m-up core sections from hole Xichen-1 and Xiyong-2, this paper discusses foraminifera distribution in reef-facies beds since late Miocene in Xisha Islands and makes a dating of late Tertiary beds. Reef sediments from late Miocene can be, from up to down, divided into 6 facies sections, each of which has a some what distinct foraminifera assemblage.1. Lime-sand-island facies, 10-14m thick, with rich foraminifera species and numbers. The tests are mostly seriously abraded. Dominant species are Calcarina spp. and A. madagascariensis d'Orbigny. Planktonic foraminifer is rarely seen. 2. Reef-flat depression facies, 4-6.5m thick, with the richest foraminifera species and numbers and highest diversity. Tests are well preserved. Dominant species appear to be the same as the above. A little number of planktonic foraminifers can be found. 3. Reef frame facies, 17-18m thick, with a little number of species and tests and relalively low diversity. Tests are mostly abraded. A. madagascariensis and Calcarina spp. dominate. Planktonic foraminifers are rarely seen. 4. Reef flat facices, 160-163m thick, foraminifera species are similar to those of the lime-sand-island facies, but with a smaller number. Diversity is low and tests mainly abraded. Dominant species are found the same as those of the lime-sand-island facies Planktonic foraminifers are rarely seen. 5. Back-reef lagoon facies, 191-196m thick, with a large number of species and a high diversity. Tests are well preserved. A. radiata dominates, and planktonic forminifers greatly appear. 6. Lagoon shore facies, located in the section under 350m in hole Xic en-1. Only two species are found and no planktonic foraminifers. Division of stratigraphic ages can be asfollows: Quaternary reef sediments in Xisha lays the bottom at the hole depth of 213-235m, according to the lower bonud of distribution of Globorotalia truncatuHnoides (d'Orbigny) and changes of coiling direction of Gr. menardii (d'Orbigny); the bottom of Pliocene sediments is determined to be at the depth of 350-387m, based on the lower bound of distribution of Globigerinoides conglobatus (Brady), Pulleniatina and Neogloboquadrina dutertrei (d'Orbigny); bottom part of late Pliocene sediments lies at the depth of 232-259m, according to the upper bound of distribution of Globigerinoides obliguus Bolli, Globoquadrina altispira Cushman and Jarvis; and bottom part of middle Pliocene sedimehts is at the depth of 289-337m, according to the upper bound of distribution of Globorotalia margaritae Bolli and Bermudez and Lepidocyclina (Nephrolepidina) sp.