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

Key words: 
• genetic types of sequence boundary /
• basin evolution /
• marine facie sequence /
• Youjiang basin /

Abstract: Based on the research of marine facies sequence stratigraphy in Youjiang basin area, 44 sequences can be distingui shed uhich can be divided into 6 ultrasequences,and they respond well to the e volution of the Youjiang ba sin as well as to basin-mountain transformation during the Devonian to the ear ly Triassic. The surface of sequence boundary is a negative sedimentary record w h ich depends on the physical property of the sediments below and above the bounda ries and/or on the different sedimentary structure. Every sequence surface of bo undary represents each sedimentation and tectonic movements in the sedimentary b a sin because the tectonic movement can enhance or weaken the marks for sequence b oundaries. According to the sequence stratigraphy and basin evolution in Youjia ng 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 unconformit y, the exposed sequence unconformity and the orogenic erosional sequence uncon formity. Uplift erosional sequence unconformity is caused by tectonic uplifting or by s e a-level falling. This unconformity represents isochronous surface of boundary w hi ch show the new basin genesis or basin-basin transformation. Transgressive onla p sequence unconformity is a transgressive surface which show synchronism bewtee n tectonic subsidence of basin and sea-level rising. The exposed sequence u nc onformity take place in the steady tectonism stage. The orogenic erosional uncon f ormity means that the sequence boundaries may be highlighted by tectonism in the course of the transformation of the basin systems into the orogenic processes.