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Sequence Architecture and Its Controlling Factors of Middle Jurassic Fluvial Successions in Western Sichuan Foreland Basin


During the depositional period of the middle Jurassic Shaximiao Formation, the Western Sichuan Depression was a typical foreland basin with its foredeep in the Daba Mountain front. Controlled by tectonic uplift, large amounts of terrestrial debris were transported from the mountains. A set of fluvio−incised valley successions was deposited under semi-arid to arid climatic conditions. Autogenic and allogenic controls on the fluvial sequence developed at different times and positions have rarely been described in detail, which is noteworthy here. In this study, by integrating 3D seismic data, loggings, cores and outcrops, we investigated the sequence architecture and its sedimentary infill of the Shaximiao Formation, analyzed the different controls on fluvial successions developed in the foreland basin, and provided a sedimentary evolutional model. Our results show that: (1) the Shaximiao Formation can be divided into two three-order sequences, and each one is recorded by a typical sedimentary cycle of incised valley−fluvial channel−delta−fluvial channel, with incised valleys 20-30 m thick and 6-13 km wide; (2) during the phase of low frequency cycles, allogenic processes, such as tectonic movements and climatic changes, primarily controlled the progradation and regradation of the fluvial system; during the phase of high frequency cycles, autogenic processes, such as vertical and lateral accretion, mainly controlled the planform geometry of the coeval fluvial system, from the upstream to the downstream direction, the downstream decrease in the stream power appears to be responsible for the decreasing trend in the incised valley dimensions; and (3) a sedimentary model is provided to be used as a reference for the interpretation of similar fluvial systems in foreland basins. Keywords: foreland basin; fluvial facies; incised valley; controlling factor; middle Jurassic; Western Sichuan Depression