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Unformatted text preview: 8-1Triassic PeriodThe Triassic is a geologic period that extends fromabout 251 to 199 Ma (million years ago). As the firstperiod of the Mesozoic Era, the Triassic follows thePermian and is followed by the Jurassic. Both thestart and end of the Triassic are marked by majorextinction events. The extinction event that closed theTriassic period has recently been more accuratelydated, but as with most older geologic periods, therock beds that define the start and end are wellidentified, but the exact dates of the start and end ofthe period are uncertain by a few million years.During the Triassic, both marine and continental lifeshow an adaptive radiation beginning from thestarkly impoverished biosphere that followed thePermian-Triassic extinction. Modern corals madetheir first appearance. The first flowering plants(Angiosperms) may have evolved during the Triassic,as did the first flying vertebrates, the pterosaurs.The Triassic was from the three distinct layers (Latintrias meaning triad) —red beds, capped by chalk,followed by black shales— that are found throughoutGermany and northwest Europe, called the 'Trias'.In Idaho, the Triassic is represented by theMartinbridge limestone which occurs in exotic terrainnear Hells Canyon.This period occurred roughly 200 to 250 millionyears ago.Modern corals, angiosperms, and pterosaurs firstappeared during this period.This limestone formation features caves andsinkholes near Riggins Idaho.PaleogeographyDuring the Triassic, almost all the Earth's land masswas concentrated into a single supercontinentcentered more or less on the equator, called Pangaea("all the land"). This took the form of a giant "Pac-Man" with an east-facing "mouth" constituting theTethys sea, a vast gulf that opened farther westward in the mid-Triassic, at the expense of the shrinkingPaleo-Tethys Ocean, an ocean that existed during thePaleozoic. The remainder was the world-oceanknown as Panthalassa ("all the sea"). All the deep-ocean sediments laid down during the Triassic havedisappeared through subduction of oceanic plates;thus, very little is known of the Triassic open ocean.The supercontinent Pangaea was rifting during theTriassic—especially late in the period—but had notyet separated. The first nonmarine sediments in therift that marks the initial break-up of Pangea—whichseparated New Jersey from Morocco—are of LateTriassic age; in the U.S., these thick sedimentscomprise the Newark Group. Because of the limitedshoreline of one super-continental mass, Triassicmarine deposits are globally relatively rare, despitetheir prominence in Western Europe, where theTriassic was first studied. In North America, forexample, marine deposits are limited to a fewexposures in the west. Thus Triassic stratigraphy ismostly based on organisms living in lagoons andhypersaline environments, such as Estheriacrustaceans....
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- Fall '09