Coal-forming paleoclimate and mechanism during the initial coal-forming period of the Late Triassic Xujiahe Formation in Northwest Sichuan

Key words: 
• Late Triassic /
• Xujiahe Formation /
• Coal formation /
• Paleoclimate /
• Sichuan Basin

Abstract: [Objective] The end-Permian mass extinction event (EPME) led to a global decline in flora and biota. The thick coal seams, prevalent during the Permian, vanished following this event, resulting in a prolonged coal shortage throughout the Triassic. In the Sichuan Basin, following the EPME, spore plants that contributed to coal formation in the lowlands were lost, with no subsequent records of coal seam development. It was only during the early sedimentary period of the Upper Triassic Xujiahe Formation (T3x1) that coal seams reemerged. In the Sichuan Basin, coal deposits resurfaced specifically in the T3x1. This section is characterized by thin coal seams and poor spatial continuity; In the middle to late stages of the Xujiahe Formation sedimentation,the thickness of coal seam sedimentation and its spatial distribution have strong regularity. At present, research on the Late Triassic paleoclimate in the Sichuan Basin mainly focuses on the middle and late stages of the Xujiahe Formation. However, there's a research gap regarding the paleoclimate during the initial coal-forming phase of the Late Triassic (early stage of the Xujiahe Formation). [Methods] To fill this gap, a focused study was conducted on the Norian T3x1 (specifically the Gongnongzhen and Wangjialiang sections) in the northwestern part of the Sichuan Basin. This study involved rock analysis of major and trace elements, combined with field outcrop descriptions and petrological analysis. The aim was to reconstruct the coal-forming paleoclimate and sedimentary environment, thereby exploring the environmental conditions crucial for the mechanisms of coal formation. [Results and Discussions] Research has shown that the sedimentary facies of the T3x1 in the Gongnongzhen section are mainly delta front subfacies, whereas those in the Wangjialiang section are chiefly delta plain subfacies. Weathering indices (CIAcorr, Rb/Sr) and climate indices (Sr/Cu and C values) of the T3x1 in the Gongnongzhen and Wangjialiang sections displayed an overall fluctuating trend: high-medium-high-medium-high. The humid and hot climates correspond to strong chemical weathering intensity, while the warm semi-arid/semi-humid climates correspond to moderate chemical weathering intensity[Conclusions] The paleoclimate during the initial coal-forming period of the Late Triassic Xujiahe Formation in Northwest Sichuan can be divided into two types: (1) hot and humid climates; (2) warm semi-arid to semi-humid alternated climates. In study area, coal seams are associated with hot and humid climates. Hot and humid conditions promote the reproduction of coal forming plants, providing abundant material sources for the formation of coal seams.The paleoclimate of dry to wet alternation during the T3x1 in Northwest Sichuan is closely related to the activity of the super monsoon. The high concentration of PCO2 combined with the influence of super monsoon activity jointly promoted the formations of humid climate and strong chemical weathering in the study area. Further research indicates that coal deposition requires not only a suitable paleoclimate but also an appropriate preservation environment. For instance, coal seams tend to be well-developed and preserved in the swamps of high-level system tracts.