Analysis Of Stratum And Sedimentary Facies In Ordovician Period In Huanghuachang Yichang, Hubei

Yichang city is located in western Hubei Province, is situated at intersection point of upstream Yangtze River and the middle reaches, the transition zone from mountain area to the plain area. Taking Qingfeng-Xiangguang fracture as demarcation line, it can divide into southern Yangzi strata area and northern South Qinling-Dabie Mountain strata area, Yichang is located on the Yangzi strata district, the geological change are few, the strata distribution is extensive, from the Proterozoic groups 1800Ma ago to Cenozoic groups 1Ma ago, also the growth integrity, appearing is complete.This research area of Yichang Huanghuachang is located the east limb of the Huangling anticline, is in the middle-north of the Yangzi carbonate platform in Ordovician. Not only the Huanghuachang Ordovician system section plane the structure is simple, the strata growth is integrity, each class of fossil is also extremely rich, the research degree is high, is also the typical profile of Ordovician system division and the contrast, but also is the boundary stratotype section of Dawan stage, simultaneously the global most perfect boundary stratotype section of middle-lower Ordovician.Through the geology survey in the open air and studying and contrasting Ordovician system strata division, the lothological characteristics, specially the sedimentary facies symbol research, connecting with predecessor's research results, it has established the Yichang Huanghuachang deposition facies model of Ordovician period, restored the environment of deposition then, has made clear the evolution of paleogeography and the fluctuation change of sea level then.Ordovician period, epicontinental sea spread out the Huanghuachang area in Yichang, Hubei province, it is characteristics of broad distribution, shallow sea water and gentle seabed slope, indicates that is stable surroundings, forming the suit stratum that is mainly carbonatite. The sediment mode is belongs to shore epicontinental sea`s carbonatite assemble principally, including basin facies, wide continental shelf facies, basin marginal facies, mesa marginal biohermal facies, mesa marginal riffle facies, wide-sea mesa facies and limited-sea mesa facies. The transgression continuous expanding and sea water constant deeping from Xilinxia Formation to Wufeng Formation, and have multi-gyre phenomenon in sea water advance and retreat. The aggradation is divided into two high level gyre of the sea level, that stands for sea water advance and retreat process respectively, and many basic gyre overprinted on the each high level gyre. The low high level gyre include three basic gyre, the sediment facies mainly focus to limited-sea mesa facies, wide-sea mesa facies and mesa marginal riffle facies; The top high level gyre include two basic gyre, the sediment facies mainly focus to wide continental shelf facies and basin marginal facies. The transgression scale that responsed to the the top high level gyre more lager than the low high level gyre, that is to say, the largest transgression taken place in Ordovician period, and the Wufeng Formation record the most high tide of the transgression.