Revisiting the Yudongzi microbialites (basal Triassic, northwestern Sichuan, South China): Fabric textures and paleoenvironmental implication

Abstract

Unusual sedimentary deposits such as stromatolites, thrombolites, dendrolites, and giant ooids are widely distributed in the Permian–Triassic boundary (PTB) successions of shallow platform facies worldwide. These microbial fabrics, however, are rarely present in the same section. The genesis of these PTB microbialites and the oceanographic conditions linking to the end-Permian mass extinction still remain disputed despite intense study. Here, we report a stromatolite-dendrolite-thrombolite-giant ooid association from the PTB beds of the Yudongzi section on the northwestern margin of the Yangtze Platform, South China, which was situated in low latitudes during the Permian-Triassic (P-Tr) transition. The Yudongzi microbialites rest on an irregular contact with underlying Permian bioclastic limestone and are comprised of lower bedded stromatolite, digitate dendrolite, upper bedded stromatolite, and thrombolite units. The bedded stromatolite is characterized by undulating laminated structures and various amalgamated and layered clots. The digitate dendrolite is made up of digitate-like branches, formed by clots distributed in an arched arrangement, and characterized by abundant branching dendroids. The thrombolite displays distinct clotted textures that comprise a mixture of dark-colored microbial fabric and light-colored interstitial sediment. In microbialite, columnar structures identified as Gakhumella-like calcimicrobe are very abundant. Each individual of Gakhumella-like structure has densely arranged segments forming a columnar- to fan-shaped micro-structure. Single segments are arch-shaped and form a thin chamber between segments. Coccoid-like, bacterial clump-like, and other spheroidal aggregates are also commonly present and suggestive of a biogenetic origin for the Yudongzi microbialites. Moreover, distinct aragonite crystal fans are also for the first time found in the PTB microbialites in South China, suggesting that the fans may have resulted from high inorganic carbon concentrations, and thus high levels of carbonate dioxide in the seawater. The Yudongzi microbialites are capped by shelly grainstone and a massive giant oolite, implying that the demise of the Yudongzi microbialites may be due to destruction by strong wave currents in a high energy setting. Some ooids are coated unevenly by micritic laminae (or envelopes) indicating the possibility of a microbe-related origin. These microbial-coated giant ooids, together with the massive microbialites, may imply the long-term of microbial bloom in the oceans immediately after the end-Permian extinction. The co-existence of abundant Gakhumella-like microorganisms and oxygen-dependent invertebrates, together with some coccoid-like spheroids, aragonite cement crystal fans, and pyrite framboids indicative of oxygen-poor conditions, implies that oxygenation state of the PTB microbialites is much more complex than previously recognized, demonstrating need for continued multidisciplinary examination of facies changes associated biotic mass extinction.