High-resolution maps of Khulsan and Nemegt localities (Nemegt Basin, southern Mongolia): Stratigraphic implications

Since the discovery of fossil-rich exposures at the Khulsan and Nemegt localities (Nemegt Basin, southern Mongolia), tens of paleontological expeditions returned to these exposures, which are considered some of the richest Cretaceous units in the world. However, limited data on the local geology and cartography have been limiting our comprehension on correlations between different sites and the stratigraphic occurrence of vertebrate remains. Although informative, hand-drawn maps from the 1940s still represent the sole tool for researchers to provide a framework for locating access path, quarries and major geological sections. In 2016, we started a project for the realization of high-resolution topographic maps of key localities within the Nemegt Basin based on drone-acquired images. Several tests were performed at pivotal localities such as Altan Uul 2 and 3, Tsagaan Khushuu, and Ulaan Khushuu providing partial maps of such localities, whereas the Nemegt and Khulsan localities were fully covered. Photogrammetric elaboration of high-resolution images resulted in digital surface models used to obtain high-resolution topographic maps and orthophotos. Rock cairns used in the 1940s for triangulation were relocated and painted with vivid colors in order to represent reliable ground control points. In this study, we introduce new topographic maps for both Khulsan and Nemegt localities as well as comprehensive geological maps. In so doing, we provide additional stratigraphic and geographic data on the approximately 25. m thick interfingering interval that characterize the transition from the Baruungoyot and overlying Nemegt formations. Data presented in this study provide a solid framework in which detailed geological and paleontological investigations can be referred in order to significantly improve our comprehension on the Late Cretaceous Nemegt ecosystems.