通过中上扬子地区寒武系野外露头实测、踏勘、前人研究成果整理以及盆地腹地露头缺乏地区的钻井资料的分析,研究了中上扬子地区中寒武世的古地理背景.白云岩的成因、平面上以及垂向上的分布特征都受控于古地理.中寒武统整个台地区为局限台地,在台地发育咸化澙湖、潮坪、局限潮下、浅滩等次一级的古地理单元.中寒武统的膏岩与白云岩不同程度互层.向台地内部,滩相发育愈少,咸化澙湖、潮坪以及局限潮下越发育;越往台地的边缘,滩相越发育.研究区中寒武统白云岩按照晶体大小可以分为泥粉晶白云岩和砂糖状白云岩.泥粉晶白云岩为潮坪准同生白云岩,机理为蒸发泵作用,白云化流体来自于澙湖浓缩的海水.砂糖状白云岩绝大多数为回流渗透白云化成因,白云化流体主要来自于蒸发泵机理富余的高镁钙比流体.回流渗透白云化过程缓慢,白云石结晶较好.寒武系岩性在垂向上有很好的叠置关系,反映古地貌随地质时代的变迁.

Based on summarizing regional data and practical investigation,the sedimentary characteristics of the slope controlled by synsedimentary faults were studied.It is found that rock types include pebble clastic limestone,micrite,cherts,shale and so on,with abundant sedimentary structures,and the thickness of sedimentary rocks in slope is much larger than that in platform and basin.Palaeogeographic division was controlled by bordering fault zone,and locations of platform,slope and basin remained stationary during different phases of the Cambrian.Region of the slope subsided in a tension environment,with a continuous increasing accommodation space and plenty of sediments from platform,which resulted in much thicker strata in slope than that in shallow platform and deep basin.

Using data from tens of measured and observed outcrop successions,thin rock slices and sample analyses,we comprehensively studied the Cambrian sedimentary environments and evolutionary characteristics in the north margin of the Middle-Upper Yangtze Plate.During the Cambrian,platform,slope,and deep sea basin environments were developed in the study area.On the platform,both clastic rocks and carbonate rocks were deposited.Clastic rocks mainly occur in the Lower Cambrian,and were deposited in marine shore and shelf environments.Carbonate rocks are dominant in the Middle and Upper Cambrian,and were deposited in the open platform,restricted platform,tidal flat,beach,and reef environments.Carbonate gravity flow deposits were developed on the slope.In the basin,mainly black shales and chert beds were deposited.The Cambrian represents one large transgression-regression cycle,and maximum transgression occurred in the Qiongzhusi Age of the Early Cambrian.Tectonics and sea level fluctuations had important impacts on sedimentary environments.The Chengkou-Fangxian-Xiangfan Fracture controlled the position of the platform,slope and basin,as well as the silica supply for chert deposition in basin.Sea level fluctuations controlled types of sediments and sedimentary facies on the platform.In the study area,there are good reservoir rocks,including dolomites,grainstones,debris flow deposits,sandstones,and conglomerates;there are good source rocks,including black shales,dark micrites,and chert beds;and there are also good reservoir-source rock assemblages.The hydrocarbon potential of the study area is great.

Based on the observation on outcrops, using the single factor analysis and comprehensive mapping method, the single factor maps of the Meishucun Stage, the lithofacies palaeogeographic maps of the Meishucun Age were compiled. The single factors in this paper mean the factors which can independently reflect some characteristics of sedimentary environments of a stratigraphic interval of a section in an area. Presence or absence as well as high or low percentage of single factors can all independently and quantitatively reflect some characteristics of sedimentary environments of the interval of the section in the area, such as water depth, water energy, nature of the sedimentary environments, etc. The thickness of the interval and its specific rock types, texture constituents, mineral composition, chemical composition, fossils and their ecological assemblages, etc, can all be taken as single factors. This study compiles lithofacies palaeogeographic maps of quantitative nature for the first time for the Meishucun Age in the Middle and Upper Yangtze Region. The model of the sedimentary facies of the Meishucun Age is studied based on the depositional setting and the lithofacies palaeogeographic maps. In the Meishucun Age, the Dianzhong Land, Chuandian Land and Niushoushan Land were developed in the west of the studied area, the Shanyang - Xichuan Land, Hannan Land and Ezhong Land were developed in the north of the studied area. The fine clastic rock shore was developed around the western land, and the carbonate platform, clastic rock shelf, shale-chert bathyaldeep sea were developed seaward. Quantitative lithofacies maps can more accurately reflect palaeogeography by its quantitative data of single factors. Therefore, quantitative lithofacies palaeogeographic maps can be very useful in the exploration of oil and gas as well as other minerals.

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