Middle Triassic tectono‐sedimentary development of Sichuan Basin: Insights into the cratonic differentiation

The Middle Triassic succession of the Sichuan Basin contains key information about the Early Indosinian Movement, during which it has changed from extensional to compressional. In order to clarify the relationship between the evolution of the Sichuan Basin during the Indosinian Movement, the Middle Triassic tectonostratigraphic environment should be determined. Detailed stratigraphy and facies analysis integrated with structural and basin analysis, allowed a full reconstruction of the tectonic and depositional environments for the Middle Triassic strata of the Sichuan Basin. Six facies associations from two rock systems of: (a) carbonate-evaporite system and (b) a mixed carbonate-siliciclastic system, were characterized. Four depositional stages highlighted by the lithostratigraphic members of the Leikoupo (Lei) and Badong (Ba) formations were divided: (a) stable subsidence and deposition of the Lei-1/Ba-1 Member, leading to an embryonic 'uplifts and sags' configuration; (b) inherited deposition of the Lei-2/Ba-2 Member, with the platform margin belt extended; (c) rapid deposition of the Lei-3/Ba-3 Member, characterized by an eastward shift of the depositional centre and formation of the central anhydrite-rich lagoon; and (d) intensification of cratonic tectono-sedimentary differentiation during the Lei-4/Ba-4 deposition, that resulted in subaerial exposure along the Luzhou-Kaijiang uplift zone and a hydrodynamically restricted depression along the western platform. The Middle Triassic tectonic framework of the Sichuan Basin was dominated by compression in the east and extension in the west. The western margin was a passive continental margin, inheriting an early rifting event, while the eastern area within the craton showed an 'uplift-sag' pattern, which is a response to intracontinental tectonic movement of the Xuefengshan uplift belt. The basin evolved from incipient low-relief uplifts and sags within the intracratonic depression to subsequent isolated foreland and restricted depression during the Middle Triassic.