Lithostratigraphy and depositional environments of the Upper Cretaceous Djadokhta Formation, Ulan Nuur basin, southern Mongolia, and its paleoclimatic implication

Non-marine Upper Cretaceous strata are widely distributed in the Gobi basin of southern Mongolia and record marked paleoenvironmental and paleoclimatic changes. The climate in this region is considered to have changed from humid during the Cenomanian to Santonian, to semi-arid during the Campanian, and returned to humid during the early Maastrichtian. The change from humid to semi-arid climate is well recorded in the "Djadokhta Formation" (as former division) in the Ulan Nuur basin of the central Gobi basin. In previous studies, the Djadokhta Formation was formally regarded as being characterized by eolian deposits formed under a semi-arid climate. However, the present study revealed that this formation comprises sub-humid fluvial lithofacies in the lower part and semi-arid eolian lithofacies in the upper part. Since the Djadokhta Formation was initially defined as a succession characterized by eolian lithofacies, it is proposed here that the lower part showing fluvial lithofacies should be separated from the "Djadokhta Formation" and is newly named as the Alagteeg Formation. The Alagteeg Formation is characterized by alternating beds of horizontally-bedded sandstone and mudstone, indicating sandy braided river, flood-plain, and ephemeral lake environments under sub-humid climate. The Djadokhta Formation (revised division) is further subdivided into two parts. The lower part is composed of large-scale cross-stratified sandstone, in situ and reworked calcretes, and thick mudstone lens, suggesting desert environments of broad sand dune fields with few inter-dune ephemeral river and pond deposits under semi-arid climate. On the other hand, the upper part comprises structureless sandstone with mature in situ calcretes, exotic pebbly sandstone and large-sized burrows. These lithofacies imply steppe environments with calcic soil development under semi-arid climate. This change from sub-humid to semi-arid climate during the Campanian is recorded not only in the Ulan Nuur basin but also in other regions of southern Mongolia as well as northern China. Hence, the broad aridification in the mid-latitudes of the Asian interior during the Campanian is inferred.