The biohermites in the Qiangtang Basin and their sedimentary model

The biohermites were widespread within the Tethys Sea,and also in the Qiangtang Basin,during the Triassic to the Cretaceous. The depositional sequences of the biohermites may be classified into following categories:(1) shoal bioclastic limestone and oncolitic limestone → biohermite → open platform micritic limestone → slope breccias;(2) open platform limestone → biohermite → restricted platform dolomitic limestone;(3) shoal bioclastic limestone → littoral clastic rocks,and (4) tidal-flat siltstone and marl → tidal-flat bar bioclastic limestone → biohermite → tidal-flat bar bioclastic limestone → tidal-flat marl. The biohermites may be subdivided into three genetic types:framestone,bafflestone and bindstone. The diagenesis comprises recrystallization,cementation,compression,pressure solution,fragmentation and dissolution. The porosity types consist of primary,fractured,solution openings and fissures. The Late Triassic reef-building organisms are dominated by corals and algae,whereas the Middle and Upper Jurassic ones by coral reefs and sponge reefs. The biohermites occur mostly in the transgressive systems tract,occasionally in the highstand systems tract and rarely in the lowstand systems tract,and consist of coral reefs,sponge reefs,algal reefs and Stromatopora reefs. The coral reefs are observed in the western part of southern Qiangtang,the sponge reefs in eastern Qiangtang,the algal reefs and Stromatopora reefs in the western part of northern Qiangtang,and coral reefs and sponge reefs within the platform. The above-mentioned organic reefs may be discriminated,according to the palaeogeographic settings,into platform-margin and intraplatform reefs. The former occur on a wide range of scale,and may be noticed in seismic profiles. All the organic reefs outlined above are controlled by the global sea-level changes.