Small microbialites from the basal Triassic mudstone (Tieshikou, Jiangxi, South China): Geobiologic features, biogenicity, and paleoenvironmental implications

摘要

We report small microbialites from the calcareous mudstone slightly above the Permian-Triassic boundary (PTB) in the Tieshikou section, southern Jiangxi Province, South China. The newly found microbialite is a bowl-like structure, which contrasts with the surrounding calcareous mudstone. The small microbialite is composed of columnar forms and fan-shaped structures. Mini-columnar structures resemble mini stromatolites. In plane view, most branches are patchy or strip-shaped, with clotted structures, resembling that of a thrombolite. Fan-shaped cement precipitates are comprised of multiple crystal fans that have a radiating texture and that show distinct growth laminae. The radiating fabrics are represented by rod-shaped filaments under SEM, which are interpreted as calcified trichomes/filaments. Prominent micropores occur within the rods, and they may be the plane views of multiple sheaths of the false branching zone of trichomes. Thus, both the columnar forms and radiating fans are probably microbial in origin. Moreover, three types of nano-sized structures: intraparticle micropores, fibrous biofilms, and filamentous sheaths are also pronounced in both the columnar structures and cement fans of the Tieshikou microbialite, all suggestive of biogenicity. These well-preserved fibrous biofilms probably represent calcified extracellular polymeric substances (EPS), and linear sheaths are interpreted as the traces of bacterial activities during the precipitation of dolomite crystals. The Tieshikou microbialite therefore shares similar biogenetic mechanisms with other PTB microbialites. The combination of the absence of pyrite framboids, slightly negative carbon isotope values, and an association with abundant ammonoids, bivalves, and gastropods indicates oxic conditions during the growth of the Tieshikou microbialite. Moreover, the nodular preservational state resembles widely distributed nodular mudstones or muddy limestones of the Lower Triassic successions worldwide. T