Late Triassic ecosystem variations inferred by palynological records from Hechuan, southern Sichuan Basin, China

Abstract

-The Late Triassic deposits of the Sichuan Basin, southwestern China are significant for hosting abundant and diverse fossil assemblages including plants (containing spores and pollen), bivalves and insects. However, the Late Triassic palaeoecological variations are still poorly documented in this region. Here we present results from a palynological study from the Upper Triassic Xujiahe Formation in Hechuan of Chongqing, southern Sichuan Basin. The palynological analysis revealed a well-preserved terrestrial palynoflora of high diversity, comprising 184 species in 75 genera of spores and pollen. Three palynological assemblages were recognized, reflecting terrestrial successions throughout the entire interval with significant changes in the vegetation. Cycads/bennettites/ginkgophytes and conifers show an increasing trend into younger deposits, while ferns and lycopsids decrease in relative abundance. The Late Triassic vegetation underwent changes from lowland fern forest to a mixed forest with more canopy trees. We applied the Spore-pollen Morphological Group (SMG) method and Sporomorph EcoGroup (SEG) model to interpret the palaeoclimate features. The results reveal that the lower part of the Xujiahe Formation was deposited under relatively warm and humid conditions with an overall cooling and drying trend from latest Norian to Rhaetian time, accompanied by a general decrease of ferns and simultaneous increase of gymnosperms, and a decline in diversity of miospores. This study presents data on variations within the terrestrial ecosystem prior to the end-Triassic extinction event in the Sichuan Basin, and therefore provides important information for understanding the changes in the vegetation preceding the end-Triassic event.

Palaeovegetation and palaeoclimate changes across the Triassic–Jurassic transition in the Sichuan Basin, China

“…A cooler and drier condition in the latest Triassic has also been revealed by palynological data from Hechuan region in the southern Sichuan Basin (Li et al, 2018). Recent sedimentary and biomarker studies suggested increased wildfire activity during the Triassic-Jurassic transition within the Sichuan Basin (Pole et al, 2018;Song et al, 2020), possibly as a result of the warmer and drier climate during this interval.…”

“…The Upper Triassic Dictyophyllum-Clathropteris macroflora of the Xujiahe Formation is replaced by the Lower Jurassic Ptilophyllum-Coniopteris flora in the Zhenzhuchong Formation (Ye et al, 1986), and the Upper Triassic Dictyophyllidites-Kyrtomisporis-Ovalipollis-Ricciisporites palynological assemblage is replaced by the Lower Jurassic Dictyophyllidites-Classopollis-Cycadopites assemblage (Lu and Wang, 1987;Wang et al, 2010). Previous palynological and macrofloral studies dated the Xujiahe Formation as Norian to Rhaetian in age (Ye et al, 1986;Lu and Wang, 1987;Wang et al, 2010;Li et al, 2016Li et al, , 2018. The Sulcocythere-Oncocythere-Darwinulla ostracod assemblage and the Burmesia-Myophora (Costatoria) Yunnanophorus-Pemophorus-Weiyuanella bivalve assemblages of the Xujiahe Formation also indicate a Norian to Rhaetian age (Wei, 1982;Gou, 1998;Wang et al, 2010).…”

“…The Precambrian to Middle Triassic marine strata are well developed in the adjacent mountain areas, while the Upper Triassic to Lower Jurassic terrestrial lacustrine-fluvial to coal-swamp sediments are mainly distributed in the eastern and northeastern margin of the basin (Wang et al, 2010;Li et al, 2018). This change from marine to continental environments is due to the collision of the South China block with North China and other Asian plates during the Middle-Late Triassic (Wang et al, 2010).…”

“…However, evidence from the eastern Tethys region (Asia) is still sparse, with only systematic studies reporting on e.g. palaeobotany, palynology, bivalve stratigraphy and palaeoclimate in the Junggar Basin, NW China Deng, 2005, 2009;Deng et al, 2010;Sun et al, 2010;Sha et al, 2011Sha et al, , 2015; palynostratigraphy in the Tarim Basin, NW China (Peng et al, 2018); Late Triassic terrestrial palynology and ecosystem variation preceding the end-Triassic mass extinction event in the Sichuan Basin, SW China (Li et al, 2016(Li et al, , 2018.…”

Triassic vegetation and climate evolution on the northern margin of Gondwana: a palynological study from Tulong, southern Xizang (Tibet), China

“…Traditionally, four major basins have been the focus of Chinese Triassic palaeobotany, all with well-exposed continental Triassic successions yielding diverse macro-and microfloras. These basins are the Junggar and Tarim basins in the northwestern part of the country (Deng et al, 2010;Sha et al, 2011Sha et al, , 2015Peng et al, 2018a), the Ordos Basin in North China (Qu, 1980;Huang and Zhou, 1980) and the Sichuan Basin in South China (Wang et al, 2010;Li et al, 2016aLi et al, , 2016bLi et al, , 2017.…”