Basin Analysis And Study Of Sedimentary Suites In The Changchengian Aulacogen Of The Middle Proterozoic Of Yanshan, North Chian

Sedimentary basin is a complex system. Regional stuctureaction under plat tectonic setting is the most importantin factors controlling basinal sedimentation and develop-ment. Structure action controls sedementary basins, boththat it opens up sediment sources and that it offers sedi-mentary places. This paper will centre on the study ofrelations between sedimentation and structure action tothe Yanshan aulacogen of Changchengian. The purpose of thiscontribution is to reconstruct the basinal paleo-environ-ments and to determine further the basin nature and itsevolution.The Yanshan basin of Changchengian belongs to a partof long and narrow aulacogen in the interior Sino-KoreanPlateform of the Middle-Late Proterozoic. Detrital rocksand carbonate rocks in approximately 5000m thickness weredeposited in the basin for a long time(1850-1400Ma).Comparing with Athapuscow and Oklahoma aulacogens ofNorth America(Hoffman, 1974), the evolution of the Yanshanbasin of Cangchengian can be divided into three stages:inter-continental graben stage, transitional stage anddownwarping stage. The difference is that this basin doesnot develop reactivation stage like the both formers. Inthe graben stage(1850-1700Ma), axiled sandy river andflank alluvial fans-braid rivers were formed in earlyterrestrial environment; and detrital turbidites, lagoonblack shales, shallow water carbonate and tempestites inmid-late marine envionments. In the transitional stage(1700-1550Ma), fan-deltas, silica-detritus-carbonate com-plex, sandy beach-bar facies were developed. There weresome volcanic action and formed alkali-basalts and trachytes,which are of the characteristics of high content of K O(6.54%) and TiO (1.67%) in the stage. Volcanic rocks showthe feature of alkali-volcanism in interiorcontinent plats.In the downwarping stage(1550-1400Ma), enomous thick blackshale(or marl)-micrite carbonate with rhythm, slumpedbreccia carbonate, gravity current sediments and a lot ofmetal-sulfides were deposited.Structure framework of the Yanshan aulacogen were formedof basin-margin normal faults and transfer faults. By in-ference, northwestern margin fault was a major detachmentfault that controls of the locus of maximum sedimentaryaccumulation-subsidence. The three transfer faults playeda role to regulate extensional rate of each block of thelong and narrow-basin, and they also exert fundmentalcontrols upon sediment distributions, specially to thenorthwest of the basin. Volcanism basically occurs at theintersected position of the both faults.The basin-fill sequence is typically asymmetric in geometry ofcross-section. The asymmetrial nature in the basin is due to continued struture tilting on the hang-ing wall of the sortheastern marginal normal fault.According to Meng xianghua'(1982, 1987) classificationof sedimentary suites(SDS), the sediment fill of theaulacogen belongs to substable type SDS in the locusand the sortheast of the basin and stable type SDS inthe northwest, mainly including bitrrigenous SDS, volca-nic monoterrigenous SDS, black shale SDS, black shale-carbonate SDS, monoterrigenous SDS and so on.Formation and evolution of the Yanshan aulacogen ofChangchengian reflectedthe early extension of the MiddleProterozoic in the North China. The weakness zone ofprovious basement structure was one of the important fac-tors to initial strcture action of the au(?)acogen. Theaulacogen was developed under the background of widerstructure-hot events on the Sino-Korean Platform andits adjacent area during the Middle-Late Proteroic.