Study Of Reef And Depositional Environment Of The Ordovician In The Southern Margin Of Ordos.

The object of this study is the reef of the Ordovician in the southern margin of Ordos. On the basis of absorbing the outcomes of predecessors, scouting field sections and collecting data of relevant wells, testing the samples with geochemistry methods. The distribution range of reef, the reef building organisms,. the reef attaching organisms and the rock types are defined, the depositional environment of reef is studied, the microfacies and evolution of the reef are analyzed, what's more, the trace element and isotope of C and O are applied to analysis the reef. In the end, the control factors why the reef developed are analyzed and the formative pattern of the reef is summarized.The reef of the Ordovician in the southern margin of Ordos is distributed to the north of Weihe, east to Fuping, west to Longxian, the extentis about 250 kilometres. On the basis of scouting field sections of Fuping Xiaoyuan-JiangJunshan, Yaoxian TaoQupo, Chunhua TieWadian, Jingyang XuJiashan, Yongshou HaoZhihe and Liquan Dongzhuang, the conclusions are made that the reef building organisms are made up of coral, algae and stromatopora, the reef attaching organisms contain cephalopod, brachiopod, gastropod, crinoid, trilobite, and conodontophoridia, the rock types which make up the reef consist of framestone, bafflestone, bindstone, intralimestone, algal limestone and dolomite. The microfacies consist of reef base,reef core and reef cover, most reefs have not the distinguish between reef core and reef flank, forereef and backreef, the Yongshou HaoZhihe and Liquan Dongzhuang are where the reef flank appears. The process of evolution is discontinuous, the reefs are made up of many layers.In late Ordovician, the sedimentary facies of the southern margin of Ordos can be divided into open marine platform facies, platform edge reef flat facies, platform front clinothem facies and deep water clinothem-trench facies, the platform edge reef flat facies is where the reef developed.The organisms absorb the 12C largely, so, the waters consist of abundant 13C, which makes that theδ13C of carbonatite in late Ordovician is high, the 813C of reef carbonatite which contain abundant organisms is lower than the usual carbonatite. The salinity is not the main factor which influences the development of reef, because there is just a little variation when reef occurs. In late Ordovician, theδ18 has a little rise, which accounts for the temperature has a little rise. The warm water is more appropriate for organisms to live, which can promote the development of reef. The content of Sr is related to organism closely, it is the abundant organisms that make the content of Sr has a rise obviously.The factors which affect the development of reef in the southern margin of Ordos include tectonic structure, geologic climate, reef building organism, foundation conditions, contemporaneous fault, sea-level change, bottom configuration, hydrodynamic force and sea water nature. Finally, we summarize the formative pattern of the reef of the late Ordovician in the southern margin of Ordos.