Clinoform Evolution in a Triassic Lacustrine Environment, Southwestern Ordos Basin: Insights from Core, Well-Log, and 3D Seismic Data

Clinoforms are inclined depositional units formed by sedimentary processes, commonly developed in a wide range of subaqueous environments. They constitute key geomorphic elements within source-to-sink systems and play an important role in the transport of terrigenous clastic sediments and the evolution of sedimentary basins. Compared with marine settings, the architecture, evolution, and controlling processes of clinoforms in lacustrine basins remain comparatively underexplored, due to the limited availability of high-resolution 3D seismic data. In this study, we integrate sedimentary cores and well logs from 16 industry wells, along with a 3D seismic dataset covering c.1500 km2 to investigate the sedimentary evolution of lacustrine clinoforms within the Triassic Yanchang Formation on the southwestern margin of the Ordos Basin. Seismic interpretation interprets eight clinoforms (C1-C8). These clinoforms are characterized by sigmoidal and oblique geometries, with slope gradients ranging from 0.77° to 1.10° for the sigmoidal type and 0.55° to 0.93° for the oblique type. The eight clinoforms record a three-phase evolution. Stage I (C1-C2) is dominated by oblique clinoforms, Stage II (C3-C5) by aggradational to progradational sigmoidal clinoforms, and Stage III (C6-C8) by oblique clinoforms. The thickness of these clinoforms shifts from the topset to the foreset and ultimately to the bottomset, indicating a progressive progradation of the depocentre from the southwestern toward the central (northeastern) lacustrine basin. Well-log interpretation shows that the base of the topset is typically composed of 2.5-10 m thick sandstones overlain by 0.5-5 m thick mudstones, with sandstone content exceeding that of mudstone. Core observation indicates that cross bedding and parallel bedding are the dominant sedimentary structures. In the foreset, well-log interpretation indicates 3-15 m thick mudstones interbedded with thin (1-5 m) sandstone layers. Core observation shows deformed bedding, flaser bedding, and homogeneous mudstones. In the bottomset, well-log interpretation reveals thick (2-10 m) sandstone packages with minor (1-5 m) mudstone intercalations. Core observation documents massive to graded sandstones, slump-related deformation structures, and flame structures. Based on seismic interpretation, well-log analysis and core observation, we interpret that the topset is dominated by meandering fluvial depositional environments, including channel and point-bar settings. The foreset corresponds to slope environments associated with headwall scarps of mass-transport complexes and background muddy sedimentation, whereas the bottomset represents deep lacustrine environments dominated by gravity-flow deposits, including slumps, debris flows, and turbidity currents. In contrast to clinoforms developed in submarine settings, where large accommodation commonly prevents preservation of complete slope infill successions, the lacustrine clinoforms of the Yanchang Formation record a complete infilling evolution of lake basins. Understanding the infilling evolution of lacustrine clinoforms therefore provides a robust sedimentological framework for reconstructing basin evolution that is difficult to resolve in submarine settings.