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Application of Reef building Organism Community Evolution in Sea Level Change Research —An example from reef of Honghuayuan Formation of Lower Ordovician in the Central Yangtze Platform

YANG Wei 1 ZHU Zhong de 2 LIU Bin li 2 XIAO Chuan tao 2 1 (Gas Geology Department of LangFang Branch Research Institute of Petroleum Exploration and Development, CNPC Langfang Hebei 065007) 2 (Jianghan Petroteum Institute , CNPC Jinzhou Hube  
A lot of reefs have been discovered in Honghuayuan Formation of Lower Ordovician in the Central Yangtze Platform. Their rock comparisons contain two types. One is baffle stone. Another is binding stone. Their reef building organisms contain five types.They are bluegreen alga (containing blue bacterium and Girvanella ), Calathium, Batostoma, Pulchrilamina , and Archaeoscyphia as representation of rock sponge. They composite five types of reef-building organism community. Is bluegreen alga community (S community), Calathium bluegreen algae community (C S community), Archaeoscyphia Calathium community (A C community), Batostoma Calathium community (B C community), Batostoma Calathium bluegreen algae community (B C S community). Ecological environment of each of reef building organism community and the most lithofacies assemblage of host rock may indicate characteristics of each community,which includes water body environment and the most depth range suitable. Each community can adapt normal shallow sea of warm, clean, rich sunshine and oxygen, and cyclic better. But the most suitable depth may be different. S community represents shallow water and small energy. C S community and B C S community may be 5~10 m and medium large water energy. B C community and A C community may be 10~20 m and large water energy. Organism belt has been discovered in reefs of thickness over 5m in the study area. Organism belt is a result of organism community evolution,and is the most intuitive evidence of organism community evolution. Organism belt within reefs from bottom to up may indicate two models of community evolution. The trend of their evolution is ①A C community or B C community→C S community or B C S community→S community, ②S community→C S community or B C S community→A C community or B C community. This is an entire course of community from growth to death. Organism richness of community interior has also been changed with time. Because the most suitable water depth of each community is different, the force of organism community evolution may be sea level change. According to the most suitable water depth of reef building organic community, detailed change of sea level can be diagnosed in reef building term. Relative sea level change is from a falling course of 20~10 m→10~5 m→5 m to a rising of 5 m →10~5 m→20~10 m. The largest change magnitude of sea level is about 15 m. This is similar to the former. So, according to the reef building organic community evolution can quantitatively study small magnitude sea level change.
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