Sedimentary environments and sequence stratigraphy of Late Triassic coal-bearing strata in Yanyuan area, Sichuan Province

Late Triassic is one of the most important coal-accumulating periods in South China, and Boda and Donggualing Formations of Late Triassic are coal-bearing strata in Yanyuan area. Boda Formation was developed in the tidal flat environment associated with the barrier of Changqiang-Qimulin Islands, and the Formation mainly consists of clastic rocks interbedded with muddy and sandy limestone, with a provenance from Kangdian Mountains to the southeast. The coal-forming swamp was developed from the coastal tidal flat environments. Donggualing Formation was formed in the fluvial plain to lacustrine delta environments developed as a result of regression and rapid uplift of the surrounding mountains. The coal-forming swamp at this stage was developed from the delta plain inter-distributary bay and fluvial flood basin environments. Tectonic unconformities were widely distributed in Yanyuan area, including the parallel unconformities between Boda and underlying Shemulong Formations, and between Boda and Donggualing Formations. Erosional surfaces at the base of incised valley channel sand bodies were developed in Donggualing Formation, especially in Members 3 and 6. According to these key surfaces, totally 5 sequence boundaries were identified and 4 third-order sequences were subdivided in the Late Triassic coal-bearing strata. Sequence 1 corresponds to Boda Formation, Sequence 2 corresponds to Members 1 and 2 of Donggualing Formation, Sequence 3 corresponds to Members 3, 4 and 5 of Donggualing Formation, and Sequence 4 corresponds to Members 6, 7 and 8 of Donggualing Formation. Coal accumulation was favored in sequence II, followed by sequence III, while within these sequences, coals were preferentially accumulated in the transgressive systems tract, and then the highstand system tract.