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

¹ Osaka City University, Department of Geosciences, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
² Peking University, School of Earth and Space Sciences, Haidian, Beijing 100871, China
³ Schlumberger K. K., Hokuyo, Nagaoka, Niigata 940-0871, Japan
⁴ Research Fellow of the Japan Society for the Promotion of Science, Miyagi University of Education, Department of Geology, Aoba, Aoba-ku, Sendai 980-0845, Japan; ezaki{at}sci.osaka-cu.ac.jp

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 thin-bedded 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, skeletal-sediment 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.