The Sedimentary Research About Reef Carbonatite In Xisha Islands Waters

This paper is mainly based on the sedimentary survey of modern coral reef of Xisha Islands and parts of drilling cores of Xichen 1 well. Making use of many geochemistry techniques, such as the carbon, oxygen and strontium isotopes, major elements and trace elements, electron microprobe, inclusion and cathodeluminescence, the modern reef facies and the drilling well facies of Xisha Islands, the geochemistry of drilling core and the diagenesis have been studied.First of all, the characteristics of modern sedimentary facies have been particularly researched, and then , they were used to classify the drilling facies. Seven subfacies, such as in-place reef facies (reef frame facies), gravel bindstone facies in the margin of the reef, reef platform facies, sand cay facies, colluvial fore-ree f facies, backreef lagoon facies and sand-mud facies fore reef in shallow sea, have been identified in the modern sedimentary facies in Xisha Islands waters. Except the colluvial fore-reef facies, the other facies have been identified in the core of four drilling wells in Xisha Islands. And the bioclastic bank on shore and off shore also have been identified in the drilling cores.The aeolianite of Sand Island has been surveyed, by the way of thin section, scanning electron microscope (SEM), carbon and oxygen isotope, major elements and trace elements, the characteristics of petrology and geochemistry. The oxygen isotope curve has been compared with the oxygen isotope curve of cave stalagmites, and the acureate formation time of the aeolian dunes and palaeolsols have been confirmed. Following the time sequence (from older to newer), the five stages of aeolian dunes had been formed during the next five stages: 73 000~58 500 a B. P., 56 000~48 000 a B. P.,45 500~38 000 a B. P.,36 000~30 500 a B. P.,28 500~25 000 a B. P.;and the four stages of palaeolsols had been formed during the next four stages: 30 500~28 500 a B. P.,38 000~36 000 a B. P.,48 000~45500 a B. P.,58 500~56 000 a B. P.. By combining the background of the last glaciation, the morphological feature of Xuande Atolls and the Eastern Asian monsoon, the sedimentary facies of aeolianite has been set up.The pedogenesis make the elements of Al, Fe, Mn and U concentration in the palaeolsols. Although the elements of Y, Cr and REE(rare-earth element) seems have little relationship with pedogenesis. We induce that such element may come from the windblown dust or volcanic ash.The carbon and oxygen isotopes are sensitive to the diagenesis, and they have the same variation trend. In the fresh water environment, they tended to negative bias obviously, and in the dolomization environment, they tend to positive bias obviously. The correlation factor between the carbon and oxygen isotope in Xichen 1 drilling cores is 0.88.Below the depth 116m of Xichen 1 drilling cores, the values of 87Sr/86Sr of drilling cores are obviously higher than the sea water at that time, which reflected the uplift of the Qinghai-Tibet plateau. Above 116m, the value of 87Sr/86Sr occurred many very low values, this may has a close relationship with the submarine volcano activities.According to the lithology, palaeontology and geochemistry characters, all the four drilling cores of Xisha Islands have been classified, and systematic comparative study has been done. We found that many important palaeo-oceanography events have been recorded in drilling cores. The dolomite in the drilling cores could well be compared, and their genesis is close to the Messinian event and formation of Arctic glacier. The carbon and oxygen isotopes looks obviously positive bias during the section 757~788m in Xichen 1 drilling well. This event has little relation with dolomization, we induced that it recorded the important enlargement of Antarctic glacier. The Calcareous nannofossils in Xiyong 1 drilling cores recorded a fast transgression event happened in the early stage of Pliocene. The obvious negative bias of carbon and oxygen isotope in Quaternary stratas recorded the fresh water eluviation event happened in the last glaciation stage. Since the later stage of Pliocene, the contain of reef platform, reef framework and sand cays has gradually increased, which show that the atoll reef became more mature, and the tectonic subsidence became more slowly.The content of Ca and Mg in the dolomite mineral looks fluctuated in the electron spectrum line-scanning-maps, which show the different state dolomization from inside to the outside of the dolomite minerals. We believe that, the difference of dolomization in dolomite mineral coresponded to the zonal structures in the cathodeluminescence figures. According to the fluid inclusion characteristics of dolimite minerals, the geoheat driving dolomization model has been built up.