Geological reassessment of syn‐rift extensional sequences in the Shine Usny Tolgod and Dzun Shakhai fossil localities, Eastern Gobi Basin, Mongolia

ABSTRACT Sedimentary infill patterns in the Eastern Gobi Basin of southern Mongolia record a complex, polyphased history. Asynchronous timing and intensities of extensional tectonism during the Early Cretaceous fragmented the Eastern Gobi Basin into a series of sub‐basins within an extensional rift (horst‐graben) setting, which likely infilled penecontemporaneously to asynchronously. Of these sub‐basins, the north‐eastern Sainshand sub‐basin preserves a nearly continuous Lower Cretaceous syn‐rift succession. However, many outstanding uncertainties concerning intra‐sub‐basinal and inter‐sub‐basinal biostratigraphic correlations persist, including stratigraphic linkages locally at the Dzun Shakhai and Shine Usny Tolgod localities, regionally across the eastern Sainshand sub‐basin, along with the adjacent Zuunbayan and Unegt sub‐basins. This study confirms that Dzun Shakhai and Shine Usny Tolgod are hosted within a horst‐graben setting with sedimentary successions composed of locally sourced (para‐autochthonous to autochthonous) detritus. Facies analysis reveals a broad suite of evolving transitional depositional environments, including alluvial, fluvial and lacustrine environments. Basin infill initiated in a retrogradational setting (underfilled‐starved stage) that transitioned to an aggradational and a subsequent progradational setting (filled stage). Based on the identification of six syn‐rift sequence boundaries (SR1 to SR6), this study determined that this portion of the Sainshand sub‐basin fits a gradual subsidence model. Additionally, this study presents significant sedimentological evidence for: (i) the designation of a new member, the Ikh Ulaan Nuur Member of the Shinekhudag Formation; and (ii) the subdivision of the Khukhteeg Formation into an informal lower and upper member. These novel sedimentological data improve lithostratigraphic and palaeoenvironmental correlations across the Sainshand sub‐basin, with strengthened correlations to the adjacent Zuunbayan and Unegt sub‐basins and more peripheral linkages to the Erlian, Yingen and Songliao basins of north‐eastern China. These findings provide an important foundation for assessing the spatiotemporal distribution of syn‐rift fossil‐bearing units across the greater Eastern Gobi Basin and the North China Block.