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Nemegt Formation
The Nemegt Formation (or Nemegtskaya Svita) is a geological formation in the Gobi desert of Mongolia, dating from the Late Cretaceous. It overlies and sometimes interfingers with the Barun Goyot Formation. Interfingering has been noted at the stratotype (Red Walls) and Khermeen Tsav. It consists of river channel sediments and contains fossils of fish, turtles, crocodilians, birds and a diverse fauna of dinosaurs. 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. It is, however, almost certainly early Maastrichtian c 71-69 Ma. Gradzinski and others considered a Campanian age possible but more recent research indicates otherwise. A Campanian age no longer seems credible, inasmuch as the Alagteegian (or lower Djadokhtan, at the locality "Chuluut Uul") has been radiometrically dated at about 73.5 Ma or even younger (a more recent K/Ar date is 71.6 +/- 1.6 Ma). The c 73.5 (or perhaps 72) Ma Alagteegian is separated from the Nemegt by the "classic" Djadokhtan (e.g. Bayan Dzag), later Djadohktan (represented by Ukhaa Tolgod) and Barungoyotian (Khulsan). All these intervening horizons almost certainly represent more than the 1.5 million years between the dated Alagteegian level and the onset of Maastrichtian time (72.1 million Ma according to current dating). Ergo the Nemegt is entirely Maastrichtian. See also Shuvalov, Sochava and Martinsson '' The Age of Dinosaurs in Russia and Mongolia.'' The presence of Saurolophus further supports an early Maastrichtian age as the same genus occurs in the early Maastrichtian Horseshoe Canyon formation.
==Description==
The Nemegt Formation (the upper beds) are composed of shales and sandstones that were deposited by ancient lakes, streams, and flood plains. The Altan Ula locality was described by Michael Novacek (1996) 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 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.〔Novacek, M. (1996). Dinosaurs of the Flaming Cliffs. Bantam Doubleday Dell Publishing Group Inc. New York, New York. ISBN 978-0-385-47775-8〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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