The Nemegt Formation (also known as Nemegtskaya Svita) is a geological

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondar ...

in the Gobi Desert of

Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million, ...

, dating to the Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

. The formation consists of river channel sediments and contains

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s of

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

s, crocodilians, and a diverse

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s, including birds.

Description

The Nemegt Formation is composed of mudstones and sandstones that were deposited by ancient lakes, streams, and flood plains. The Altan Uul 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

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...

associated with it was wetter than when preceding formations were deposited; there seems to have existed at least some degree of

forest cover Forest cover is the amount of forest that covers a particular area of land. It may be measured as relative (in percent) or absolute (in square kilometres/square miles). Around a third of the world's surface is covered with forest, with closed-canop ...

. Fossilized trunks have been also found. These petrified wood, and the remains of

Araucariaceae Araucariaceae – also known as araucarians – is an extremely ancient family of coniferous trees. The family achieved its maximum diversity during the Jurassic and Cretaceous periods and the early Cenozoic, when it was distributed almost worldw ...

conifers indicate that the forests of the Nemegt were thickly wooded, with a high canopy formed by tall conifer trees. When examined, the rock facies 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.

Stratigraphy

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 Paludal is derived from the Latin word ''palus'' ("marsh"). *Paludal, in geology, refers to sediments that accumulated in a marsh environment. *Paludal, in ecology Ecology () is the study of the relationships between living organisms, inclu ...

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

, 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. It overlies and sometimes interfingers with the

Barun Goyot Formation The Barun Goyot Formation (also known as Baruungoyot Formation or West Goyot Formation) is a geological formation dating to the Late Cretaceous Period. It is located within and is widely represented in the Gobi Desert Basin, in the Ömnögovi Pr ...

. Interfingering has been noted at the

stratotype A stratotype or type section in geology is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the stan ...

(Red Walls) and Hermiin Tsav. 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 (= Svita) has been suggested as

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. ...

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...

, and Shuvalov (2000) found K-Ar dating of basalts that they referred to the mid and upper Baruungoyot to be 75 to 80 million years old.

Paleobiota of the Nemegt Formation

Stratigraphic positions are based on Eberth (2018) who correlated localities to their approximate position within the formation.

Amphibians

Crocodylomorphs

Fish

Flora

Invertebrates

Mammals

Pterosaurs

Turtles

Dinosaurs

Ankylosaurs

Alvarezsaurs

Birds

Dromaeosaurs

Hadrosaurs

Ornithomimosaurs

Oviraptorosaurs

Pachycephalosaurs

Sauropods

A Very large Titanosaur Informally called "Mongolian titan" is present by footprints

Therizinosaurs

Troodontids

Tyrannosaurs

References

{{Reflist Geologic formations of Mongolia Upper Cretaceous Series of Asia Cretaceous Mongolia Sandstone formations Shale formations Conglomerate formations Mudstone formations Lacustrine deposits Fluvial deposits