UPPER PERMIAN REEF AND SHELF FACIES OF THE SOUTHERN MARGIN OF THE YANGTZE PLATFORM, SOUTH CHINA PROVIDE INSIGHT INTO REEF ECOLOGY AND ENVIRONMENTS PRIOR TO THE END-PERMIAN MASS EXTINCTION

Upper Permian reef facies along the southern margin of the Yangtze Platform, in the Nanpanjiang Basin of south China, are massive boundstone with a primary framework of inozoan and sphinctozoan sponges bound by laminated microbial crusts, Tubiphytes and marine cements. Secondary frameworks are constructed by rugose corals, branching bryozoans and Tubiphytes. Organisms dwelling within the framework include fusulinids, smaller foraminifers, brachiopods, bivalves, and dasycladacean algae. We found no evidence of bioerosion.

Backreef strata consist of finely comminuted and rounded skeletal grainstone, laminated dasycladacean grainstone and peloidal microtube grainstone, fine skeletal wackestone, mud-rich skeletal packstone with fragments of large open-marine organisms, and minor sponge-coral boundstone. The finely comminuted and rounded grainstone represents the shallowest subtidal high-energy environments to the north, whereas the packstone and boundstone represent deeper, low energy open-marine environments to the south. Point counts show a significant increase in skeletal diversity to the south.

Facies patterns in the overlying basal Triassic microbialite parallel trends in the uppermost Permian: Thinner microbialite development in the north represents lesser creation of accommodation space over shallower, higher energy areas in the Upper Permian, thicker microbialite represents greater accommodation developed over the deeper low energy areas of the Upper Permian to the south. Basal Triassic microbialites contain a greater proportion of skeletal grainstone interbeds indicating greater current energy on the ramp facing the open ocean to the south.

Despite the dramatic biotic turnover associated with the end-Permian extinction, the Upper Permian and Middle Triassic reefs of south China are similar. Sponges are the dominant metazoan framebuilder in both. Both contain framework of tubular micrite walled fossils and are bound by microbial and marine cement crusts. Even though the boundstones have a significant metazoan framework, they also have a large (in some cases dominant) volume of microbial and abiotic (cement) precipitates. Seawater chemistry (anoxia and carbonate saturation) rather than evolutionary trends likely explains the large microbial and abiotic component.