Calcilobes wangshenghaii n. gen., n. sp., microbial constructor of Permian–Triassic boundary microbialites of South China, and its place in microbialite classification
Authors: Kershaw Stephen , Zhang Tingshan , Li Yue 
Published: September 2021

AbstractPermian–Triassic boundary microbialites (PTBMs) that formed directly after the end-Permian extinction in the South China Block are dominated by one structure, a lobate-form calcium carbonate construction that created extensive very thin (ca. 2–20 m thick) framework biostromes in shallow marine environments, effectively occupying the ecological position of the prior pre-extinction Permian reefs and/or associated carbonates. In the field, vertical sections show the microbialite is dendrolite (branched) and thrombolite (clotted), but because thrombolite may include branched portions, its structure is overall best classed as thrombolite. In the field and in polished blocks, the microbial material appears as dark carbonate embedded in lighter-coloured micritic sediment, where details cannot be seen at that scale. In thin section, in contrast to the largely unaltered micritic matrix, the microbial constructor is preferentially partly to completely recrystallised, but commonly passes gradationally over distances of a few mm to better-preserved areas comprising 0.1–0.2 mm diameter uneven blobs of fine-grained calcium carbonate (micrite to microsparite). The lobate architecture comprises branches, layers and clusters of blobs ca. 1–20 mm in size, and includes constructed cavities with geopetal sediments, cements and some deposited small shelly fossils. Individual blobs in the matrix may be fortuitous tangential cross sections through margins of accumulated masses, but if separate, may represent building blocks of the masses. The lobate structure is recognised here as a unique microbial taxon and namedCalcilobes wangshenghaiin. gen., n. sp.Calcilobesreflects its calcium carbonate composition and lobate form,wangshenghaiifor the Chinese geologist (Shenghai Wang) who first detailed this facies in 1994. The structure is interpreted as organically built, and may have begun as separate blobs on the sea floor sediment (that was also composed of micrite but is interpreted as mostly inorganic), by microbial agglutination of micrite. Because of its interpreted original micritic–microsparitic nature, classification as either a calcimicrobe (calcified microbial fossil) or a sedimentary microbial structure is problematic, soC. wangshenghaiihas uncertain affinity and nature.Calcilobessuperficially resemblesRenalcisandTarthinia, which both form small clusters in shallow marine limestones and have similar problems of classification. Nevertheless,Calcilobesframework architecture contrasts both the open branched geometry ofRenalcis, and the small tighter masses ofTarthinia, yet it is more similar toTarthiniathan toRenalcis, and may be a modification ofTarthinia, noting thatTarthiniais known from only the Cambrian. Calcilobesthus joinsRenalcis,Tarthiniaand alsoEpiphyton(dendritic form) and others, as problematic microbial structures.Calcilobeshas not been recognised elsewhere in the geological record and may be unique to the post-end-Permian extinction facies.C. wangshenghaiioccurs almost exclusively in the South China Block, which lay on the eastern margin of Tethys Ocean during Permian–Triassic boundary times; reasons for its absence in western Tethys, except for comparable fabrics in one site in Iran and another in Turkey, are unknown.