Microbialite of anoxic condition from Permian-Triassic transition in Guizhou, China

Abstract

Spherical microfossils are present in the Hindeodus parvus zone of the Lower Triassic in Ziyun, Guizhou Province. They generally range from 0.15 to 0.30 mm across, with micritic wall and filled by micro-sparry calcites, and are evenly scattered in micritic matrix. Their abundance makes the rock assigned to microbialites. The accompanied organisms include ostracods and algal mat, but no gastropods or bivalves. Presence of small (<7 μm) pyrite framboids indicates that this bed formed in anoxic conditions. In some sections, this bed is overlain and underlain by tidal-flat micritic limestone with microgastropods and small burrows. Occurrence only in deposits on reef top indicates that the microbial organism was benthic, and needed sunlight in life. The size of the microbial fossil exceeds that of any bacteria or cyanobacteria. Thus, it does not belong to these two phyla. It may belong to lower green algae, and is assigned to a new species in a new genus, Ziyunosphaeridium sinensis gen. et sp. nov. Up to now, the rocks described as microbialites from the Permian-Triassic transition include six types: (1) porous micritic limestone such as that descried from Japan by Sano and Nakashima (1997), (2) limestone with rich globular microfossils such as that described from the Nanpanjiang Basin in China by Lehrmann (1999), (3) dendroidal limestone such as that described from the vicinity of Chongqing by Kershaw et al. (1999), (4) spherical microbial limestone adapted to anoxic environments described from Ziyun, Guizhou reported in this paper, (5) limestone with cyanobacterial fossils such as that described by Wang et al. (2005), and (6) stromatolites. All these microbialites are not reefs in the proper sense, and the argument that Permian reef ecosystems extended into the Mesozoic is incorrect.