Ordovician reef systems and settings in South China before the Late Ordovician mass extinction

Abstract

Ordovician reefs of the South China Block occur chiefly in three stratigraphic units: (1) the middle Tremadoc Fenhsiang Formation and the upper part of the Lunshan Formation; (2) the late Tremadoc Hunghuayuan Formation; and (3) the middle Ashgill Xiazhen, Sanjushan and Daduhe Formations. The region therefore records part of the Early Palaeozoic reef expansion episode and permits a broader assessment of change in Ordovician reef facies. During middle Tremadoc time, lithistid sponge–bryozoan– Calathium–calcimicrobially dominated patch reefs of the Fenhsiang Formation occur in the high-energy belt of the central Yangtze Platform. Columnar nonskeletal stromatolites of the Lunshan Formation (coeval with Fenhsiang Formation) occur on the southeastern margin of the platform, where low diversity reef-attached organisms, and the generally fine-grained character of the sediment, are consistent with a depth below normal wave base. Calathium and lithistid sponges were the principal reef builders of late Tremadoc reefs and were widespread in the areas of Yichang (Hubei Province) and Dongzhi (Anhui Province), from the platform centre to its margin settings. Bryozoan reef builders occur only in the platform centre with a high diversity of reef dwellers such as brachiopods, trilobites and nautiloids. However, during the same time, at the platform margins, microbes played an important part in reef building together with Calathium and lithistid sponges. The middle-late Tremadoc lithistid sponge–bryozoan– Calathium–microbial community was replaced by a middle Ashgill coral–stromatoporoid community and shows that the succession of community replacement during the Ordovician took a relatively long time. Restricted by the black shales of the main part of the Yangtze region, middle Ashgill reef complexes can be found only on the northeast platform of Cathaysian Land, between the Yushan (Jiangxi) and Changshan (Zhejiang) regions. Carbonate mudmounds are present on the western margin of the Yangtze Platform. On the northeast platform of Cathaysian Land, patch reefs (some higher relief) with talus and biostromes of the Xiazhen Formation consist of high-diversity biotas of corals, stromatoporoids, calcimicrobes, brachiopods and gastropods. The Sanjushan Formation is age-equivalent to the Xiazhen Formation and contains carbonate mudmounds that are composed of abundant calcareous algae and calcimicrobia. Uplift forced a northward extension of Cathaysian Land and caused a regional relative sea-level fall, eliminating this reef complex prior to the first extinction event of the Late Ordovician. The Daduhe Formation carbonate mudmounds occur in the nearshore belt of Kangdian Land (western margin of the Yangtze Platform) and are paraconformably overlain by the Hirnantian Nancheng Formation. In total, the Ordovician reefs of south China show a range of habitats and controls on growth and demise.