Iren Dabasu Formation

The Iren Dabasu Formation is a Mesozoic geologic formation in the Iren Nor region of Inner Mongolia. Dinosaur remains diagnostic to the genus level are among the fossils that have been recovered from the formation. It is located in the Iren Nor region of China, and dates from the Upper Cretaceous.

It is typically referred to continental clastic sediments consisting of light grey fine sandstones, coarse sandstones and glutenites as well as mottled claystones and siltstones. This lithological unit has yielded plentiful dinosaur remains in quantity and group, also including a great deal of microfossil material. The stratigraphic sequence, sedimentary system, and the morphologies and assemblages of the paleontological taxa are summarized on the basis of previous studies on the Iren Dabasu Formation. The fine-grained floodplain sediments and the coarse-grained sediments of the point bar formed a series of repeated frequently binary sedimentary rhythms. The “binary structure” of the sedimentary rhythms strongly indicates meandering stream deposits rather than braided river deposits as previously thought. The ostracod and charophyte assemblages of the Iren Dabasu Formation has suggested a potential correlation with those of the Sifangtai Formation and the basal Mingshui Formation in the Songliao Basin (mid/late Campanian age). Vertebrates point to an older date than middle-late Campanian. The turtle Khunnuchelys is known from both Iren Dabasu and Baynshirenian-equivalent units such as the Bostobe and Bissetky. In addition, a giant Caenagnathid similar to Gigantoraptor is now known from the Baynshirenian beds of Tsagaan Teg. Like the coeval Baynshiree (or possibly Javkhlant) Formation in the Gobi, the dinosaur fauna of the Iren Dabasu Formation includes tyrannosaurs, ornithomimids, therizinosaurs and oviraptors. Gigantism, such in the case of Gigantoraptor, may have been due the humid climate of the Iren Nor region during the Late Cretaceous. It may also have been due to weak contemporary tyrannosaurs, as predator decline tends to maximize herbivore diversity.