Sedimentary facies and palaeogeography of the evaporates in the middle-upper Yangtze area

The evaporates as the cap rocks of the Early Palaeozoic marine oil and gas accumulation occur on a wide range of scales and in tremendous thickness in the central-upper Yangtze area,and are assigned to the Longwangmiaoan and Maozhuangian-Zhangxian strata to the east of Chengdu in Sichuan,north of Zunyi in Guizhou and west of the Wuling Mountains in Hunan and Guizhou.Four depocenters have been distinguished for the evaporates: Changning in Sichuan,Yongchuan-Jiangjin in Chongqing,Lichuan and Wuhan in Hubei.The Cambrian evaporates composed of gypsum and gypsum intercalated with halite are grouped,as revealed by the deep-seated drilling,into three rock types: silty dolostone,interbeds of dolostone and gypsum,and gypsum intercalated with halite.In the surficial outcrops,the main indications include evaporate-solution breccias,halite pseudocrystals and secondary gypsum.In addition,salt springs are studded as well in eastern Sichuan.Two barriers including tectonic-palaeogeography and sedimentary facies barriers were organized for the deposition of the evaporates in the study area during the Cambrian.Apart from the island-chain old lands in front of the Longmen Mountains,the central Sichuan old uplift came into being during the latest Early Cambrian,and extended further eastwards.During the Early Cambrian,the central Guizhou old uplift may lie buried and thus appeared as the subaqueous non-depositional area as a restricted barrier till the Middle to Late Cambrian.Moreover,the carbonate shoals were developed on the eastern flank of the tectonic barrier during the Early to Middle Cambrian.The features of tectonics,palaeogeography and sedimentary facies are responsible for the restricted,enclosed or semi-enclosed tectonic-palaeogeographic settings,and thus are favourable for the deposition of the evaporates in the study area.