Early Triassic Griesbachian microbial mounds in the Upper Yangtze Region, southwest China: Implications for biotic recovery from the latest Permian mass extinction

Abstract

Early Triassic microbialites are distributed widely in the shallow marine facies of the Tethys Region, especially in the carbonate platform where they were deposited immediately after the latest Permian mass extinction (LPME). Ten Griesbachian domed microbial mounds were found in an outcrop of the uppermost first member of the Feixianguan (FXG) Formation at Baimiaozi, which is located in Beibei in the Upper Yangtze Region of southwest China. Field investigations and thin-sections analyses indicated that oolitic limestone, bioclastic limestone, microbialite, marl, and mudstone deposits are present in the first and second members of the FXG Formation, among which the thickness of the microbial mound above the massive oolitic limestone at the carbonate platform was approximately 3–4 m. Three facies were identified at the microbial mounds, namely, a mound base, mound body, and mound cap. Irregular laminae were found in the brown-colored microbialite of the mound base. The main mound body, which is composed of gray microbialite, is 1.0–1.5 m high and 2.0–3.0 m in diameter at the base. Dark gray grainstone found in the mound cap is covered by a thin layer of shelly limestone containing intact fossils of bivalves and gastropods, which are indicative of a simple ecosystem consisting of microbes and primary consumers. Brown-colored mudstone and marl layers of the second member of the FXG Formation overlie the microbialite, and this indicates that growth of the microbial mounds was halted by a sudden increase of terrestrial inputs and rapid transgression. Early Griesbachian conodonts of Hindeodus parvus? were identified from the mound limestone and the overlying strata of the second member of the FXG Formation, which is suggestive of the presence of a microbialite-dominated ocean in the Upper Yangtze Region during a certain interval after the LPME.

Disentangling microstructure and environmental conditions in high‐altitude Andean microbialite systems (Catamarca, Argentine Puna)

“…Microbialites have notably caught the attention of the scientific community for its extensive evidence in the geological record after massive extinctions (Chen et al, 2011; Mata & Bottjer, 2012; Wu et al, 2017; Yang et al, 2011) and important climate changing events (Le Ber et al, 2013; Lipar et al, 2017; Pollier et al, 2021). Therefore, these structures are known as opportunistic and disaster‐related forms in stressed environments (Ezaki et al, 2008), also known as ‘unusual facies’ or ‘disaster form’ (see Duan et al, 2018).…”

Insight to the geological character of the Early Triassic strata in the Western Salt Range, Pakistan: A comparative study

“…These shales are analogous to the laminated, dark shales with high organic content reported from South China and India by Galfetti et al (2008). The rocks of the Triassic period in China are represented by relatively pure carbonates containing small biostromes and potentially reef builder calcified microbial communities (Duan et al, 2018; Li, Song, Woods, Dai, & Wignall, 2019; Tian et al, 2015). Because of the demise of a large proportion of frame builders in the aftermath of P–T mass extinction, it is highly unlikely for a characteristic rimmed shelf.…”

“…Because of the demise of a large proportion of frame builders in the aftermath of P–T mass extinction, it is highly unlikely for a characteristic rimmed shelf. Duan et al (2018) have described shallow marine platform carbonates containing bioclastic limestones, mudstones, marls, and microbialites with at least 10 microbial mounds within the Feixianguan Formation in BeiBei, Upper Yangtze region, Southwest China. The bioclastic limestone bears a simple fauna of bivalves and gastropods indicating a simple ecosystem consisting of microbes and primary consumers (Duan et al, 2018).…”

“…Duan et al (2018) have described shallow marine platform carbonates containing bioclastic limestones, mudstones, marls, and microbialites with at least 10 microbial mounds within the Feixianguan Formation in BeiBei, Upper Yangtze region, Southwest China. The bioclastic limestone bears a simple fauna of bivalves and gastropods indicating a simple ecosystem consisting of microbes and primary consumers (Duan et al, 2018). The prevailing shallowing‐upward conditions and calcified microbial communities present in the Early Triassic rocks of South Tibet (Brühwiler et al, 2010; Li et al, 2019) are analogous with the facies and shallowing‐upward trend of the current study.…”

“…The Early Triassic Luolou Formation of Nanpanjiang Basin, South China, essentially has communities of low‐diversity bivalves, gastropods, ostracods, and rare foraminifera (Galfetti et al, 2008). The colonization of microbial limestones and development of a mixed siliciclastic‐carbonate environment in the Triassic Luolou Formation, South China (Baresel et al, 2017), dolomicritic‐microbialites‐dominated carbonate platform of Li et al (2019) in South Tibet, and microbialite‐dominated carbonate platform (Feixianguan Formation) of Duan et al (2018) in southwest China is in coincidence with the Early Triassic algal‐bearing mixed siliciclastic‐carbonates of Pakistan. This system had the presence of bryozoans, brachiopods, and plicate bivalves.…”