An Early Triassic (Smithian) stromatolite associated with giant ooid banks from Lichuan (Hubei Province), South China: environment and controls on its formation

Here we report the thickest Early Triassic stromatolite so far recorded, which developed within giant ooid banks in the upper Smithian succession (Lower Triassic) of the Lichuan area, western Hubei Province, South China. Approximately 16. m of stromatolite is embedded within ca. 30-m-thick oolitic limestones that crop out in the upper Daye Formation. The associated conodonts suggest a late Smithian (Early Triassic) age for the stromatolite-ooid complex. Stromatolites exhibit domal, stratified columnar, wavy laminated, cabbage-shaped, roll-up, and conical structures. Stromatolites are overlain by thick oolitic limestone, implying that the demise of the Lichuan stromatolite may be attributed to an environmental change to agitated shallow waters. Four types of microbially-induced microstructures are recognizable in the stromatolites, including diffuse laminated, reticular, intraclastic, and irregular clotted microstructures. Co-occurrence of these microbe-induced microstructures indicates a biogenic origin for the Lichuan stromatolite. The ooids can be categorized into circular, compound, superficial, and irregular types. Some interior layers within ooids exhibit intense fluorescence, indicative of microbial organomineralization, which may have contributed to the formation of the ooids. Moreover, abundant nanometer-scale textures and particles are usually interpreted as microbial involvement during the formation of the dolomite. These nano-textures in both stromatolites and ooids, and authigenic quartz grains commonly preserved in stromatolite can be attributed to abundant organic matter in seawater, resulting from microbial proliferation. The stromatolites described here are evidence of flourishing microbial communities on both eastern and western margins of the Paleo-Tethys Ocean during the middle Early Triassic, suggesting episodic degradation of marine ecosystems after the Permian-Triassic mass extinction.