Sequence Stratigraphy And Lithofacies Paleogeography Of The Cambrian In The Middle-eastern North China Platform

Marine carbonate rocks are well developed during the Cambrian period in the mid-eastern North China Platform,which are of great potential for petroleum accumulation and great prospect for exploration.The understanding of the deep hydrocarbon reservoirs,especially the developing mechanism of the inner buried hill accumulations,has been restricted severely by the limited knowledge concerning the distribution pattern and evolution of the carbonate rocks.Based on the analysis of sections,cores,well loggings,seismic profiles,thin sections and geochemical characteristics,this paper establishes the sequence stratigraphic framework of the Cambrian succession in the mid-eastern North China Platform.The research also discusses the lithofaices palaeogeography within the sequence stratigraphic framework and the sedimentary systems evolution.Then the main controlling factors on sequence stratigraphy and sediments distribution are analyzed for a better understanding.The Cambrian succession is composed of many vertically stacked meter-scale cycles,which includes the open type formed in the open water masses and the limited type developed in limited depositional settings.Based on the recognition of typical sequence boundaries such as the unconformity,lithofacies transition surface and karst surface,according to the stacking pattern of meter-scale cycles and the geophysical response features,this paper determine the 3^rd sequence boundaries and maximum flooding surfaces,and accordingly divide the Cambrian succession into seven 3^rdd sequences.The SQ1 refers to the Changping Formation,which is equal to the Zhushadong Formation and the Fujunshan Formation.The SQ2 is composed by the Mantou Formation and the SQ3 refers to the Maozhuang Formation.The SQ4 includes the Xuzhuang Formation and the Zhangxia Formation.The SQ5,SQ6 and SQ7 refer to the Gushan Formation,Changshan Formation and Fengshan Formation,respectively.Each 3^rd sequence constitutes the transgressive system tract and highstand system tract,which are well laterally distributed.According to the mineral compositions,textures and typical structures,the Cambrian sediments in the mid-eastern North China Platform is divided into three categories,referring to the carbonate rocks,clastic rocks and the peperites,and fourteen lithofacies.Seven main types of subfacies are identified,namely the low subtidal zone,microbial mats,supratidal zone mudflat,grainstone shoal,shoalface beach,tempestite and intraplatorm basin,according to the vertical combination of different lithofacies in the system tracts.In this study,subfacies with a thickness exceeding 30%of the total thickness are selected as the predominant subfacies of the system tracts and the combination of subfacies are applied to reconstruct the depositional environments.The Changping succession were deposited in restricted inner gentle slope,which evolved into the mixed shelf for the early Cambrian Stage 3.The Zhangxia succession dominated by grainstone beach aggradated to become the rim of the shelf in the eastern North China Platform,contributing to a rimmed platform background for the late Stage3 and the Furongian,which are dominated by the subtidal deposits.The sequence stratigraphy and the evolution of the depositional system of the Cambrian for the mid-eastern North China Platform is comprehensively influenced by the global sea-level fluctuations,regional tectonism,seawater physicochemical properties,storm processes and the terrigenous input.The SQ1 and SQ5 are tectonic sequences controlled by the global sea-level fluctuations and regional tectonism.The intra platform basin firstly reported in this study is induced by the contemporaneous normal faults.The frequent storms,the paleoclimate and the terrigeneos supply determined the pattern and distribution of the dominated subfacies.