The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China

Peloids are major contributors to modem and ancient limestones, although their origins are not yet comprehensively understood. Upper Permian to Lower Triassic carbonate successions are well preserved at the Bangeng area in Guizhou Province, South China, where microbialites (thrombolites), coccoidal microbes and peloids are preserved together. These peloids are classified into three types (Peloid-A, B, C) that typically occur together along with coccoidal microbes and restricted bioclasts in a variety of microenvironments. Peloid-A is most common and exhibits close relationship with coccoidal microbes. Peloid-A1 has spherical gain and a size range typical for coccoidal microbes (10-60 mum in diameter), whereas Peloid-A2 is marked by larger spherical and subrounded grains with diffuse margins, ranging from 70 to 200 mum in diameter. Peloid-A1 is in part produced by the complete micritic filling in coccoidal microbes by their metabolic activities, whereas Peloid-A2 is related to the calcification of a colony of coccoidal microbes by their metabolic activities, or simply to the aggregation of individual peloids and coccoidal microbes. Peloids of this type might also originate from other carbonate precipitation processes. Peloid-B is derived merely by micrite filling in ostracodes and gastropods skeletons, whereas Peloid-C is of micritized-bioclast origin. These peloidal origins do not explain those of all environments and ages. Peloids are polygenetic in origin, irrespective of the presence of coccoidal microbes. However, it is emphasized that the peloids formed after the end-Permian extinction are dominated by microbes related ones. Those kinds of peloids might have occurred repeatedly and predominantly, especially in severely deteriorated environments such as those after the end-Permian mass extinction. (C) 2003 Elsevier B.V. All rights reserved.