Middle Jurassic–Lower Cretaceous Depositional Facies Evolution and a Fan-Delta–Beach-Bar Model Based on a Case Study from the Anjihai Section in the Southern Junggar Basin

Lake-margin successions preserve interactions between terrestrial sediment supply and lacustrine deposition. We examined the Toutunhe–Qingshuihe succession at the Anjihai section in the western sector of the southern Junggar Basin using outcrop logs, lithofacies descriptions, sedimentary structures, and assemblage relationships. Eighteen lithofacies were organized into eighteen lithofacies assemblages (A–R), interpreted as depositional microfacies and grouped into four depositional facies: lacustrine (FA1), fluvial channel–floodplain (FA2), alluvial-fan (FA3), and shallow-water delta (FA4). Depositional systems constitute the highest level and comprise the stacking of depositional facies. The succession records Toutunhe shallow-water delta–lacustrine deposition (FA4–FA1), Qigu fluvial channel–floodplain deposition (FA2), Kalaza alluvial-fan–lacustrine deposition (FA3–FA1), and Qingshuihe lacustrine deposition (FA1). Repeated sandy–gravelly beds and their association with lacustrine fines support a possible gravelly beach-bar interpretation for parts of the Kalaza Formation. A fan-delta–beach-bar model is proposed for the Anjihai section. The documented facies changes provide a record of depositional evolution for comparison with regional models.