An in-situ stump (Fig. 4a, c) and several in-situ hollows (Fig. 4b) were founded on the paleosol surface (11m above the conglomerate, 28.7m below Tr-J lithological boundary) in 5th member of Xujiahe at Zilanba section. Particularly, the lodging direction of these in-situ trunk fossils is opposite to the paleocurrent direction. As a long-time exposed environment indicator, paleosol, as well as the in-situ concomitants above, are unlikely to be affected by high-energy currents. One of the most important external agencies that force trees to fall in the same direction is a strong wind. Hence, it can be inferred that strong southeasterlies prevail in the northern Sichuan Basin at the time when paleosol informed in 5th member of Xujiahe formation. Nevertheless, according to the classical definition of monsoon, it refers to the phenomenon of the opposite direction of surface wind in winter and summer49. Unfortunately, even if we find evidence of the southeasterlies in the geological record in Xujiahe Formation at Zilanba section, we cannot provide an annual resolution to conclude that both of them are in opposite directions. So, a storm rather than megamonsoon was identified in upmost Late Triassic Xujiahe Formation.
On a global scale, due to the Pangea and the Tethys Ocean were symmetrically divided by the equator during the Late Triassic with the equal area in both hemispheres, as well as contrast in the solar radiation received by the hemispheres in both seasons. Over Pangaea, a year-round heating contrast means a year-round air pressure contrast, characterized by a low-pressure cell in each summer hemisphere and a high-pressure cell in each winter hemisphere50. Therefore, surface winds alternate between the different hemispheres in both winter and summer. Meanwhile, it was controlled by Coriolis Force when surface wind crosses the equator. The ITCZ typically refers to the surface wind convergence zone in the tropics and also indicates the location of maximum persistent rainfall51. A seasonal reversal of circulation and large-scale migration of the ITCZ was identified as a monsoon climate2,52. In present days, most monsoons regions occurred between the ITCZ belt53. According to the global seasonal surface wind direction and the ITCZ reconstructed by the serval numerical simulation over the Pangea2,54, The displacement of ITCZ spans 40 latitudes in a year49 and its extent even involved the whole Tethyan area over Tethys2, including the South China Block. Nevertheless, neither of reconstructed surface wind over Pangea2 and in Early Jurassic54 were not insufficient to prove the existence of strong southeasterlies in South China, because these models were run at the extremely low resolution of global paleogeography.
Another distinguished layer was found just below the paleosol horizon, a set of sandstones containing a large amount of branch or trunks fragment fossil with a thickness of 10m (Fig. 5a-e). Particularly, sandstones were branch-supported with branch or trunks contacting each other as a particle. Generally, a fining-upward sequence with numerous erosional bases (Fig. 5d, f) and sandstone lenses stacked laterally or vertically showed a braided river system. Branch-supported sandstones are usually well stratified, but displayed mostly with wavily bedding (Fig. 5e), this may be related to branches or trunks filling causing an uneven bed. The dip of most branch fossil fragments was oriented (Fig. 5a), but lack of mudstones, suggesting the sediments has been transported with a long – distance. A relatively complete impression leaf fossil, Pterophyllum sp. (Fig. 5g), was found insert in sandstone with a random dip, indicating a fast unloading process in a high load stream. All these sedimentary evidences together with the roundness boulder in upmost 4th member of Xujiahe Formation, indicate an alluvial fans deposit during a flood period.
In fact, the gravel in 4th member of Xujiahe Formation has a wide distribution, covered in front of the northwestern, northern, and northeastern mountains of Sichuan Basin, named Longmen Mountains, Micang Mountains, and Daba Mountains, respectively55 (Fig. 1c). The provenance of these gravels usually considered as a consequence of mountain uplift56,57, although the lack of the absolute age confined. The branch-supported sandstones in 5th member of Xujiahe Formation witness a massive runoff at Zilanba section, nevertheless, the reason for the occurrence of these branch fossil fragments is still unknown. Despite the Triassic – Jurassic boundary are not exposure in Zilanba section, however, according to the lithographic data from a section of Xujiahe Formation in Xujiahe village, 10Km northeaster of Zilanba section. The top of the branch-supported sandstones is 28.7m below Tr-J lithological uncomfortable boundary. Meanwhile, we found the same branch-supported sandstone (Fig. 5h) in Sangjiang section (Fig. 1c) just upon the gravel of 4th member of Xujiahe Formation, and the straight-line distance between the two sections is 84.6 Km, where located in front of Micang Mountain. Considering that many alluvial fans are only a few kilometers long, some can reach a length of more than 50 Km58. Particularly in modern alluvial deposits, the largest alluvial fan in the world spans the desert where located in Taklimakan desert, XinJiang, China, with 56.6 kilometers wide and 61.3 kilometers long59. Therefore, the same scenario occurred between such a long-distance may reflect deforestation was forced by climate disturbance rather than a sedimentary change in the latest Triassic. During the Triassic – Jurassic interval, increased atmospheric pCO2 concentration induced by CAMP was expected to 2100 ~ 2400 ppm22,60, as a consequence, 3–4℃ of warming was created22. Increasing temperature directly leads to the increase of thermal contrast between land and sea, under the general circulation, hydrologic activity subsequently increased and transport more rainfall to the monsoon area, where large-scale runoff occurred. Moreover, increased atmospheric pCO2 means acidification in the terrestrial realm, acid rain could reduce the ability of trees to consolidate the soil, and severe acid rain can cause aluminum poisoning in trees, which can cause severe root blight and lead to tree dieback61. Therefore, the branch-supported sandstones in 5th member of Xujiahe Formation may record a deforestation in the latest Triassic. Moreover, the record of massive runoff, catastrophic soil loss62, deforestation63, and intense weathering64–66 in Triassic – Jurassic interval on both sides of Tethys may be due to CAMP-derived sedimentary feedback. Nevertheless, more data including palynology, chronology, and geochemistry need replenish around the Sichuan Basin to examine the deforestation in Triassic – Jurassic interval.