Permian–Triassic boundary microbialites (PTBMs) in southwest China: implications for paleoenvironment reconstruction

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• 作者：Hao Tang ; Stephen Kershaw ; Hong Liu ; Xiucheng Tan ; Fei Li ; Guang Hu ; Cheng Huang…
• 关键词：Permian–Triassic boundary microbialites (PTBMs) ; Conodont ; Anoxic upwelling ; Redox condition ; Early Triassic recovery ; Yudongzi ; Southwest China
• 刊名：Facies
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：63
• 期：1
• 全文大小：6,858 KB
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Sedimentology
  Biogeosciences
  Geochemistry
  Paleontology
  Ecology
  
• 出版者：Springer Berlin / Heidelberg
• ISSN：1612-4820
• 卷排序：63

文摘

Permian–Triassic boundary microbialites (PTBMs) are commonly interpreted to be a sedimentary response to upwelling of anoxic alkaline seawater and indicate a harsh marine environment in the Permian–Triassic transition. However, recent studies propose that PTBMs may instead be developed in an oxic environment, therefore necessitating the need to reassess the paleoenvironment of formation of PTBMs. This paper is an integrated study of the PTBM sequence at Yudongzi, northwest Sichuan Basin, which is one of the thickest units of PTBMs in south China. Analysis of conodont biostratigraphy, mega- to microscopic microbialite structures, stratigraphic variations in abundance and size of metazoan fossils, and total organic carbon (TOC) and total sulfur (TS) contents within the PTBM reveals the following results: (1) the microbialites occur mainly in the Hindeodus parvus Zone but may cross the Permian–Triassic boundary, and are comprised of, from bottom to top: lamellar thrombolites, dendritic thrombolites and lamellar-reticular thrombolites; (2) most metazoan fossils of the microbialite succession increase in abundance upsection, so does the sizes of bivalve and brachiopod fossils; (3) TOC and TS values of microbialites account respectively for 0.07 and 0.31 wt% on average, both of which are very low. The combination of increase in abundance and size of metazoan fossils upsection, together with the low TOC and TS contents, is evidence that the Yudongzi PTBMs developed in oxic seawater. We thus dispute the previous view, at least for the Chinese sequences, of low-oxygen seawater for microbialite growth, and question whether it is now appropriate to associate PTBMs with anoxic, harsh environments associated with the end-Permian extinction. Instead, we interpret those conditions as fully oxygenated.