Genesis of Sequence Boundary and Basin Evolution in Youjiang Basin during the Devonian to the Middle Triassic

Based on the research of marine facies sequence stratigraphy in Youjiang basin area, 44 sequences can be distinguished uhich can be divided into 6 ultrasequences,and they respond well to the evolution of the Youjiang basin as well as to basin\|mountain transformation during the Devonian to the early Triassic. The surface of sequence boundary is a negative sedimentary record which depends on the physical property of the sediments below and above the boundaries and/or on the different sedimentary structure. Every sequence surface of boundary represents each sedimentation and tectonic movements in the sedimentary basin because the tectonic movement can enhance or weaken the marks for sequence boundaries. According to the sequence stratigraphy and basin evolution in Youjiang basin, 4 genetic sequence boundaries can be reconstructed during the Devonian to the early Triassic, which suggest different basin evolution stage. They are uplift erosional sequence unconformity, transgressive onlap sequence unconformity, the exposed sequence unconformity and the orogenic erosional sequence unconformity. Uplift erosional sequence unconformity is caused by tectonic uplifting or by sea\|level falling. This unconformity represents isochronous surface of boundary which show the new basin genesis or basin\|basin transformation. Transgressive onlap sequence unconformity is a transgressive surface which show synchronism bewteen tectonic subsidence of basin and sea\|level rising. The exposed sequence unconformity take place in the steady tectonism stage. The orogenic erosional unconformity means that the sequence boundaries may be highlighted by tectonism in the course of the transformation of the basin systems into the orogenic processes.