Lithofacies And Paleogeography Of The Yingshan Formation Of The Middle And Lower Ordovician In The Platform Basin Of The Tarim Basin

Based on the theory of ancient geography and carbonate sedimentology,the comprehensive application of core,thin section,logging,outcrop and geophysical data,the combination of macro and micro,the stratigraphic characteristics,the petrologic feature,the sedimentary facies types and the longitudinal and transvers distribution of the Middle-Lower Ordovician Yingshan formation of the Tarim Basin were studied;combined with the regional geological background,"the single factor analysis and multifactor cornprehensive mapping method" adopted,the Yingshan formation thickness,deepwater sedimentary rock content,limestone content,dolomite content,granular rock content as five single factor serve as the key control factors,established the early to middle Ordovician Yingshan stage lithofacies palaeogeography map in Tarim Basin,analysised the paleogeographic evolution and distribution in different periods of the Yingshan formation.According to the characteristics of the rock and the fauna and in combination with the main structure line or main present geographic boundaries,the Tarim basin Ordovician can be divided into several stratas:Bachu-Tazhong-Manxi-Lunnan area,Yingmaili area,Tangguzibasi area,Keping area,Tadong-Kongque River area.Yingshan formation is mainly distributed in the five areas,from top to bottom is divided into four lithologic sections.The third and fourth formation belong to the lower Ordovician,the first and second formation belong to the middle ordovician.Differences in the thickness of the Yingshan formation in different regions is very large,between 0-850m,which reflects during the deposition process and after the deposition the formation has suffered more than once tectonic transformations.The rock types in Yingshan formation manily include carbonate rock and clastic rock two categories.Carbonate rocks are mainly distributed in the Bachu-Tazhong-Manxi-Lunnan area,Yingmaili area,Tangguzibasi area and Keping area,the main rock types are granular limestone,micritic limestone;clastic rocks are mainly distributed in Tadong-Kongque River area,the main rock types are mudstone and shale.Tarim Basin Yingshan formation sedimentary facies types include open platform facies,semi-restricted platform facies,platform edge facies,slope facies and basin facies,microfacies types include intraplatform shoal,intraplatform depression,platform edge shoal,the beach between the sea,lagoon and so on,vertically more developed interaction deposition of the intraplatform shoal and the beach between the sea,the intraplatform shoal and lagoon,overall were the grain shoal deposits,followed by the beach between the sea.From the west to the East,the sedimentary facies of Yingshan formation varys from platform,platform edge,slope to basirr and gradually translates to the Rossi area.From north to south,there is no obvious sedimentary facies transition.Through the thickness of strata(m)contour map,deepwater sedimentary rock content(%)contour map,limestone content(%)contour map,dolomite content(%)rock content contour map,grainstone content analysis(%)5 single factors,the lithofacies palaeogeography is established in study area:based on the deepwater sedimentary rock content,higher than 50%of the area viewed as a deepwater sedimentary area,namely basin;two slopes were drawed in the vicinity of 50%areas,less than 50%of the area seen as a shallow sedimentary area,namely the platform;in the platform,according to the limestone content and dolomite content open platform and half restricted platform were divided;in the carbonate platform,on the basis of grainstone content,some high-energy grain beach were determined.The early to middle Ordovician lithofacies palaeogeography in the Tarim Basin showed western carbonate platform facies in west,basin facies in east,as a whole a pattern of" West-platform,East-basin;the main lithofacies palaeogeographic units were western Tarim platform and Luoxi platform two platforms and a eastern Tarim Basin.Carbonate grain beaches were mainly distributed in northern Tarim,the Tazhong-Bachu area and Bashakema field profile.