Search

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. [ABSTRACT FROM AUTHOR]

Abstract A new Permian–Triassic boundary microbialite (PTBM) is described from the Xiajiacao section of western Hubei Province, South China. The new microbialite, 3.16 m thick, comprises a thin layer of stromatolite and a thick thrombolite unit. An irregular contact separates the uppermost Permian skeletal packstone from the post-extinction stromatolite, but it is not yet possible to discriminate whether it was formed by submarine solution in the wake of ocean acidification or subaerial exposure due to regional regression, or a combination of both. The stromatolite shows "cabbage-like" morphology, and the thrombolite is characterized by centimetric clotted texture. Abundant columns of the microproblematica structure Gakhumella , coccoid-like spheroids, bacterial clump-like spheroids, and hollow spheroids are recognized in both stromatolites and thrombolites and may have played an important role in accretion of the microbialites. Pyrite framboid analysis indicates that microbialites may have been affected by lower dysoxic to upper dysoxic conditions in the immediate aftermath of the Permian–Triassic extinction. Stratigraphic abundance of both high-temperature grains (β-quartz and glassy balls) peaked ~ 20 cm below the biotic extinction horizon, implying that either intensive volcanic eruption occurred only just prior to biotic extinction, or volcanism was still intensive during biotic extinction, but volcanic grains were not deposited in the Xiajiacao locality, likely due to the shallow, agitated environment caused by the regional regression. Overall, microbial bloom, indicated by the widespread PTBMs, seems to have been little affected by the contemporaneous volcanism. Highlights • Gakhumella,and coccoid-like, bacterial clump-like and hollow spheroids are crucial in accretion of microbialite; • Pyrite framboid size and morphology show that the microbialite may have been deposited in lower to upper dysoxic conditions. • Microbial bloom, indicated by the widespread PTBMs seems not to be linked with the contemporaneous volcanism. [ABSTRACT FROM AUTHOR]

A late Smithian ichnoassemblage is reported from the Lower Triassic succession from the Lichuan section, western Hubei Province, South China. This ichnoassemblage comprises 13 ichnogenera, which include simple, horizontal traces ( Cochlichnus , Cosmorhaphe , Didymaulichnus , Gordia , Circulichnis , and Planolites ), vertical traces ( Arenicolites , Laevicyclus ), oblique- or horizontal branching traces ( Palaeophycus ), slightly complex burrow networks ( Thalassinoides , and Treptichnus ), and grazing traces ( Cosmorhaphe and Gyrochorte ). These traces are also categorized into fodinichnia, domichnia, pascichnia, and repichnia ethologic types, suggesting a fairly high degree of behavioral complexity. Of these, the fodinichnia traces are most abundant. The ichnoassemblage horizons usually have rather high ichnofabric indices (ii), reaching ii 4–5. Bedding plane coverage is up to 70–90%, resulting in high bedding plane bioturbation index as well (BPBI 4–5). The Lichuan ichnoassemblage traces have mean and maximum diameters of 4.8 mm and 9 mm, respectively ( n = 342). The traces penetrate to a depth of up to 30 mm into the sediment, with an average depth of penetration of 15 mm, indicating third to fourth tiering levels (3–4). Overall, the Lichuan ichnofauna shows an abrupt increase in ichnodiversity, burrow width, behavioral complexity, and ecologic tiering level, which are indicative of recovery stages of 2–3 in the late Smithian. When compared to five coeval ichnoassemblages from other sections in South China (Yashan, Susong, Daxiakou, Gaimao, and Tianshengqiao) and coeval ichnoassemblages from elsewhere (northern Italy, Western Australia, western US), the Lichuan trace-fossil assemblage reaches similar level of diversity during this time interval. Paleoenvironmental analysis indicates that the late Smithian recovery pulse is typically confined to the offshore transition setting, whereas more distal zones record much lower levels of recovery. Therefore, time and environmental conditions were the two crucial factors controlling the rate and degree of recovery of trace-making organisms in the aftermath of the latest Permian mass extinction. [ABSTRACT FROM AUTHOR]