Study On The Characteristics And The Depositional Environment Of The Oolitic Limestones Both From The Fenxiang Formation(Lower Ordovician) In Yichang,Hubei And The Nanjiang Formation(Llandovery) In Wangcang,Sichuan

This paper presents the detailed comparative study on the characteristics and the depositional environment of the oolitic limestones both from the Fenxiang Formation(Lower Ordovician)in Yichang County,Hubei Province,and the Nanjiang Formation(Llandovery,Early Silurian)in Wangcang Coutnty,Sichuan Province.The Lower Ordovician succession in Yichang is a series of typical epicontinental sea carbonate-dominated deposits,but the Lower Silurian in Wangcang consists of mixed terrigenous-carbonate deposits.In different environment,the characteristics of the oolites are quite different.The oolitic limestones of the Fenxiang Formation in Yichang appear as oolitic shoal deposits,which are medium to thick alternating beds with oolitic limestones and bioclastic limestones.Oolites,detritus,calcirudites and bioclasts are mixed together,but the oolites are original deposits that are preserved very well.Petrochemical analyses A1,Ti,Ca show that the content of CaCO3 is up to 69%in the Fenxiang Formation,containing rare terrigenous deposits,which indicates that the depositional setting should be in clean water,which provid a good condition for the autochthonous deposition of oolites.Under the microscope,very rare minor pressure solutions and recrystallization can be found,and the oolites are not dolomitized.With the Mn/Sr ratios,the oxygen isotopic comptositions,as well as the relativity between ?13C and ?18O,it can determine that all the samples are weak diagenesis,in which the original isotopic information can be preserved quite well.But the oolitic limestones from the Nanjiang Formation in Wangcang are interbedded in the ramp succession with the thickness less than 10-20cm in general.Almost all the oolites are broken,with different sizes,mixed with plenty of subhedral,allotriomorphic bioclasts and terrigenous(quartz).All the particles were poorly sorted with different sizes and shapes..Petrochemical analyses show that the content of terrigenous deposits is up to 42%in the Nanjiang Formation,which implies that the environment should not be suitable for the deposition of oolites.All the features indicate that the oolites layers are allochthonous ones.Under the microscope,we can see obvious compaction,pressure solutions and various suture lines.Some oolites recrystallized seriously,and became single crystal oolites.The oolites in the NanjiangFormation were dolomitized seriously.With the Mn/Sr ratios,the oxygen isotopic compositions,as well as the relativity between ?13C and ?18O,it can also conclude that all the oolites from the Nanjing Formation are strong diagenesis,in which,the isotopic data can not represent the original information of the depositional water.Hence all the data need to be corrected prior to infer them.The ratios of Sr/Ba indicate that the paleosalinity of the Fenxiang Formation is higher than that of the Nanjiang Formation.On the other hand,the oxygen and carbon isotope analyses of the oolites in the Fenxiang Formation indicate that the paleotemperature is 27?33?,with average of 31 V,and that of the Nanjiang Formation is 22-34?,with average of 26?.It suggests a tropical to subtropical climates in the mediate to low latitude both during the Early Ordovician and the Early Silurian at the Upper Yangtze Region.Combining with the analysis of the palaeogeographical data and the depositional model,it may conclude that the oolitic limestones of the Fenxiang Formation in Yichang should deposit autochthonously in an open platform facies zone in southwestern Hubei.On the contrary,the thin interbedded oolitic limestones of the Nanjiang Formation in Wangcang should be shifted by periodic strong currents from an inferred oolitic shoal at the platform margin,which might be located in somewhere near Guangyuan and Ningqiang during the Early Silurian.Hence we would like to further infer that the oolitic shoal,the dark rocks underlying and the mudstone overlying should constitute a natural source-reservoir-cap assemblage for this area.