Arizona Geological History Chapter 4: Triassic Period

With this chapter we begin the Mesozoic (middle life) Era which extended from 251 million years ago to 65 million years ago. The Mesozoic is divided into three Periods: the Triassic (251- to 202 million years ago), the Jurassic (202- to 145 mya), and the Cretaceous (145- to 65 mya).

The preceding Paleozoic Era (542- to 251 mya) ended with a mass extinction and with most of the landmass forming a massive continent called Pangea. Arizona was just barely north of the equator, and once again, emerging from the sea which still existed in California and Nevada.

Paleomap 237

By Triassic time, dinosaurs, pterosaurs (flying reptiles), lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock. In Arizona, there were Phytosaurs, crocodile-like animals (2- to 12 meters long) which inhabited streams and ponds.

Triassic sedimentary rocks, well-exposed on the Colorado Plateau, are represented by the Moenkopi Formation and the Chinle Formation. The Moenkopi consists of continental redbeds (sandstones, shales, and conglomerates) in the northeastern part of the plateau, and minor mixed carbonates of fluvial (river), tidal flats, and shallow marine origin in the west. After a period of erosion, continental sandstones, mudstones, and lake-formed carbonates of the Chinle Formation were deposited. Most Triassic sediments represent deposition well-inland from the sea. The climate was semi-arid in the interior and wet and swampy in the lowlands. Temperatures were 15 -to 20 F warmer than today.


Southern Arizona was a major volcanic province. Many of the mountain ranges contain Triassic volcanic rocks. In the Santa Rita Mountains, for instance, almost 10,000 feet of volcanics were deposited. The Recreation Redbeds in the Tucson Mountains represent an inter-volcanic period of erosion in upper Triassic time.

Volcanism and the high-energy continental deposits made poor hosts for fossils of terrestrial animals and plants. However, the Chinle Formation contains the silicified trunks of large trees preserved and exposed in the Petrified Forest of Arizona, and colorful Chinle rocks are exposed in the Painted Desert.

In mid-Triassic time, the mega-continent of Pangea began splitting into two parts: Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north. This split caused massive volcanism along a rift that would become the Atlantic Ocean.

The Triassic Period ended with another mass extinction of about 76% of marine species and some terrestrial species. Again, the reason is not known, but speculative theories attribute it to comet impacts and volcanism. According to The Resilient Earth: ” At least two impact craters have been found from around the time of this extinction. One is in Western Australia, where scientists have discovered the faint remains of a 75 mile (120 km) wide crater. The other is a 212 million year old crater in Quebec, Canada, forming part of the Manicouagan Reservoir. The Manicouagan impact structure is one of the largest impact craters still visible on the Earth’s surface, with an original rim diameter of approximately 62 miles. Others have suggested that a sudden, gigantic overturning of ocean water created anoxic conditions causing the massive die-off of marine species.”


Blakey, R.C., 1989, Triassic and Jurassic Geology of the Southern Colorado Plateau, in Geologic Evolution of Arizona, J.P. Jenney and S. J. Reynolds, eds. Arizona Geological Society Digest 17.

Hayes, P.T. and Drewes, Harald, 1978, Mesozoic Depositional History of Southeastern Arizona, in Land of Cochise, New Mexico Geological Society Guidbook 29.

Moore, R. C., 1958, Introduction to Historical Geology, McGraw-Hill.



  1. And it was during this epoch that conservatives evolved in their present form, remaining essentially unchanged through the ages. 

  2. And it was during this time that the liberals came out of the trees. But they still walked on all fours and still liked to swing on vines. They never got past their preference for bananas and coconuts. Things haven’t changed much since.

  3. Sadly, it is now illegal AND culturally frowned upon for conservatives to kill and eat liberals, thereby protecting trees, bananas AND coconuts from their predations.  Also, liberals don’t taste particularly good, and squeal like small animals when being dismembered. Hardly worth the effort.

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