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Exploring the Fossils of Arizona USA

Arizona is the sixth largest state located in the southwestern region of the United States and is known for its warm and dry climate, vast deserts, rugged canyons, and towering mountains.

Petrified Forest National Park Arizona - Petrified wood and butte - Image NPS
Petrified Forest National Park Arizona - Petrified wood and butte - Image NPS

Arizona has both a diverse fossil record and geological history. Whether that is having Palaeozoic Era Trilobites found within the limestone formations of the Grand Canyon or Late Cretaceous dinosaur remains found in the Petrified Forest National Park, the Grand Canyon, and the Navajo Nation or the remains of Pleistocene mammoths and mastodons that once roamed the grasslands of Arizona.



During the Palaeozoic Era dated between 541 - 252 million years ago the area known as Arizona was covered by shallow seas and sedimentary rocks were formed from the presence of calcium carbonate-rich shells and skeletal remains of marine organisms such as algae, crinoids, brachiopods, and bryozoans.


The Mesozoic Era (252 - 66 million years ago) saw Arizona as part of a vast inland sea called the Western Interior Seaway where the deposition of thick marine sedimentary shales and sandstones were formed. From about 66 million years ago and from the start of the Cenozoic Era the region including Arizona experienced significant tectonic and volcanic activity.


The Grand Canyon
The Grand Canyon

The geological and fossil record of the Arizona stretches back to the Precambrian and tells a wonderfully diverse natural history story of shallow warm seas, volcanic activity, mountain building as well as iconic arid deserts.


Arizona as we know it today became a U.S. state on 14th February, 1912, making it the 48th state to join the Union under the 27th President of the United States, William Howard Taft.


There is a lot of debate over the origins of the name Arizona but the oldest residents of the lands were the first nation of Tohono O’odham who for thousands of years lived on the lands known as Papagueria.



Among the state's most recognisable natural landmarks is the Grand Canyon, a massive gorge carved during the last 5 million years by the Colorado River that stretches over 270 miles long and is up to 18 miles wide. Arizona is also home to a number of natural wonders such as the red rock formations of Sedona, the Sonoran Desert, the Painted Desert and Petrified Forest.


Arizona geologically has been subjected to intense tectonic forces and has undergone significant metamorphism, resulting in the formation of minerals such as copper, gold, and silver that have been economically important to the state.


This intense activity in some area such as in the Coronado National Memorial has destroyed much of the fossil record from the disturbance created from the metamorphism of igneous intrusions, volcanic activity, deformation from faulting, and recrystallization of limestone to dolomite.

Compiled by Jason Kenworthy and Philip Reiker (NPS Geologic Resources Division) from ESRI Arc Image Service, National Geographic Society TOPO Imagery.
Compiled by Jason Kenworthy and Philip Reiker (NPS Geologic Resources Division) from ESRI Arc Image Service

Above is the map of physiographic provinces of the western United States. The Colorado Plateau is relatively undeformed area rough cantered on the Four Corners area where Utah, Colorado, New Mexico, and Arizona come together. The map labels the high mountains of the Rocky Mountain province east of the Colorado Plateau, the parallel valleys and mountains of the Basin and Range province west and south of the Colorado Plateau. National parks adjacent to Glen Canyon National Recreation Area are Canyonlands National Park, Rainbow Bridge National Monument, Grand Canyon National Park, and Capitol Reef National Park.


The officially designated gemstone of Arizona is Turquoise a mineral that is known for its blue-green colour and a hydrous phosphate of aluminium and copper. The copper star in the states flag identifies Arizona as the largest copper producing state in the United States.


The official designated fossil of Arizona is petrified wood called Araucarioxylon arizonicum the designated state fossil. This is a 225-million-year-old primitive and extinct araucarian conifer.


Petrified Forest National Park - Image NPS
Petrified Trees in the Petrified Forest National Park, Arizona - Image NPS

Arizona’s petrified wood was formed from trees that grew during the Triassic Period in the high mountain ranges and Badlands of central Arizona. The petrified wood in Arizona is particularly well-preserved and is often used as a symbol of the state's natural beauty and geologic history.


Petrified Insect Borings, Petrified Forest National Park - Image NPS
Petrified Wood Insect Borings - Image NPS

This wood comes from trees that grew in a tropical forest approximately 213 to 237 million years ago.


These trees were buried by sediment and volcanic ash, which eventually led to their preservation through a process called permineralisation.


This is a type of fossilisation that happens when minerals transported by ground water fill in all the organic open spaces and mineral deposits form an internal cast of the original structure and shape of the wood including any insect borings.


By the process of petrifaction, the wood was buried by sediments and over millions of years the mineralisation mainly from silica has replaced the organic matter leaving an agate fossil.


The agate crystal is a microcrystalline-quartz mineraloid that has a lustrous display of multi-colour banding of white, grey, red, green, and blue as a result from long-term accumulations of siliceous groundwater deposits in silicified cavities of plant tissue.


Unlike other types of fossils which are typically impressions or compressions, petrified wood is a three-dimensional mould of the original plant.

Close up of petrified wood, Petrified Forest National Park - Image NPS
Close up of petrified wood, Petrified Forest National Park - Image NPS

The petrified wood can be seen in Arizona at the Petrified Forest National Park found within the Colorado Plateaus Province a 130,000 square mile region of the western United States spanning parts of Arizona, Colorado, New Mexico, and Utah.


The Petrified Forest National Park not only has unique and stunning landscape but it is also known for its exposures of the Upper Triassic Chinle Formation part of the larger Chinle Group that also includes the Moenkopi Formation and Shinarump Conglomerate.



The Chinle Formation is composed primarily of sedimentary rock layers, including sandstone, mudstone, and shale, that were deposited in a range of environments, including rivers, lakes, and floodplains. The different layers of the Chinle Formation vary in colour, with shades of red, brown, and purple being particularly prominent.


The Chinle Formation is particularly notable for its rich fossil record including early dinosaurs, crocodiles, mammals, and a variety of plants and invertebrates. It is globally significant because the Upper Triassic fossil flora and fauna fossil record provides a distinct record of diverse terrestrial ecosystems during “the dawn of dinosaurs”.

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