Study of Palaeoecology of Middle Jurassic Reefs in Suoxian and Baqing area of Northern Tibet

Key words: 
• organic reefs /
• paleoecology /
• way of reef-building /
• Middle Jurassic /
• Northern Tibet

Abstract: Suoxian and Baqing county are located in the south side of Tangula mountain and belong to the Qiangtang stratigraphic area. However, Liuwan Formation of Suoxian and Buqu Formation of Baqing are especially characterized by a lot of organic reefs. Reef-building organisms are chiefly hexacorals Schizosmilia, Stromatoproids Parastromatopora and Bivalve Liostrea. The reef-building organisms mentioned above are majority preserved at the original growth state, which all require warm, clean and normal shallow water environment with abundant light and better circulation. 
  Through the study of individual ecology and analysis of assembled of reef-building organisms, five reef-building communities are recognized, namely Schizosmilia-Parastromatopor，Parastromatopora， Schizosmilia，Liostrea and Cyanobacteria-Liostrea community. Among these five reef-building communities，First three communities formed reef-building community evolutionary sequence of Suoxian county area, while, Liostrea and Cyanobacteria-Liostrea community made up reef-building community evolutionary sequence of Baqing county area. Community evolution research indicates that it exists two kinds of community evolution in the studying areas, the first one is community succession that mainly developed in reefs of Liuwan Formation at Suoxian county, the second one is community replacement that developed in reefs of Buqu Formation in Baqing county.
  The developing characteristic of biological-baffling. Among these three reef-building, frame-making, baffling are developed in reefs of Liuwan Formation at Suoxian county, baffling and bind-baffling are seen in reefs of Buqu Formation in Baqing county. The authors hold that the existenc of reef communities is of great significance for further study of merging time of Bangongcuo-Nujiang River suture zone. The authors consider that oceanic crust subduction time of middle-east Bangongcuo-Nujiang River suture zone(ie. Suoxian and Baqing county region) should be in early age of Middle Jurassic or before the deposition of Liuwan Formation of Middle Jurassic. The existence of reef communities of Middle Jurassic showed that the study area may belong to a part of shallow continental shelf of the remaining back-arc basin after Bangongcuo-Nujiang River oceanic crust subduction.