Biotic structure and morphology of patch reefs from South China (Ningqiang Formation, Telychian, Llandovery, Silurian)

Summary Ningqiang Formation (late Telychian, Llandovery, Silurian), characterized by nearly 3000 m of shales in tercalated with carbonates, is situated between Ningqiang (S. Shaanxi Province) to Guangyuan (N. Sichuan Province) adjacent to the northwest margin of the Yangtze Platform. The high diversity “Xiushan Fauna”, and abundant reef development, illustrate a relatively warm and persistent shallo marine environment in these early Silurian sediments. The sequence shows reef radiation after recovery from the end Ordovician mass extinction envents. Multiple horizons of reef-building occurred within a relatively short geological interval and resulted in more than 30patch reefs up to 200 m in diameter and 1–50 m vertically, composed of abundant fossils. Reef biota include frame-building corals, stromatoporoids, bryozoans, and microbialites, and reef-associated oranisms such as crinoids, brachiopods, trilobites, gastropods, nautiloids and ostracods. Three reefrelated biotic associations are recognised: a) reefs dominated by framework with crinoids and microbia; b) reefs dominated by only crinoids and microbia; and c) crinoiddomainated facies. Seven representative reef examples illustrate different morphologies and growth styles. A high terrigenous debris input and shallow epicontinental ramp, which lacked obvious topographic variation, were major controls which resulted in rather simple reefs; sedimentation was apparently the main constraint on lateral and vertical extension of reefs, and prevented large-scale reef complexes developing.