Sedimentary facies and sandstone distribution in the 4th member of the Upper Triassic Xujiahe Formation, Deyang, Sichuan

The fan delta depositional systems are developed in the 4th member of the Upper Triassic Xujiahe Formation in the Xiaoquan-Xinchang-Hexingchang zone, western Sichuan depression. The sandstone types consist dominantly of fan delta frontal subaqueous braided distributary channel (gravel-bearing) medium-to coarse-grained sandstone intercalated with debris flow conglomerates. The 4th member of the Upper Triassic Xujiahe Formation comprises one long-term, three medium-term and eighteen short-term base-level cyclic sequences grouping into six sets of sandstones. The distribution of individual sandstone sets are closely bound up with the ratios of sediment supply and accommodation spaces.The sediment input was much greater than the accommodation spaces due to the slow rise of the base levels during the deposition of the lowstand systems tract. The deposition occurred in the active progradational modes, and the sandstone bodies migrated towards the lake basin.The sediment supply was much smaller than the accommodation spaces due to the rapid rise of the base levels during the deposition of the transgressive systems tract. There were gradations from the progradation to the aggradation and retrogradation modes of deposition. Finally the sediment supply was also much greater than the accommodation spaces due to the rapid fall of the base levels during the deposition of the highstand systems tract. The deposition took place in forced progradation modes.