Arizona Geological History Chapter 4: Triassic Period
Chapter 4: Volcanos, dinosaurs, and petrified wood
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 covered California and Nevada.
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. For more information on Triassic life in Arizona go to Ron Ratkevich’s quirky, but very informative site: http://www.t-rat.com/ and click the “Triassic” button.
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.
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. The Chinle Formation hosts the Petrified Forest. The Chinle Formation preserves a suite of lowland terrestrial environments that includes river channels, floodplains, swamps, and small lakes operating in a strongly seasonal subtropical climate. The rocks show that with time, the climate became arid.
The Arizona Geological Survey has a new geologic map (65Mb) and 18-page report (9Mb) on Petrified Forest National Park in Arizona (located 25 miles east of Holbrook). Both can be downloaded for free here.
The petrification process begins with wood transported by stream becoming trapped and buried in shallow water and covered by mud and volcanic ash. Burial cut off oxygen and delayed decay. Ground water percolating through the sediments dissolved silica from the volcanic ash. As the silica filtered through the logs, it precipitated from solution as microscopic quartz crystals in the woody tissues where air, water, and sap were originally present in the living tree. In some logs, cell structure remained intact, albeit entombed. Where the logs were hollow, woody tissue did not limit crystal growth; large crystals of rose quartz, smoky quartz, amethyst, and other gemstones or large masses of amorphous (noncrystalline) chalcedony and chert lined the cavity walls.
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, and early Jurassic 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.”
For more geologic history see:
Global paleomaps by Christopher R. Scotese (http://www.scotese.com/)
North American paleomap by Ron Blakey,
Colorado Plateau Geosystems.com
Arizona Geological Survey Geosnaps
Copyrighted by Jonathan DuHamel. Reprint is permitted provided that credit of authorship is provided and linked back to the source.