PERMIAN-TRIASSIC BOUNDARY MICROBIALITES AT YUDONGZI SECTION, SICHUAN PROVINCE, SOUTH CHINA: GEOBIOLOGY AND PALAEOCEANOGRAPHIC IMPLICATIONS

Unusual microbialite deposits are widely distributed in shallow platform facies Permian-Triassic boundary (PTB) beds, particularly within the Palaeo-Tethys region. However, the genesis of these PTB microbialites still remains enigmatic despite intense study and the association of widespread microbialites with the PTB mass extinction. Herein we present a new example of microbialites from the PTB beds of the Yudongzi section, which is situated along the northwestern margin of the Upper Yangtze Platform of the South China Block. The Yudongzi microbialites are mainly bedded or have a dendritic morphology. Bedded microbialite is characterized by undulating laminated structures, and various amalgamated and layered clots. Dendritic microbialite is bush-like in longitudinal section and formed by clots distributed in an arched arrangement characterized by abundant branching dendroids, which are enclosed in micrite with some shell debris. Abundant microproblematic calcimicrobe structures, preliminarily identified as Gakhumella, are preserved in dark colored, laminated areas of the dendritic microbialite. Each “Gakhumella” individual has densely arranged segments, which form a columnar- to fan-shaped structure. Single segments are arch-shaped and form a thin chamber between segments. “Gakhumella” individuals in the dendritic microbialites are slightly different from each other, but are readily distinguished from the “Gakhumella”- and Renalcis-like calcimicroalgae reported from other PTB microbialites elsewhere in South China, in that they are smaller in size, have columns that do not branch and are made up of densely arranged, arch-shaped segments. Coccoidal aggregates are also observed in the dendritic microbialites, and are suggestive of a biogenetic origin for the Yudongzi microbialites. The PTB microbialites are capped by massive oolite beds that yield giant ooids, implying that the demise of the Yudongzi microbialites may be due to destruction by strong wave currents in high energy habitats.