Three Basic Provinces
of the Southwest
Think in 3s.
regions: Basin and Range, Central Highlands and the Colorado Plateau.
Basin and Range
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 Geology
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
Map of Arizona
Map of the reservations and Four Corners