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Duoni Formation in the studied area is a set of coaly grey to black lithic quartz sandstones, feldspar-sandstones, carbonous siltstones and shale. The biostratigraphic characteristics indicate the age of Duoni Formation is Berriasian to Valanginian of Early Cretaceous. The analyses of lithologic features and clastic rock size show that the sedimentary environment of Duoni Formation is delta deposit, including prodelta, delta front and delta plain, and the third-order sequence presents gradual transgression feature in general.