Geobiological Aspects of the Earliest Triassic Microbialites Along the Southern Periphery of the Tropical Yangtze Platform: Initiation and Cessation of a Microbial Regime

Abstract

Microbialite masses within the lowest Triassic strata along the southern periphery of the tropical Yangtze Platform were produced in a post-Permian microbial regime. These microbialites (the Hindeodus parvus Zone), which are represented by thrombolites, occur only where terrigenous sediment influx was rare, even in shallow-marine settings. The lower parts of the thrombolites lie upon a distinctly unconformable Permian-Triassic boundary and exhibit a thinbedded to thick-bedded planar structure. In contrast, the upper parts of the thrombolites contain domed macrostructures that interact in complex ways with skeletal grainstones and packstones. Irregular frameworks of thrombolite bodies differ in degree of lateral and vertical accumulation and in the amalgamation of mesoclots of microbial origin; they exhibit marked variations in texture. A transgressive episode occurred in the earliest Triassic following the mass extinctions, and this included the initiation of microbial regimes that usually formed planar thrombolite masses in lower-energy, deep subtidal environments. The varied textures and structures of thrombolites during deposition may reflect a combination of sea-level fluctuations, physicochemical ocean conditions, microbial activity, skeletalsediment influx, and other factors. These earliest Triassic, uniquely microbial regimes collapsed in stepwise fashion and were succeeded by the Isarcicella staeschei and I. isarcica zones, which contain a predominance of mudstones, suggesting a marked sea-level transgression. Space-specific and time-specific, the earliest Triassic microbialites record short-term, high-resolution paleoenvironmental fluctuations immediately after the end-Permian extinctions.

Role and lifestyle of calcified cyanobacteria (Stanieria) in Permian–Triassic boundary microbialites

“…Many previous studies have also documented the presence of very small (b0.05 mm) coccoid fossils in these microbialites (Ezaki et al, 2003(Ezaki et al, , 2008Wang et al, 2005;Yang et al, 2011;Kershaw et al, 2012), and related them to cyanobacteria. Some researchers (Ezaki et al, 2003) suggested that the precursor microbes of the coccoid fossils formed the mesoclots constituting the microbialites.…”

“…It is generally 2 to 10 m thick and consists of two parts (Kershaw et al, 1999(Kershaw et al, , 2007(Kershaw et al, , 2012Lehrmann, 1999;Ezaki et al, 2003Ezaki et al, , 2008Wang et al, 2005Wang et al, , 2012Jiang and Wu, 2007a,2007bYang et al, 2011). The lower part consists of gray limestone with scattered irregular brown spots 0.5 to 1.5 cm wide, and has been described as spotted rock.…”

“…Their external diameters based on measurement of 567 specimens (441 from Laolongdong Section 1, 69 from Laolongdong Section 3, and 57 from fig. 9B of Ezaki et al (2008)) are between 2.8 and 49.1 μm, with most being in the range of 25-30 μm (Fig. 3).…”

“…Of the 57 fossils shown in fig. 9C of Ezaki et al (2008), spherical forms account for 93% and other forms for 7%.…”

Permian–Triassic boundary microbialites (PTBMs) in southwest China: implications for paleoenvironment reconstruction

“…The models for their generation consist of a series of viewpoints, including: (1) reduced benthic grazing pressure; (2) upwelling of anoxic alkaline waters; (3) elimination of skeletal CaCO 3 sinks; (4) increased nutrient levels from erupted volcanic ash; and (5) a rebound in carbonate saturation following a transient ocean acidification event (Lehrmann et al 2015 and references therein). The upwelling model, in particular, has been applied by numerous workers in the Early Triassic (Kershaw et al 1999;Ezaki et al 2003;Lehrmann et al 2003;Pruss and Bottjer 2004;Pruss et al 2005;Kershaw et al 2007Kershaw et al , 2009Kershaw et al , 2011Ezaki et al 2008;Mary and Woods 2008;Mata and Bottjer 2011;He et al 2013;Woods 2014). Lehrmann et al (2003) also drew attention to the possibility of PTBM formation in oxygenated seawater conditions.…”