The Nemegt Formation is a geological formation in the Gobi Desert of Mongolia, dating to the Late Cretaceous. It overlies and sometimes interfingers with the Barun Goyot Formation. Interfingering has been noted at the stratotype and Khermeen Tsav. The formation consists of river channel sediments and contains fossils of fish, turtles, crocodilians, and a diverse fauna of dinosaurs, including birds. The climate associated with it was wetter than when preceding formations were deposited; there seems to have existed at least some degree of forest cover. Fossilized trunks have been also found. There has been no absolute dating of the Nemegt Formation. Historically the Nemegt has been considered late Campanian to Maastrichtian, based on comparisons of fossils present, but no exact dating has been performed. The age for the underlying Baruungoyot Formation has been suggested as Santonian to Campanian, and Shuvalov found K-Ar dating of basalts that they referred to the mid and upper Baruungoyot to be 75 to 80 million years old.
Description
The most recent stratigraphy divides the Nemegt Formation into three informal members. The lower member is dominated by fluvial deposits, while middle and upper members consist of alluvial plain, paludal, lacustrine, and fluvial deposits. The colour of the sediments is usually light grey to tan in colour in comparison to the typically red colour of the underlying Barun Goyot Formation. The Nemegt Formation is composed of mudstones and sandstones that were deposited by ancient lakes, streams, and flood plains. The Altan Ula locality was described by Michael Novacek as "a canyon carved out of a very rich series of sedimentary rocks" with "steep cliffs and narrow washes". The presence of petrified wood, and the remains of Araucariaceae conifers indicate that the forests of the Nemegt were thickly wooded, with a high canopy formed by tall conifer trees. When examined, the rockfacies of this formation suggest the presence of stream and river channels, mudflats, and shallow lakes. Sediments also indicate that there existed a rich habitat, offering diverse food in abundant amounts that could sustain massive Cretaceous dinosaurs.
