Early Silurian (Telychian) bryozoan reefs in the epeiric sea of South China: Are heterotrophic metazoan buildups promoted by internal waves?

Abstract

After the mass extinction at the end of the Ordovician period, metazoan reefs recovered and became widespread on the Yangtze carbonate platform by the late Aeronian, as recorded by a diverse and complex reef system in the Shihniulan Formation. In the overlying siliciclastic-dominated Hanchiatien Formation, the reefs declined in size and biodiversity along with a sudden facies change during the lower Telychian. Without any phototrophic organisms, small reefs were primarily built by abundant bryozoans, which crop out in the lower part of this formation at the southern tip of Chongqing, Southwest China. These reefs occur as massive argillaceous limestones, varying in size from 1.6 to 2.8 m in thickness and 6 to 16 m in diameter. The reefs are surrounded by gray-green shales and silty shales. Distinct vertical ecozonation is present, and four growth stages have been recognized, represented by four facies types from bottom to top: (1) auloporid coral-echinoderm-bryozoan packstone/floatstone, (2) fenestrate bryozoan-echinoderm bafflestone, (3) Fistulipora-Asperopora framestone, and (4) Fistulipora framestone. In tropical mid- to outer-ramp settings, these bryozoan reefs showcase an unusual reef community in heterozoan carbonates, which were probably influenced by internal waves.