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Central Highlands

Three Basic Provinces


Geology of the Southwest

 Think in 3’s.

Three main regions: Basin and Range, Central Highlands and the Colorado Plateau.

Basin and Range

 

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Basin and Range meets Grand Staircase.JPG (38318 bytes) On the edge of the Grand Wash Cliffs where Basin and Range and the Grand Staircase meet.  Dangle your feet over the edge of this!  Stunning views.

The Basin and Range is found uniquely in the southwestern United States and northern Mexico. It is very well developed in southern Arizona and a lot of Nevada. In fact, it is easier to spot the basin and range terrain in Nevada than any other place on Earth.

Also, look closely at where the basin and range is located. The basin and range sort of wraps around the Colorado Plateau. At one time, Arizona was also a plateau, not exactly like the Colorado Plateau but similar. As extension occurred, and Arizona was pulled apart, the old plateau was turned into the basin and range. Today, the Colorado Plateau represents the remaining land that was not pulled apart into the washboard of the basin and range.

Central Highlands

 Click to enlarge map.

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Central Highlands Geology

The Central Highlands region has been termed the Transition Zone by geologists as it  bisects the state into two major geologic provinces, the Colorado Plateau to the north and the Basin and Range to the south.  The region is characterized by numerous mountain ranges separated by several basins including Chino Valley, the Verde Valley, Tonto Basin, San Carlos Basin, and the Safford Valley.

Unlike the Colorado Plateau to the north, igneous and metamorphic rocks are well exposed in many areas in this region. In several areas granitic plutons have intruded into overlying sedimentary rocks. The heat and water associated with this magma caused intense mineralization of nearby rocks, particularly limestone, and copper minerals formed. These valuable copper deposits have been mined historically and are being mined today in the Clifton-Morenci area and the Globe-Miami area. The mines contain low-grade ore (not very concentrated) so huge open-pit mines have been dug to extract enough rock to gather the copper.

It is also responsible for the richest mineralized zone in the state, the Bradshaw Mountains.  These mountains have yielded over 150,000 ounces of gold.  Rich Hill, Potato Patch, Stanton, Lynx Lake, Bumble Bee and numerous others are gold mining areas made famous by this mineral.  Many of the towns are now disserted.  Today, gold prospecting clubs are still actively panning the area.

Interestingly, some quartzite pebbles associated with ranges south and below the Mogollon Rim have been found in stream deposits atop the rim and to the north. How did they get there? It appears that at one time at least one of the ranges of the Central Highlands, the Mazatzal, was once above and connected to the Colorado Plateau. The Mogollon Rim did not yet exist, and sediment from this range was shed northward onto the plateau.

Colorado Plateau / Mogollon Rim

The Colorado Plateau Geologic Province in Arizona stretches from the Grand Wash Cliffs on the west, southward to the Mogollon Rim, and eastward to the New Mexico border. It is famous for its colorful sedimentary rocks, which cover most of the region. These rocks, so dramatically exposed in the Grand Canyon and in other areas such as Monument Valley and Canyon de Chelly, formed tens of millions of years ago when a vast ocean covered Arizona and much of the western United States. Sandstones, shales, and limestones formed along the periphery and underneath this ancient sea, accumulating in great thicknesses. Some of the more interesting rock units which were deposited during this invasion of the sea include the Coconino Sandstone, which formed from sand dunes giving it it's distinctive "swirled" look, and the Redwall Limestone, which forms many of the spectacular cliffs in the Grand Canyon.

In many areas the great sedimentary beds of the Colorado Plateau have remained intact as flat-lying rock layers, undisturbed by faulting. However, in other places the horizontally-layered rocks have been warped and folded, often forming what are known as monoclines. These monoclines form great "steps" on the earth' surface. An excellent example is the East Kaibab Monocline, which bounds the high Kaibab Plateau on its east side. The lack of vegetative cover in many areas makes these great folds quite apparent.

Numerous volcanoes dot the region, adding variety to the landscape. The Uinkaret Mountains (Mt. Trumbull area), the Hopi Buttes, Vulcan's Throne in the Grand Canyon,   Sunset Crater and the  San Francisco Peaks near Flagstaff are all volcanoes found in the area. Sunset Crater erupted in 1064 AD.  The highest point in the state is atop 12,633 foot Mt.Humphreys in the San Francisco Peaks. The "peaks", as locals call them, supported glaciers about 15,000 years ago during the last major glacial advance, but today only scattered snowfields manage to survive into the warm summer months.

Despite abundant precipitation in certain areas, surface water is rare on the Colorado Plateau. Much of the bedrock is limestone absorbs water into solution cavities. So where does the water go? It sinks through the ground and eventually reaches the a harder subsurface which pushes the water laterally.  This results in  springs at lower elevations. Many springs in the Grand Canyon and at the base of the Mogollon Rim are fed by water falling high atop the Colorado Plateau.

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Map of Arizona

 

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Map of the reservations and Four Corners 

 

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