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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， no longer appear following this event， resulting in a prolonged coal shortage throughout the Triassic. In the Sichuan Basin， following the EPME， peat-forming spore plants that contributed to coal-forming period were lost in the lowlands， with no recorded subsequent coal seam development. This period is called "coal gap".The climate was hot during the Lower and Middle Triassic， and the turbulent sedimentary environment in the Sichuan Basin suppressed the growth and development of peat-forming plants. Coal seams re-appear in the Sichuan Basin during the deposition of the First member of the Upper Triassic Xujiahe Formation （T3x1）， characterized by thin coal seams and poor spatial continuity. During the middle to late sedimentation stages in the Xujiahe Formation， thicker coal seams developed with highly regular spatial distribution. Current studies of the Upper Triassic paleoclimate in the Sichuan Basin have mainly concentrated on the middle and late stages of Xujiahe Formation sedimentation， leaving a gap in research on paleoclimatic conditions during the initial coal-forming phase of the Late Triassic （early stage of the Xujiahe Formation）. To fill this gap， a focused study was conducted on the T3x1 （specifically the Gongnongzhen and Wangjialiang sections） in the northwestern part of the Sichuan Basin. Methods Conducted on the paleoclimate and coal-forming mechanism of the coal bearing strata （Gongnongzhen and Wangjialiang sections） of the Xujiahe Formation in the northwest Sichuan Basin based on field outcrop observation， petrological microscopy analysis， and principal and trace element analysis. Results It was found that the sedimentary facies of the T3x1 in the Gongnongzhen section are mainly delta front subfacies， whereas those in the Wangjialiang section are mainly 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. The humid and hot climate corresponds to strong chemical weathering intensity， whereas the period of warm semi-arid/semi-humid climate corresponds to moderate chemical weathering intensity. Conclusions The paleoclimate during the initial coal-forming period of the Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin is of two types： （1） a hot， humid climate； and （2） alternating warm， semi-arid to semi-humid climates. Coal seams in the study area are associated with Type 1. The hot， humid conditions promoted the reproduction of peat-forming plants， providing abundant source material for the formation of coal seams. The paleoclimate alternating between dry to wet during the T3x1 in the region is closely related to super-monsoon activity.After the Middle Triassic， the South China Block gradually moved closer to the mid-latitude humid climate zone，amd the high concentration of pCO2 combined with the influence of monsoonal activity in the study area jointly promoted the formation of a humid climate and consequent strong chemical weathering. Further research indicated that coal deposition requires both a suitable paleoclimate and an appropriate preservational environment. For instance， coal seams tend to be well-developed and preserved in the swamps of high-level system tracts.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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