Djadochta Formation

The Djadochta formation (sometimes transcribed and also known as Djadokhta, Djadokata, or Dzhadokhtskaya) is a highly fossiliferous geological formation in Central Asia, Gobi Desert, dating from the Late Cretaceous period, during the Campanian to the Maastrichtian. The type locality is the Bayn Dzak locality, famously known as the Flaming Cliffs. Reptile (including dinosaur) and mammal remains are among the fossils recovered from the formation.

The Djadochta formation was first documented and explored—though only a single locality—during paleontological expeditions of the American Museum of Natural History in 1922–1925, which were part of the Central Asiatic expeditions. The expeditions were led by Roy Chapman Andrews, in company of Walter Willis Granger as chief paleontologist and field team. The team did extensive exploration at the Bayn Dzak (formerly Shabarakh Usu) region, which they nicknamed flaming cliffs, given that at sunset the sediments of this locality had a characteristic reddish color. Notable finds included the first known fossils of Oviraptor, Protoceratops, Saurornithoides, and Velociraptor, the first confirmed dinosaur eggs (a partial nest of Oviraptor), as well as fossil mammals. Some of these were briefly described by Henry Fairfield Osborn during the ongoing years of the expeditions. In 1927 the formation was formally described and established by Berkey and Morris, with Bayn Dzak as the type locality.

In 1963 the Mongolian paleontologist Demberelyin Dashzeveg reported the discovery of a new fossiliferous locality of the Djadochta Formation: Tugriken Shireh. During the 1960s to 1970s, Polish-Mongolian and Russian-Mongolian paleontological expeditions collected new, partial to complete specimens of Protoceratops and Velociraptor at this locality, making these dinosaur species a common occurrence in Tugriken Shireh. Some of the most notable excavations made at Tugriken Shireh include the Fighting Dinosaurs (Protoceratops and Velociraptor locked in combat), and abundant articulated, in situ (in the original pose), and sometimes complete skeletons of Protoceratops.

During the 1980s a joint Soviet-Mongolian paleontological expedition discovered several Mesozoic fossil-rich localities in the Gobi Desert of Mongolia. Among these sites, Udyn Sayr was discovered and examined by the expedition, regarding its age as Late Cretaceous. This new locality was predominantly rich in avimimid fossils, with a lesser abundance of mammal and other dinosaur fossils.

In 1993 a collaborative expedition by the Mongolian Academy of Sciences and the American Museum of Natural History discovered a new fossil site within the Djadochta Formation, named Ukhaa Tolgod, which translates to "Brown Hills." This site has yielded a significant number of well-preserved fossils, including those of mammals, dinosaurs, lizards, and eggs. Most specimens are found in near-complete articulation, indicating excellent preservation conditions. Compared to other Mesozoic fossil sites, Ukhaa Tolgod stands out for its high fossil diversity.

The modern-day Djadochta Formation is set in an arid habitat of sand dunes with little freshwater apart from oases and arroyos, in the Gobi Desert. The dominant lithology of the Djadochta Formation is represented by non-marine, cemented reddish-orange and pale orange to light gray, medium to fine-grained sands and sandstones, which include minor deposits of calcareous concretions and orange-brown silty clay. Less abundant sedimentation comprises conglomerates, siltstones, fluvial (water-deposited) sandstones, and mudstones. The entire thickness of the formation in the Ulan Nur Basin is at least 80 m (260 ft). Several aeolian processes (wind works) indicate the presence of large straight-crested dune-like structures, and smaller barchan (crescent-shaped) and parabolic (poorly U-shaped) dunes across the formation. Reddish sandstones are observed in numerous localities.

The Djadochta formation occurs in the Late Cretaceous period of the Campanian and Early Maastrichtian stages. Magnetostratigraphic datings from 2006 initially suggested a Campanian age for the formation. But that same study indicated their findings could not be completely substantiated. More recent studies have suggested that the Djadochta, Barun Goyot and Nemegt Formations might instead represent different contemporary environments instead of strict temporal succession from each other. If the Djadochta Formation is the same age as the Nemegt Formation, it would be Maastrichtian in age. Because of this uncertainty, the age of the Djadochta is most parsimoniously Campanian to Maastrichtian.

The upper boundary of the Djadokhta strata has been identified at the erosional surface overlain by gravels of the Palestine Khashaat (Gashoto) Formation.