Sedimentary Microfacies in the Lower Ordovician strata in Shizhu,Chongqing

Thirteen sedimentary microfacies and five sedimentary subfacies have been distinguished for the Lower Ordovician strata in Qiliao,Shizhu,Chongqing.The restricted platform subfacies are recognized in the Lower Ordovician Nanjinguan Formation and the lower part of the Fenxiang Formation,where there occur limestone intercalated with terrigenous clastic lenses and oolitic limestone.The open platform subfacies are arranged into the middle and upper parts of the Fenxiang Formation,in which ooiltic shoal and argillutite are accentuated.The basal part of the Honghuayuan Formation dominated by alternating bioclastic limestone and mudstone is assigned to the platform-margin beach subfacies,whereas the foreslope subfacies occur in the middle and upper parts of the Honghuayuan Formation occupied by thick-bedded dark grey cherty nodular limestone and calcirudite intercalated with bioclastic limestone.The Dawan Formation built up of alternating bioclastic limestone and shale belongs to the inner shelf subfacies.Multidisciplinary analysis indicates that the Lower Ordovician strata in the study area are considered as the deepening-upward depositional cycles from the Lower Ordovician Nanjinguan Formation to the Dawan Formation.