GEOLOGY OF UTAH
Salt Lake City's State Street in the flood of 1983
Diverse, spectacular, and dynamic are three words that describe the
geology of Utah. Generations of geologists have chosen to study geology
in Utah because erosion and climate have exposed an extraordinary diversity
of rocks and geologic structures. This allows scientists to unravel secrets
of the earth's past and predict its future; it also allows prospectors to
search for the earth's resources, and it provides others the opportunity
to appreciate the grandeur and subtleties of their physical environment.
Imposing mountains and flat-layered plateaus in Utah spectacularly display
over two billion years' accumulation of rock, often with little vegetation
concealing their story. These features document ongoing processes of wind
and water erosion, the formation and disappearance of lakes, glaciers, and
the periodic occurrence of powerful earthquakes. Utah's landscape exposes
a variety of sedimentary, igneous, and metamorphic rocks, more than 500
mineral species, and fossils of widely diverse lifeforms including worms,
trilobites, shellfish, corals, fish, dinosaur footprints and bones, and
plant and animal remains, including ice-age mammoths. Utah's rocks provide
clues to the earth's environment during every period of geologic time. Exposed
geologic structures range from simple to complex, often displayed with textbook
clarity in spectacular vistas. Scenes of beauty and wonder match Utah's
wealth of metals, energy resources, groundwater, and industrial materials.
Diverse geologic hazards--especially earthquakes, flooding, and landslides--command
respect for ongoing geologic processes.
Geographers divide the United States into approximately twenty regions based
on contrasting landforms and underlying structure. Portions of three provinces
(the Basin and Range, the Colorado Plateau, and the Rocky Mountains) lie
within Utah's boundaries. No wonder Utah has such diverse mineral resources,
scenery, and geologic hazards! The three provinces contrast dramatically.
For instance, processes of erosion dominate the Colorado Plateau today while
sedimentation dominates the Basin and Range. The massive metamorphic rocks
of the Uinta Mountains of the Rocky Mountain province contrast markedly
with the red, flat-layered rocks of the mesas of the Colorado Plateau or
the gray, folded, faulted, complex mountain-blocks of the Basin and Range.
Each province has its own suite of resources, hazards, and scenic attractions.
Even the properties of the rocks deep within the earth's crust differ among
Geologic History of Utah
Just as human history builds on the past, so present-day geologic relationships
result from as much as four billion years of geologic history. The rocks
of Utah developed in a wide variety of physical environments. They reflect
volcanic eruptions like Mount St. Helens, glaciers like those of the Canadian
Rockies, swamps like those of Florida, sand dunes like the Sahara desert,
shallow seas and islands like the Bahamas, and coastal areas similar to
those of present-day Texas. Geologists, much like historians and archaeologists,
divide time into phases, label periods of significance, distinguish different
rock horizons by formation names, and interpret what they find on the surface
and subsurface into a story of events and processes that explain today's
The rocks exposed in Utah have enabled geologists to identify and name almost
600 rock units in order to understand Utah's geologic history. One simplified
rendition (by L.F. Hintze) of Utah's geologic past divides Utah's story
into eight phases. The first is the longest and least understood. The oldest
rocks in Utah, more than 2,500 million years old, are known to exist only
in northern Utah. These and other rocks 1,600 million years old were so
deeply buried that heat and pressure within the earth changed them to metamorphic
rocks. This process integrated them into the assemblage of rocks that forms
the core of the North American continent and underlies much of Utah.
Eight hundred million years of younger rocks have been deposited on this
"basement" foundation. Rocks recording this phase of Utah's geologic
history can be studied in some mountain ranges of northern Utah, including
the Wasatch and Raft River ranges and the eastern end of the Uinta Mountains.
During the second phase a feature developed that has influenced much of
the geology from that time to the present, where it now demarcates the eastern
boundary of the Basin and Range Province. This long-time boundary has defined
regions of different geologic character for more than 750 million years
and is commonly known as the Wasatch line because in the north of the state
it approximately coincides with the Wasatch Range.
During the second phase of Utah's geologic history, warm, shallow-water
conditions deposited thick accumulations of sediments on the downward, subsiding
side of the Wasatch line. and also in an east-trending trough now occupied
by the Uinta Mountains. These rocks now form much of the gray-rock landscape
of western Utah and the high Uinta Mountains, and they contain many fossils
that record life of these seas, such as the trilobites of the Wheeler Shale
west of Delta. Across the Wasatch line, generally flat topography near sea
level was alternately inundated and exposed alternately as seas encroached
and retreated, leaving beach deposits now deeply buried by more recent sediments
in eastern Utah.
During the third phase, two areas of Utah accumulated extraordinarily thick
deposits of sediments while most other areas were either eroded or accumulated
only thin deposits of sedimentary rocks. In the Paradox Basin, hot conditions
and shallow-lake environments deposited thousands of feet of shales and
evaporites, while in the Oquirrh Basin shallow seas deposited sandstone,
shale, and limestone as much as three miles thick. Rocks in the Paradox
Basin produce oil, gas, phosphate, and potash. Oquirrh Basin sediments now
form the craggy ledges of Mount Timpanogos and the Oquirrh Mountains. The
more easily eroded Paradox Basin sediments outcrop only in some valleys
of the Colorado Plateau but can be studied as cores of rock retrieved from
the subsurface in wells drilled in the exploration for oil.
The rocks deposited during most of the fourth phase are non-marine, although
two shallow seas did cover parts of the area for relatively short times.
During this phase, extensive sand deserts developed in Sahara-like conditions,
and rivers, mudflats, and other shallow-water environments deposited conglomerates,
shales, and sandstones that now make up the magnificent red rock country
of the national parks of Utah. Land plants and animals, including many dinosaurs,
lived and died in this environment. The Morrison Formation is famous for
its dinosaur fossils, such as those quarried at Dinosaur National Monument.
Much of the uranium produced in Utah comes from rocks of this phase.
The compression of western Utah as the North American continent collided
with land masses to the west drastically affected the fifth phase. The crust
on either side of the Wasatch line responded differently. The crust east
of the line remained rigid. The crust to the west buckled and folded, mountains
rose, and western Utah's landscape shortened by tens of miles as older rock
layers were folded and thrust eastward over younger rock. The complex geologic
structures that formed during this phase are exposed in faulted and folded
rocks over much of western and northern Utah. Sediments shed from the eastern
side of these newly created highlands formed a wedge of conglomerates and
sandstones across central Utah. These graded onto the gentler, coastal-plain
topography of eastern Utah, that looked much like present-day Texas and
gradually stretched into a broad, shallow seaway. Extensive swamps, now
Utah's richest coal deposits, flourished along the margin of the seaway
and the slow-moving, gentle-gradient rivers of the coastal plain. The sedimentary
rocks deposited during this phase cover much of central and eastern Utah
and grade from coarse conglomerates to the dull gray shales deposited in
the shallow Mancos seaway. These shales erode easily and have provided today's
transportation and utility corridors from Colorado into Utah.
The sixth phase's uplifting of the Uinta Mountains and downwarping of the
Uinta Basin resulted from the creation of the Rocky Mountains. Smaller uplifts
created such scenic attractions as Waterpocket Fold, the San Rafael Swell,
Monument Upwarp, and the Circle Cliffs anticline. Lakes occupied large basins
east of the Wasatch line. The Green River lake beds were in one of these
basins, and now are an important source of oil and oil shale. Much of the
area not covered by these lakes eroded.
The seventh phase must have been rather exciting. A period of widespread
igneous activity began about 40 million years ago. Caldera explosions erupted
thousands of cubic miles of volcanic rocks from several locations. Volcanoes
spewed ash and lava. For 20 million years these extrusive volcanic rocks
smoothed the landscape, filling depressions with accumulations of ash, flows,
and debris literally miles thick. These mostly pastel-colored extrusive
rocks still blanket much of the high areas of central and southwestern Utah.
During this seventh phase, not all of the molten rising igneous material
erupted as volcanic rocks; some material, along with its mineral-bearing
fluids, congealed in the earth's crust. Several of these intruded masses
having been exposed by erosion or encountered out by exploration drilling
became great mining districts, such as at Alta, Brighton, Bingham, Park
City, and Cedar City. In the Colorado Plateau, bodies of intrusive rocks
domed the overlying sedimentary rocks to form the La Sal, Abajo, and Henry
The eighth phase created our present topography. Regional uplift of much
of the western North American continent raised Utah to its present elevation,
on average about one mile above sea level. Steepened river gradients greatly
accelerated erosion, and several rivers still sculpt the great canyonlands
of the Colorado Plateau and carry incredible volumes of sediment to the
Colorado River toward the Gulf of California. As the western coast of the
North American continent moves slowly westward relative to the continent
east of the Wasatch line, the east-west stretching has broken the crust
along north-south faults, creating elongated basins and ranges, disrupting
drainages, isolating mountain ranges, and creating closed basins that have
been filling with sediments ever since. Active faults such as the Wasatch
fault accommodate this stretching and tilting in jarring, potentially destructive
readjustments called earthquakes. Volcanism continues, particularly in southwestern
Utah. Substantially cooler and wetter climate periods created glaciers at
high elevations and lakes in basins. One such was Lake Bonneville, which
reached its highest level, in places more than 1,000 feet deep, about 15,000
years ago. Significantly drier conditions intervened. Today we live in one
of the drier periods. The glaciers have retreated and hotter conditions
have almost dried up the extensive lakes, leaving the Great Salt Lake as
the largest remnant of the lake that once covered most of northwestern Utah.
Utah's varied geologic history endowed Utah with a remarkable array of geologic
resources, and uplift and erosion have made locating and exploiting them
relatively easy in some areas. Many mines, mostly in the western part of
the state, have produced important amounts of metals. The most famous Utah
mine is the huge open-pit mine at Bingham Canyon, one of the largest copper
mines in the world. Much of the world's beryllium is produced from a mine
near Topaz Mountain. Central and eastern Utah contain tremendous energy
resources, including coal, oil, natural gas, oil shale, tar sand, and uranium.
Deposits of salt and phosphate are important sources of chemicals, as are
Utah's saline lakes. Less glamorous but important in support of local industry
are abundant construction materials--these include immense deposits of sand
and gravel, limestone for cement, dimension stone, and other industrial
minerals. Utah's wide variety of gemstones, rocks, and minerals interest
professional and amateur collectors.
It should come as no surprise that the geologic processes that blessed Utah
with an abundance of material resources and a variety of natural features
distributed an equally diverse suite of geologic hazards across the state.
The geologic processes that shaped the landscape of Utah present significant
hazards to people and property. Utahns are exposed to earthquakes, landslides,
mud flows, rock falls, avalanches, flooding of rivers and lakes, radon,
and problem soils that shrink, swell or compact. These hazards can be costly,
and some threaten lives. For instance, during the five-year period from
1982 to 1987 landslides, rising lake levels, debris flows, high groundwater
levels, and floods caused hundreds of millions of dollars in property damage
along the Wasatch Front and in central Utah and killed three individuals.
Some hazards are rare events with high risk such as earthquakes. Others
are generally not life-threatening but are more frequent and cause considerable
damage, particularly when they are ignored or exacerbated by construction
practices. Earthquakes are the most destructive, but not the most frequent,
geologic hazard in Utah. Large earthquakes have occurred and will continue
to occur in the western two-thirds of the state, and geologic evidence and
the historic seismicity indicate that such events are more frequent in a
zone trending along the Wasatch line. Displacements along a zone of faults
account for the location of the Great Salt Lake and Utah Lake on the down-dropped
side and impressive mountain fronts on the upside. Present scientific understanding
of the faults does not provide a basis for predicting when and where the
next earthquake will occur. Estimates of the maximum magnitude of a Wasatch
Fault earthquake range from 7.0 to 7.5 on the Richter scale. This type of
earthquake will affect some area of the Wasatch Fault on the average of
once every 300-400 years. Ground-shaking over a broad area is the single
greatest hazard associated with earthquakes because shaking causes buildings
to collapse, and the falling materials kill people and destroy property.
Surface rupture, the shifting of location of lakes, failure of dams, landslides,
lateral spreads, mudflows, liquefaction, piping, other hydrologic changes,
and waves on enclosed bodies of water also can and will cause extensive
damage depending on the location and magnitude of an earthquake.
Landslides and flooding are the two most common geologic hazards in Utah
and annually cause significant economic losses. Approximately 45 percent
of the state is mountain, hill, and steep-valley terrain conducive to landslides.
Also, some geologic formations in Utah are particularly prone to develop
landslides. Summer cloudbursts and rapid snowmelt have flooded many Utah
communities. Fortunately, the conditions that produce landslides and flooding
are quite well understood, and intelligent use of geologic information in
land-use planning can minimize the negative impact of landslides and flooding.
The geology of Utah has contributed much to the economic development of
the state and offers many recreational opportunities to residents and visitors.
It is a major factor in making Utah an attractive place to live and visit.
The geology must be respected, however, or it can cause great property damage
and loss of life. Also, much of the geology is fragile and must be protected
from abuse if it is to be available to future generations. Wise development
of the state requires a knowledge and a respect for its geology.
See: William Lee Stokes, Geology of Utah (1986).