TECTONIC EVOLUTION AND SEDIMENTARY CHARACTERISTICS OF THE FORELAND BASIN IN THE NORTHERN PART OF LOWER YANGTZE AREA

The northern part of lower Yangtze area is controlled by the interaction of the Yangtze platform,the North China platform and the Qinling micro-platform.The Yangtze platform starts to dive to the Qinling platform during the late Permian and the northern part of lower Yangtze area accomplishes the tectonic environment conversion from extension to extrusion.The foreland basin in the northern part of lower Yangtze area is formed during the middle Triassic.The evolution of the foreland basin can be divided into 3 stages:1)sprout and budding stage(P₃-T₁),during which the passive continental margin basin is formed,characterized by a set of typical deep-water to semideep-water flysch;2)flourishing stage(T₂-T₃),during which the marine foreland develops to continental foreland,forming marine molasse and continental molasse;3)continued and extinction stage(J_1-2),during which the intracontinental foreland is formed,characterized by coal-bearing clastic rock sediments of continental facies.The foreland basin experiences the evolution from marine to continental facies gradually,and the sedimentary characteristics diverge with the different stages.Though the sedimentation and subsidence center migrates continually,it is characterized by continental facies superposed to marine facies.