中上扬子寒武系蒸发岩岩相古地理

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

关键词:  中上扬子  寒武系  蒸发岩  沉积相  古地理

Key words:  middle-upper Yangtze area  Cambrian  evaporate  sedimentary facies  palaeogeography

摘要:

中上扬子区寒武系蒸发岩层位上属龙王庙阶和毛庄阶至张夏阶,分布在成都以东、贵州遵义以北和武陵山以西,形成四川长宁、重庆永川-江津、湖北利川、武汉4个沉积中心,是早古生代海相油气的重要盖层。经深井揭露,寒武系蒸发岩为石膏和石膏夹石盐,有三种岩石类型:粉砂质白云岩、白云岩与石膏互层、石膏夹石盐。地表以盐溶角砾岩、石盐假晶、次生石膏为标志,川东有少量盐泉出露。寒武纪时中上扬子区蒸发岩沉积具有构造和古地理障壁及岩相障壁,西部除龙门山前分布有岛链状的古陆外,早寒武世末川中古隆起已具雏形,并由西向东迁移扩大;黔中古隆起至中晚寒武世可能为水下无沉积区,具有局限障壁作用。早中寒武世碳酸盐浅滩分布在构造障壁的东侧。构造、古地理和沉积相展布,使中上扬子区转化为局限、封闭或半封闭构造古地理环境,是该区沉积蒸发岩的关键条件。

Abstract:

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.