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. 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 rock units in order to understand Utah's geologic history. One simplified rendition by L. 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, million years old, are known to exist only in northern Utah. These and other rocks 1, 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 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.
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. 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. Intrusions Another source of high temperatures inside the Earth is magma intruding cooler rock. These temperature increases are localized near the intrusion, but also metamorphose rock this is called contact metamorphism.
Pressure or more properly, stress can also change rock. There are two main kinds I want you to know about: Confining Pressure Pressure due to the weight of overlying rock. This kind of pressure is roughly the same in all directions this is like water pressure when scuba diving , and is the kind which compacts rock during diagenesis.
Confining pressure changes rock by compaction and by changing the crystal structure of minerals from relatively open forms to more densely-packed forms. One mineral which does this is olivine, which changes from olivine isolated silica tetrahedra to spinel a much more tightly-bonded structure to perovskite a still more highly compressed structure. This kind of pressure is usually due to tectonic forces. It changes rocks by changing the structure of minerals and by changing the orientation of mineral grains, particularly platy minerals like mica or clay.
Fluids which metamorphose rock are not pore fluids remaining from when sedimentary rocks were deposited. Instead, they come from two main sources: hydrothermal fluids from magmatic intrusions and dehydration of minerals, like clay, which contain water in their structures hydrous minerals. Whatever the source, fluids contain ions dissolved from other rock or from their original source. As fluids percolate through rocks, they can exchange ions with the existing minerals and thus change the chemical makeup of those minerals.
The other way fluids change minerals is by hydrating minerals which previously did not contain water. Either way, fluids change the chemical makeup of minerals, turning them into new minerals, which changes the rocks which were made of the previous minerals. This process of change by fluids is called metasomatism. Types of Rock Metamorphism. Some kinds of metamorphism: Burial Bury rocks deeply enough and they will warm up and change. This form of metamorphism is found anywhere where sediments and rocks are buried deeply, and should strike you as being pretty similar to diagenesis, which we discussed last time.
The line between diagenesis and burial metamorphism is fuzzy. Regional Caused by widespread moderate-to-high temperatures and pressures, as opposed to localized changes along faults or near magmatic intrusions. You find this type of metamorphism in mountain building regions and near subduction zone volcanism. Contact Caused by high temperatures near magmatic intrusions. Found in volcanic regions subduction zones, hot spots and mountain building zones.
Cataclastic Caused by grinding along fault zones. Found along major faults like the San Andreas Fault in California , in mountain building zones, and in deformation regions associated with subduction zones. Hydrothermal Caused by hot fluids percolating through rocks.
Found anywhere where hot fluids can percolate through rocks, notably along mid-ocean ridges. Metamorphic Rocks and Rock Textures. Three major texture and rock types for metamorphic rocks that you need to know: Foliated Rocks Characterized by parallel planes formed through directed pressure and preferred growth orientations of certain platy minerals.
Two common kinds are schist and gneiss, which have been used in a great many really bad geological puns. Non-foliated Rocks Don't have those planes, usually because they are made of mineral grains which are cubic or spherical, and therefore have no preferred orientation.
Two common examples are marble and amphibolites. Deformational Caused by cataclastic metamorphism. The most common rock of this kind is called a mylonite; there is a big mylonite belt in the mountains south of Palm Springs, CA. Metamorphic Grade. Geologists who study metamorphic rocks have come up with the concept of metamorphic grade to describe how 'metamorphosed' a rock is. More specific distinctions can be made through lab experiments in which various kinds of rocks are squeezed and heated up and the changes observed.
Through this kind of work, geologists have found a set of index minerals , which are common minerals which form under particular combinations of pressure and temperature. Armed with knowledge from these experiments, field geologists can go out and make maps of mineral location to determine how metamorphism is distributed over large regions of rocks. Metamorphic Facies. Keep that in mind: you get different minerals from different rocks under the same conditions and you get different minerals from the same rocks under different conditions.
Geologists have formalized these statements into a system of classifications for rocks by pressure and temperature conditions, so that a given combination of pressure and temperature will give a specific class of rocks. These classifications are called metamorphic facies. This is vital information for figuring out past tectonic conditions in the region, since certain facies form in certain plate tectonic environments.
For example, blueschists form under low temperatures and moderate-to-high pressures, which indicates that the material which metamorphosed was shoved down into the Earth so quickly it didn't have much time to warm up.
What kind of plate tectonic environment displays these features? Subduction zones! Once material is weathered from rocks, it is transported away and later deposited somewhere else, and eventually is turned into new rocks.
Such rocks are called sedimentary rocks , and they're the subject of this lecture. What are sediments? Schist is a metamorphic rock type that is commonly formed by the pressure of overlying sediments over a period of millions of years.
The rocks of the Vishnu schist are typical of their type, having elongated minerals that can easily be separated into flakes. Some igneous rock is present in the Vishnu complex, though it represents an intrusion that took place considerably later than the original sediment deposits.
The oldest rocks within the Grand Canyon are exposed within Granite Gorge aria are characteristically dark somber gray. They respond to erosion to form a steep-walled V-shaped gorge Text-fig. The original sedimentary or volcanic characters have been extensively modified and in some cases obliterated. Early Precambrian rocks are not stratified but possess a planar structure known as foliation, resulting from reorientation of platy minerals, crystals, and grains in response to deformation.
Foliation throughout most of Granite Gorge is nearly vertical which contrasts with the horizontal stratification of the overlying younger rocks. Three major rock bodies are found within the Early Precambrian complex. The first encountered on the river trip consists of metamorphosed sedimentary rocks in which some relict.
This body of baked and altered rocks is known as the Vishnu Schist and is exposed downstream beyond Hance Rapids to near Zoroaster Canyon. They represent part of the older rocks of the earths crust. Very little detailed information can be gained about their environment of deposition since the original character of the rock has been nearly completely obliterated.
Downstream from Zoroaster Canyon is a sequence of metamorphic rocks which differ in composition, color, and texture from the Vishnu Schist but superficially appear similar to it because of their degree of metamorphism, These rocks are known as the Brahma Schist and probably represent metamorphosed volcanic rocks. The area of principal interest is in sections 26 and 36, T.
Salt Lake Meridian. I bisects the area and offers easy accessto many excellent outcrops. Shibboleth Sign In. OpenAthens Sign In. Institutional Sign In. Sign In or Create an Account. User Tools.
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