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 the provinces.

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 conditions.

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 Mountains.

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.

Geologic Resources

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.

Geologic Hazards

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.

Conclusion

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).

Genevieve Atwood