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

The Nanpanjiang Basin of south China is an embayment in the southern margin of the Yangtze microcontinent that was located in the tropical eastern Tethys Sea during the Permian. The basin is bordered to the north by the Yangtze Platform, a vast shallow-marine carbonate shelf. We investigated the biota and ecological guild structure of reef facies and detailed microfacies of back reef strata deposited immediately preceding the end-Permian mass extinction.

Upper Permian reef facies at Guntianao are massive boundstone with a primary framework of Inozoan and Sphinctozoan sponges bound by laminated microbial crusts and marine cements. Additional frameworks are constructed by branching bryozoans. Tubiphytes encrusts frameworks and forms secondary framework. Organisms dwelling within the framework include foraminifers, fusulinids, brachiopods, bivalves, and dasycladacean algae. Upper Permian reef facies at Yungan are massive boundstone with a primary framework of Inozoan sponges and bryozoans with Rugose corals and Sphinctozoan sponges serving as secondary framebuilders. Dwellers include foraminifers and dasycladacean algae. Reefs at Guntianao are heavily bound by microbial encrustations and marine cement, whereas reefs at Yungan lack these encrustations. Sponges at Guntianao are smaller (up to 1 cm diameter) whereas those at Yungan are larger (up to 5 cm in diameter). In both the Guntianao and Yungan areas we found no evidence of bioerosion (destroyer guild). Backreef strata consist of coarse skeletal grainstone-packstone with diverse open marine biota, fine foraminifer microproblematica grainstone, skeletal wackestone-mudstone, and minor coral sponge boundstone.

Despite the dramatic biotic turnover associated with the end-Permian extinction, the Upper Permian and Middle Triassic reefs of south China are very similar. Sponges are the dominant metazoan framebuilder in both. Both contain Tubiphytes frameworks, and are heavily bound by microbial and marine cement crusts. Even though these boundstones have a significant metazoan framework component, 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.