Extreme events of alternate dry–wet climatic cycles and lacustrine deposits: A case study from the Yanchang Formation (Triassic), Ordos Basin, China

ABSTRACT The Middle‐Late Triassic Yanchang Formation is the main oil‐bearing unit of the Ordos Basin, China. Based on integrated field outcrop observations, laboratory analyses and seismic data, this study reconstructs the palaeoclimatic evolution and sedimentary characteristics of the Yanchang Formation. Results indicate a tripartite partitioning. Arid conditions dominated during the Ladinian Stage, followed by an abrupt shift to warm and humid conditions during the Carnian Stage, likely associated with the Carnian Pluvial Episode. Subsequently, semi‐arid to subhumid and highly fluctuating regimes characterised the Norian–Rhaetian stages. Contrasting climatic conditions exerted a strong control on sediment transport processes and depositional products. During arid phases, seasonal torrential floods dominated the formation of large‐scale sandbodies in the basin centre, comprising ephemeral fluvial systems and sheet flows. Conversely, during the humid CPE, intensified rainfall enhanced clastic delivery via hyperpycnal flows, turbidity currents and sandy debris flows, generating sand‐rich fluvial delta and gravity flow deposits. Concurrently, the lake basin underwent rapid transgression episodes followed by oscillatory progressive regression episodes, jointly enhancing organic productivity and preservation, facilitating organic‐rich source rock accumulation. Two distinct sedimentary models were proposed for the accumulation of the Yanchang Formation. During arid periods, limited catchment areas developed fluvial fan systems dominated by aggradational floodplains, seasonal channels and sheet flows, lacking progradational architecture. In contrast, during humid periods, expanded catchments hosted a complete fluvial‐delta–lacustrine‐gravity‐flow system, where stable perennial discharge maintained relatively fixed channel pathways, with sedimentation dominated by progradation and minor retrogradation, while aggradation was negligible. An intraformational Xenoconformity, triggered by an abrupt climatic shift, exerted a first‐order control on the configuration of lithologic–stratigraphic hydrocarbon traps. A combination of arid‐phase low‐stand fluvial fan sandstones and humid‐phase transgressive organic‐rich mudstones established a pervasive basin‐wide sand‐rich, depression‐wide oil‐bearing configuration. This study advances understanding of lacustrine basin evolution, sequence stratigraphy and hydrocarbon accumulation mechanisms in the Yanchang Formation while providing critical insights into continental sedimentary archives of Late Triassic global climatic extremes.