Geology of Wyoming
When visitors first see Wyoming's vast prairies and mountain ranges, their interest
is normally attracted to the spectacular views, the open space, the accompanying
wildlife, and the sparse vegetation. Often, little if any thought is given to
the geologic history that produced much of what they are seeing. The state's
present landscape resulted from a long series of events reaching back in
geologic time almost to the inception of the Earth as a planet and continuing to
the present.
Scientists can determine the ages of rocks by measuring the radioactive decay of
particular elements in the rocks. In Wyoming, the oldest dated rocks are certain
granites and granite gneisses which occur in the cores of the larger mountain
ranges and are at least 2.8 billion years old. Relatively little, however, is
known about these most ancient rocks.
The better documented geologic history of Wyoming dates back about 570 million
years, to a time when sedimentary layers covered the ancient granite. The
relationship of sediments overlying crystalline rocks is clearly evident around
the margins of the mountains where erosion has exposed the interface between
these rock units. The sedimentary layers are distinguishable from each other
because they have different color, texture, and mineral composition, and because
they often contain different assemblages or fossils (the preserved remains of
animals and plants that lived during the time these rocks were deposited).
To
understand the origin of the landforms in Wyoming, one must accept the fact that
the Earth's crust, which is some 22 miles thick and composed of dense,
crystalline igneous and metamorphic rocks, subsided relative to sea level for a
long period of time. While Wyoming was below sea level, several thousand feet of
relatively flat-lying sedimentary rock accumulated below the oceanic waters or
along the shoreline areas.
Using the erosional surface that separates underlying Precambrian rocks from
overlying sedimentary rocks as a datum plane, the following events can be
deduced. The greatest subsidence below sea level occurred in western Wyoming, as
evidenced by the 30,000 feet of sediment that accumulated in a
north-south-trending trough since the beginning or Cambrian time. A broad area
east of this trough on a shelf adjacent to deeper water received much less
sediment, perhaps 10,000 feet, during the same time interval. Throughout this
long period of time, there were minor upward and downward oscillations of the
crust, but the principal activity in Wyoming was subsidence
and accumulation of sediments. Wyoming was last at or near sea level during the
close of the Cretaceous Period, approximately 66 million years ago.
Structural cross section of Wyoming today.
Physiographic provinces of Wyoming.
(Click image to download PDF)
Today,
most of the sedimentary rock units have been tilted from their originally
horizontal positions by large-scale crustal movements described by the terms
diastrophism and tectonic activity. A major period
of crustal activity began in the Late Cretaceous and has continued
intermittently until the present. This crustal unrest warped and fractured the
crust and the overlying sediments, outlining the mountain ranges and basins and
establishing the geologic framework that we see today.
This episode of mountain building elevated the rocks above sea level and,
together with further elevation in late Tertiary time, provided the necessary
stream gradients whereby the streams of the region could proceed to dissect the
rocks into the existing landforms. The major event that ultimately governed the
position of most larger streams in Wyoming was the accumulation of a vast sheet
of younger sedimentary rocks in the basin areas, climaxed by a widespread sheet
of volcanically-derived, fine-grained sediments. The major streams developed on
this aggraded surface. Later, as these streams cut
downward, their courses were locked into the harder underlying rocks and, as a
result, such great canyons as the Wind River, the Bighorn, and the Platte
evolved.
Major drainages in Wyoming.
(Click image to download PDF)
As a result of this geologic history, Wyoming is now divisible into three major
physiographic categories: mountains, the Great Plains of eastern Wyoming, and
basins. The landscape and underlying geology of features in each of these
categories are very different.
In most of Wyoming's mountainous areas, the difference of elevation is in part
due to uplift of large segments of the Earth's crust in the form of folds or wrinkles, or blocks bounded by fractures,
or a combination of both. Most of Wyoming's larger mountain ranges also have an
exposed core of very ancient Precambrian metamorphic and igneous rocks.
Physiographic provinces of Wyoming. Major drainages,
the Contential Divide, and the highest and lowest points in the State are shown.
(Click image to download PDF)
The frequently asked question, “How old are the mountains?” may be answered in two
ways. We might consider that the age of the oldest rocks exposed to the elements
and in the process of being eroded is the age of the mountains. In another sense
we might consider the time elapsed since the rocks were elevated so that they
began to be carved by erosion.
Most mountains in Wyoming were elevated about 60 million years ago, which is quite
recent by geologic standards, since the oldest rocks in the cores of these
mountains are about 2.8 billion years old.
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Stage 1: Mountains and basins formed by folding and faulting.
Extensively
modifiedby erosion. (Click image above to enlarge.) |
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Stage 2: Basins partially filled with younger sediments derived from
adjacent
mountains and intermittent volcanic ash falls. (Click image above
to enlarge.) |
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Stage 3: Basins filled to overflowing at low places on
divides, followed by regional uplift
with titling and faulting. Present master
streams developed on rejuvenated surfaces.
(Click image above to enlarge.)
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Stage 4: Present cycle of erosion - basins excavated.
Youthful canyons cut accross
resistant cores of mountains. Basin floors are
lowered as canyons are deepened.
(Click image above to enlarge.) |
The Tetons, which rise with spectacular grandeur from the floor of Jackson Hole, are
a medium-sized but unique mountain range. The bold east front, which stands
above the glacial lakes at the base, resulted from intermittent but major
movement of more than 20,000 feet on a steeply inclined fracture plane that
slopes to the east. The rugged mountain peaks have been carved from the elevated
segment of the crust. Glacial processes have produced the matterhorns and
U-shaped valleys that are the characteristic landforms.
The Tetons are extremely young by geologic standards, having attained their height
less than 10 million years ago. This fact places them among the youngest ranges
in the Rocky Mountains. Numerous minor earthquakes within historic time in
Jackson Hole attest to the fact that these mountains may still be growing.
Although the high Yellowstone Plateau and Absaroka Range of northwestern Wyoming present
a mountainous terrain, these areas are remnants of a plateau that coincided with
the top of a vast pile of nearly horizontal sheets of rock materials derived
from nearby volcanic vents in the Yellowstone area. Once the pile of volcanic
debris had accumulated, the region was subjected to the ever-present forces of
erosion, which cut deep valleys. The mountains are those of erosion – the deep
dissection of relatively flat lying rock layers – and are a classic example of
this type of development.
Even now, the Yellowstone Plateau is a thermally and seismically active area with
molten rock perhaps no more than 1.9 miles beneath the surface in some places.
While the geysers of Yellowstone also attest to the relatively shallow thermal
activity beneath the plateau, researchers have documented recent uplift of
portions of the Yellowstone Caldera (remnants of a large volcano), rising at rates up to 15 millimeters per year. The most recent
volcanic activity on the plateau, however, has been dated at 600,000 years
before the present.
Map of major structural elements in Wyoming.
(Click image to download PDF)
Scenically, the basins of Wyoming are less satisfying than the other two
geologic provinces, and some persons may feel that the long, rather monotonous
vistas of low buttes and mesas and sparse vegetation are like a lost world. A
trip across the basins during the heat of the day in August can be a less than
exciting experience, but if the same trip is made in early morning or early
evening, the landscape has a very different appearance. At these times of the
day, low-level lighting can accentuate the landscape and make it very beautiful.
Despite the rather limited vegetative cover, the wide lonesome stretches of Wyoming's
basins provide pasturage for a large number of cattle, sheep, and wildlife,
particularly in the winter months when snowmelt provides water for animals.
Viewed from a geological perspective, these same basins are much more stimulating than
some of the uplifts, and are just as profound in terms of the movements of the
Earth's crust. In fact, the amount of downfolding of the crust usually exceeds
the uplift of the adjacent mountains if sea level is used as a datum. The
basins, which are surface topographic depressions, are at the same time compound
downfolds in which the layered sedimentary rocks dip toward the lowest point or
trough. Many of the exciting aspects of these downwarps are hidden from the eye
because they lie at great depths.
In terms of economics and natural resources, Wyoming
would not be as prosperous without the raw materials that exist in the rocks of
these basins. Along the basin margins, the sedimentary strata often
crop out in hogbacks, exposing limestone, gypsum, bentonite, phosphate rock, and
building stone. Farther out in the basin, at the surface, one finds some of the
great coal fields of Wyoming, and also uranium deposits. The rock units that
extend under the basins at depth act as reservoirs for oil and gas
accumulations. Trona and oil shale are found in the central part of one of the
Green River Basin.
A discussion of Wyoming's geology would not be complete without elaborating on
the treasure house of valuable minerals and rocks located in the state. Wyoming
is perhaps best known for its abundance of energy minerals. Vast reserves of
oil, natural gas, coal, and uranium underlie many areas of the state. Wyoming's
more than 1,000 oil and gas fields produce more than 50 million barrels of oil
and more than 2.4 trillion cubic feet of natural gas each year: the state ranks
first in the Rocky Mountain region. Wyoming has more than doubled its proved
reserves of natural gas in the last 10 years, thanks mainly to the development
of coal bed natural gas (CBNG) in the Powder River Basin, and the development of
tight gas sands in the Lance Formation at Jonah and Pinedale fields in the
northern Green River Basin. Wyoming's 29.7 trillion cubic feet of proved natural
gas reserves is second to only Texas, and is enough to supply the entire United
States for more than a year and a half. The state's petroleum industry also
produces large quantities of helium and carbon dioxide, as well as by-product
sulfur from sour-gas processing plants.
The state also hosts 25 percent of the nation's coal resources, or more than one
trillion tons of coal. From this resource, coal companies mine more than 450
million tons of coal each year, ranking Wyoming first in coal production since
1988. More than 98 percent of Wyoming's coal production comes from Campbell and
Converse counties in northeastern Wyoming's Powder River Basin. This Powder
River Basin production comes from mines on the Wyodak coal deposit. Fifty to 110
feet thick where it is mined, the Wyodak is the thickest coal mined in the
United States.
Although Wyoming’s annual uranium production has declined substantially from its
peak in the early 1980s, the state is still a major uranium producer. In
addition to these actively mined energy resources, Wyoming has a vast untapped
resource of oil shale beneath the Green River and Washakie basins of
southwestern Wyoming.
Wyoming's mineral wealth does not stop at energy resources: the world’s largest
known trona resource is found in the Green River Basin of southwestern Wyoming.
Trona is a naturally-occurring hydrated sodium carbonate and sodium bicarbonate
used to produce industrial soda ash. Soda ash is vitally important in the glass,
paper, soap, petroleum-refining, and textile industries. Baking soda is also a
product of soda ash. Presently, five underground trona mines in Wyoming together
produce approximately 17.5 million tons of trona each year, more than 95 percent
of U.S. production. Soda ash is also exported to a number of countries. Wyoming
is also the largest producer of bentonite, a valuable expanding clay, in the
country. When mixed with water, Wyoming bentonite expands up to 15 times its dry
volume. This unique clay is very useful in drilling muds used by the petroleum
industry, for pelletizing taconite in the iron ore industry, and for binding
foundry sands.
Limestone, gypsum, construction
aggregate, jade, and some placer gold are
also mined in Wyoming. Many other valuable mineral resources occur in Wyoming,
including copper, iron ore, silica sand, molybdenum, feldspar, phosphate rock,
anorthosite (an aluminum-rich rock), and diamond. Although the first seven of
these mineral resources have been mined in the past, the economic value of
Wyoming's diamond occurrences is still being evaluated.
The diversity of Wyoming's mineral resources constitutes one of the state's
greatest long-term economic assets. Few other states have such a variety of
presently or potentially economic resources with which to meet changing market
demand.
References
Geology of Wyoming: G.B. Glass and D.L. Blackstone, Jr., 1999, 12 p.