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

Gobi Desert The Gobi Desert (, , ; ) is a large, cold desert and grassland region in North China and southern Mongolia. It is the sixth-largest desert in the world. The name of the desert comes from the Mongolian word ''gobi'', used to refer to all of th ...

Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...

, dating to the Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. 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 preserve ...

s of

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

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

crocodilian Crocodilia () is an Order (biology), order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorp ...

s, and a diverse

fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

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

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 a region, 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 meteoro ...

associated with it was wetter than when preceding formations were deposited; there seems to have existed at least some degree of forest cover as indicated by petrified fossil trunks. When examined, the rock

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

of the upper section consist of braided meanders, and

oxbow lakes An oxbow lake is a U-shaped lake or stream pool, pool that forms when a wide meander of a river is meander cutoff, cut off, creating a free-standing body of water. The word "oxbow" can also refer to a U-shaped bend in a river or stream, whether ...

. Alteration of sediment grades indicates frequent hydrodynamic condition changes (seasonal rain).

Siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

pellets indicate desiccation at the channel margins in river low-stand periods. Bank erosion features would form through river channel migration. Generally rivers consisted of low

flow regimes Flow may refer to: Science and technology * Flow (fluid), Fluid flow, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathematics), a group action of the real numbers on a set ...

with sporadic upper flow regimes indicated by exotic pebbles. The lower sections of the formation may represent Lacustrine or

Palustrine Palustrine wetlands include any inland wetland that contains ocean-derived salts in concentrations of less than 0.5 parts per thousand, and is non- tidal. The word ''palustrine'' comes from the Latin word ''palus'' or marsh. Wetlands within this ...

deposition; perhaps the vestiges of an

endorheic lake An endorheic lake (also called a sink lake or terminal lake) is a collection of water within an endorheic basin, or sink, with no evident outlet. Endorheic lakes are generally Saline water, saline as a result of being unable to get rid of solutes ...

? A study of

oxygen-18 Oxygen-18 (, Ω) is a natural, stable isotope of oxygen and one of the environmental isotopes. is an important precursor for the production of fluorodeoxyglucose (FDG) used in positron emission tomography (PET). Generally, in the radiopharm ...

isotopes from Theropod teeth show seasonal patterns similar to present day continental climates. Mean annual temperatures are estimated to be around with seasonal precipitation patterns compared to meteorological data from

Shijiazhuang Shijiazhuang; Mandarin: ; formerly known as Shimen and romanized as Shihkiachwang is the capital and most populous city of China's Hebei Province. A prefecture-level city southwest of Beijing, it administers eight districts, three county-le ...

, China; suggesting a monsoonal, cold semi-arid climate.

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 An alluvial plain is a plain (an essentially flat landform) created by the deposition of sediment over a long period by one or more rivers coming from highland regions, from which alluvial soil forms. A ''floodplain'' is part of the process, bei ...

, 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. It overlies and sometimes interfingers with the

Barun Goyot Formation The Baruungoyot Formation (also known as Barun Goyot) 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 Province of Mongolia. Descriptio ...

. Interfingering has been noted at the

stratotype In geology, a stratotype or type section 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 ...

(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 m ...

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. Campa ...

, 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

Ornithischians

= Ankylosaurs

= Hadrosaurs

= Pachycephalosaurs

Sauropods

Theropods

= Alvarezsaurs

= Birds

= Dromaeosaurs

= Ornithomimosaurs

= Oviraptorosaurs

= 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