Sequence-based Lithofacies and Paleogeographic Characteristics of Upper Triassic Xujiahe Formation in Sichuan Basin

Through the analysis of the tectonic framework and Coupling Process between Basin and Mountain,it can be ascertained that the Sichuan Basin is an analogous foreland basin,of which tectono-sedimentary framework was composed of depressions and one uplift,formed by systems of coupling process between basin and mountain in the Xujiahe epoch of Late Triassic. Based on the analysis of outcrops,drilling sections,sedimentary facies and sequence stratigraphy of the Xujiahe Formation or the Xiangxi Group,combining with the base-level cycle sequence and isochronic correlation technique,the Xujiahe Formation is divided into two super long-term sequence cycles(SLSC1,SLSC2) and five long-term sequence cycles(LSC1～LSC5),which can be correlated in the whole Sichuan Basin. Choosing the rising and falling base-level phases of each long-term sequence cycles as a unit,the sequence-based lithofacies and paleogeographic maps of the Xujiahe Formation have been mapped. The results indicate that the paleogeography,regulation of sedimentary facies distribution,subsidence—sediment center and migration direction are controlled strictly by nonsynchronous thrusting—napping process of the Longmen Mountains,Micang Mountains—Daba Mountains and so-called three depressions and one uplift tectonic pattern,which is composed of western Sichuan depression,northeastern Sichuan depression,eastern Chongqing—southwestern Sichuan depression and central Sichuan Palaeo-uplift. Coupling process between basin and mountain possess the features of deroofing in orogenic zone and material cycle between growth,subsidence and filling in a basin. The peculiar tectono-sedimentary pattern entirely control the depositional processes,palaeogeographic features and the source—reservoir—seal combination of the Xujiahe Formation,as well the oil and gas accumulation regularity around the central Sichuan Palaeo-uplift.