Abstract
Permian–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 named Calcilobes wangshenghaii n. gen., n. sp. Calcilobes reflects its calcium carbonate composition and lobate form, wangshenghaii for 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, so C. wangshenghaii has uncertain affinity and nature. Calcilobes superficially resembles Renalcis and Tarthinia, which both form small clusters in shallow marine limestones and have similar problems of classification. Nevertheless, Calcilobes framework architecture contrasts both the open branched geometry of Renalcis, and the small tighter masses of Tarthinia, yet it is more similar to Tarthinia than to Renalcis, and may be a modification of Tarthinia, noting that Tarthinia is known from only the Cambrian. Calcilobes thus joins Renalcis, Tarthinia and also Epiphyton (dendritic form) and others, as problematic microbial structures. Calcilobes has not been recognised elsewhere in the geological record and may be unique to the post-end-Permian extinction facies. C. wangshenghaii occurs 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.
Highlights
Introduction and aimMicrobialite deposits formed directly following the endPermian extinction in shallow marine carbonate environments in Tethys, and constitute a very thin unit of presumed organic origin, for a short period crossing the Permian–Triassic boundary, described in a wide variety of publications (e.g. Ezaki et al 2003; Baud et al 2005; Pruss et al 2006)
Understanding of the nature of the Permian–Triassic boundary microbialites (PTBMs) in the South China Block is hampered by extensive recrystallisation, leaving only remnant fabrics in most samples (Fig. 3)
The material commonly has sufficient preservation of a micritic–microsparitic texture to allow recognition that this is likely to be the original fabric (Figs. 4, 5, 6, 7, 8) and detailed examination leads to the conclusion that it is a unique microbial structure in the fossil record, which warrants its own taxon
Summary
Introduction and aimMicrobialite deposits formed directly following the endPermian extinction in shallow marine carbonate environments in Tethys, and constitute a very thin (ca 2–20 m thick) unit of presumed organic origin, for a short period crossing the Permian–Triassic boundary, described in a wide variety of publications (e.g. Ezaki et al 2003; Baud et al 2005; Pruss et al 2006). Ezaki et al 2003; Baud et al 2005; Pruss et al 2006) These Permian–Triassic boundary microbialites (PTBMs) vary in nature in the different parts of Tethys, with notable contrasts between western and eastern Tethys (Kershaw 2015). The South China Block was a distinct small continental mass during that time (Fig. 1), where the microbialite was developed separately from other areas. Understanding of the nature of the PTBMs in the South China Block is hampered by extensive recrystallisation, leaving only remnant fabrics in most samples (Fig. 3). This study aims to emphasise the distinctness of the South China Block microbialite by establishing a new microbial taxon, which may be limited to the Permian–Triassic interval. Because of copious illustration in the literature of field, polished blocks and thin sections of the lobate fabric, only essential images are presented here, to support the taxonomic treatment and discussion of the nature of the material
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