Sichuan Basin is a superimposed basin experiencing two transformation processes (i.e. from marine carbonates to marine clastics and from marine clastics to terrigenous clastics). The end of the two processes finally took place in the west of Sichuan Basin. The former transition process (from marine carbonates to marine clastics) happened in Late Triassic Carnian and is easier to be recognized compared with the latter one as the differences between carbonates and clastic rocks are definite. The latter process happened in Late Triassic and its corresponding lithostratigraphic unit is the Xujiahe Formation. Based on the study of three outcrop sections (the Guangyuan Gongnong section, the Chongzhou Wanjia section and the Dayi Tiangongmiao section), this paper illustrates the differences from the north to the south in front of the Longmen Mountains in the aspects of facies, depth and the distribution of conglomerate. The result indicates that the transition happened in the final depositional stages of the Xujiahe Formation and it occurred in turn from the north to the south, suggesting that the Longmen Mountains lifted in the north first, then to the south. The transition from marine to nonmarine deposition during Late Triassic causes differences in fundamental geologic conditions from north to south, and thus plays a significant role in oil-gas exploration in the west of Sichuan Basin. The key to hydrocarbon exploration in the Xujiahe Formation and overlying layers is to identify and locate the sand bars formed in estuary as well as channel sand bodies on the delta plain.%四川盆地是一个叠合盆地,经历了海相碳酸盐岩到海相碎屑岩、海相碎屑岩到陆相碎屑岩的转换和变迁.这两个转换过程最终结束均发生在四川盆地西部.海相碎屑岩到陆相碎屑岩的转换发生在晚三叠世末,相对应的地层单位为上三叠统须家河组.通过对须家河组厚度和沉积相变化,以及龙门山前砾岩的分布特征研究,认为这一转换主要发生于须家河组沉积末期(T_3x~4-T_3x~6),时间上随地区不同而不同,空间上具有由北往南的先后顺序.这揭示龙门山造山带的造山隆升过程具有由北往南的特点.这一转换过程导致川西前陆盆地内南、北段油气地质条件存在差异.寻找河口沙坝、三角洲平原河道砂等形成的优质储层应是须家河组油气勘探的关键.
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