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

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.

Duke Scholars

Author William Freimuth Biology