Composition and evolution of the Upper Triassic organic reef in the Riganpeicuo Formation of Gaize County, Tibet

Organic reefs of the Upper Triassic are well developed in Gaize County of Tibet. The reef-building organisms consist of hexacorallia, calcispongiae and calcareous algae, whereas the reef-inserted organisms consist of foraminifera, brachiopods, bivalves, gastropods, echinoderms and bryozoans. Biological sequence from the bottom to the top of a single reef was developed through five stages: calcareous algae, bivalves and gastropods→calcareous algae, bivalves and hexacorallia→hexacorallia→hexacorallia, calcispongiae and bryozoans→bioclast. Correspondingly, the lithological sequence from the bottom upward can also be divided into five members: bioclastic limestone→bafflestone→frame stone→bind stone→Bioclastic and intraclastic limestones. The development of platform margin reef generally includes three cycles, each of which has the same structural composition and can be divided into reef base, reef core and reef cap from the bottom upward. It is shown that the history of organic reefs has experienced four evolutionary stages, i.e., reef foundation, early flourishing, prosperousness and reef declining. Cycle Ⅰ and Ⅱ are at a large scale, and have complicated construction, whereas Cycle Ⅲ is small and simple comparatively. All of the three cycles have formed the secondary prograde sequence under a transgressive background.