Abstract: The theory and technology of the highresolution sequence stratigraphy, According to data of seismic, logging and outcrops, the authors study retrogradational Fan Deltas in the lower section of T3X4 in the middle part of West Sichuan Depression, threegrade baselevel cycle secquence is figured out in the section: the forth, the fifth, and the sixth class cycle secquence,and the characteristics of these different grade baselevel cycle secquence is analized. On the, basis, the distribution law of the baselevel cycle secquence in horizone and lengthways inside retrogradational Fan Delta is summarized, and the distribution model of baselevel cycle secquence is put forward. The lower section of T3X4 can be regarded as a forth class cycle secquence, and the rising cycle is a typical model of retrogradational Fan Deltas. The retrogradational Fan Delta plain is dominated by the semirising cycle base level, but the retrogradational Fan Delta front develops all kinds of baselevel cycles. The western Longmen Mountain thrusting belt develops the semirising cycle, the integrated cycles exists in the depression area and the Eastern slope far form the source area,the former develops the incomplete symmetry dominated by uprising semicycle and the latter develop the completenearly symmetry type.

Key words: the retrogradational fan deltas, highresolution sequence stratigraphy, baselevel cycle sequence, flooding surface, the lower section of T3X4