Early Ordovician sponge-<i>Calathium</i>-microbial reefs on the Yangtze Platform margin of the South China Block

Abstract

Early Ordovician (late Tremadocian to early Floian) reefs on the Yangtze Platform margin of the Hunghuayuan Formation at Zhangzhai section in southeastern Guizhou, South China show three types of reefs in two broad categories: microbial-dominated (stromatolite and lithistid sponge-Calathium-calcimicrobial) reefs and metazoan-dominated (lithistid sponge-Calathium) reefs. These reef types represent different communities controlled by varying environmental settings. Stromatolites tended to develop in the shallow subtidal zone, whereas unlaminated calcimicrobial mounds and metazoan-dominated reefs thrived at greater depths. This is the first report on metazoan-dominated reefs at the platform margin of the Hunghuayuan Formation.