Growth characteristics and sedimentary mode of Permian reefs, Lengwu, Tonglu, Zhejiang Province, southern China

Organic reefs are favourable accumulation spaces for hydrocarbons and various mineral resources. A complete Permian organic reef profile about 44 m thick with distribution area no more than 1 km2 is exposed near Lengwu, Tonglu, Zhejiang Province. Examination of outcrops and thin sections revealed that the main reef-building organisms are calcisponges, with inozoans as dominant type. Five types of rocks have been recognized in the reef, and they are calcisponge framestone, calcisponge bafflestone, bindstone, rudstone and bioclastic wackestone. The profile was constructed in three reef-building stages. The thickness of the second stage is largest, followed by the first stage and the third stage is smallest. Each stage started with a framestone or bafflestone, ended as the reef grew near the sea-level, and died because of influx of terrigenous sediments. The development of the Lengwu reef is controlled by biological factors and sea-level changes. Based on the study a reef sedimentary model is established.