Microbial carbonates as contributors to Upper Permian (Guadalupian-Lopingian) biostromes and reefs in carbonate platform margin setting, Ziyun County, South China

摘要

Permian (Guadalupian and Lopingian) reefs in Ziyun County, southern Guizhou, southwestern China, occur in a carbonate platform margin setting. Guadalupian reefs (Maokouan) are characterized by sponges, calcimicrobes, microbialites (as microencrusters) and syndepositional calcite cement. Global sea level falling in the latest Guadalupian changed the reef biotopes. Lower Lopingian (Wuchiapingian) deposits reflect a rapid transgressive-regressive cycle; reefs were not developed during that time, although coral biostromes are common. However, microbial carbonates occur commonly in these biostromes, including dark-coloured, homogeneous microbialite, free-growing microbes, Shamovella and Archaeolithoporella (interpreted to be problematic microbial deposits). Upper Lopingian (Changhsingian) reefs were formed by sponges, microbialites, Shamovella, Archaeolithoporella, automicrite and syndepositional calcite cement. Common primary encrustations consist of thin, homogeneous, subparallel layers of Archaeolithoporella, which is an important reef builder particularly throughout the Lopingian reef succession. Secondary encrustations are characterized by dark-coloured, homogeneous microbialite containing thin thalli that alternate with light-coloured microspar/pseudospar. Reef-building organisms (e.g., sponges, Archaeolithoporella, calcimicrobes and hydrozoans) were bound, lithified and preserved by syndepositional calcite cement and microbially precipitated micrite (automicrite). In Changhsingian reefs of the Shitouzhai Limestone, microbial carbonate (e.g., micritic peloidal crusts and automicrite layers) encrusted the top, sides and undersides of in situ organisms and also grew on their upper surface as thick accumulations. Radiaxial fibrous calcite cement is present, but is not common. Abundant microbial carbonates in the Guadalupian to Lopingian reefs in Ziyun indicate that microbial precipitation of calcium carbonate played a vital role in the development of Permian reefs in this platform margin setting. (c) 2004 Elsevier B.V. All rights reserved.