Sedimentary Characteristics of Gravity Flows and Depositional Evolution Model of a Prodelta in an Open‐Lacustrine Basin: A Case Study of the Upper Triassic Yanchang Formation in the Zhidan Area, Ordos Basin

ABSTRACT

Prodelta sedimentary dynamics in open‐lacustrine depression basins remain poorly understood compared to marine models. To decipher the interplay between hyperpycnal and gravity flows, this study investigates the Upper Triassic Yanchang Formation, Ordos Basin. Integrating 888 well logs and over 2500 m of cores, a high‐resolution sequence stratigraphic framework was established. The results reveal a wedge‐shaped progradational geometry controlled by palaeogeomorphic slope breaks, challenging traditional ‘layer‐cake’ correlations. Five identified microfacies (gully fill, lobe/lobe fringe, plume fallout mud drape, slump and tuffaceous event beds) record fluid transformations driven by flood‐generated hyperpycnal flows and slump‐induced gravity flows. A sequence‐constrained depositional model demonstrates that the Highstand Systems Tract features a supply‐driven ‘channel‐lobe’ framework under confined progradation, whereas maximum flooding periods transition to fine‐grained suspension settling and unconfined slump deformation. These findings establish a generic paradigm for progradational wedges in terrestrial depression basins, providing a predictive template for unconventional hydrocarbon ‘sweet spot’ exploration globally.