Sedimentary processes, rocks, and mineral deposits
Background (Encyclopedia of Global Resources)
Most sediments originate from the weathering of existing rocks. weathering is the breakdown of earth material by physical and/or chemical processes. Physical weathering only breaks the original material into smaller sizes. This is accomplished through mechanical means such as freezing and thawing, plant-root wedging, differential heating of rocks, and crystal growth in rock cracks. Chemical weathering, on the other hand, actually changes the composition of the original material into completely different components through solution, oxidation, hydration, and/or hydrolysis. The by-products of physical and chemical weathering provide the different types of sediment and ion-bearing solutions that create sedimentary rocks and minerals.
Biological activity can also create sediments. Many invertebrates and some algae utilize calcium carbonate in seawater to make shells. Organisms, such as coral, live in warm, shallow seas and construct reefs. In addition, both macro- and microscopic shells of organisms that do not live in reef communities become sediments that blanket the seafloor after the organisms’ deaths. Also, some algae and bacteria, as well as swamp vegetation, can become organic sediments under certain conditions of oxygen deficiency, creating valuable fossil fuels.
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Transportation (Encyclopedia of Global Resources)
Once particles of rock are loosened and broken free by weathering processes, those particles can be carried off by water, wind, or glaciers. Ions released through chemical activity are also free to travel in moving water, but they will never settle out of the water unless a specific chemical reaction occurs that causes the ions to precipitate as solid particles, or until shell-bearing organisms use the ions to build shells that later become sediments.
The amount of energy available in a transporting medium determines which rock particles are picked up (eroded) and moved along. For example, more energy is usually required to move large (or more dense) grains than small (less dense) grains. If a transporting medium loses energy, the sediments being carried will drop out in order of relative size, the largest ones first. Sediment sizes range from boulders to clay. Each has a specific size determination for the name: boulder (more than 256 millimeters), cobble (256 to 64 millimeters), pebble (64 to 4 millimeters), granules (4 to 2 millimeters), sand (2 to 0.062 millimeter), silt (0.062 to 0.0039 millimeter), and clay (less than 0.0039 millimeter). This separation of grains according to size by the transporting medium is called sorting. A well-sorted sediment is one that contains nearly all the same size grains. The farther sediments travel, the better sorted they become. Thus, a poorly sorted sediment probably is still fairly...
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Deposition (Sedimentation) (Encyclopedia of Global Resources)
All particles eventually are carried to specific environments where the transporting energy decreases to the point of no longer being able to carry the material, and the grains are deposited. If not destroyed by further erosion, they may harden into various sedimentary rocks. There are three main depositional systems: marine, transitional, and terrestrial.
Marine environments are important mainly, but by no means exclusively, for deposition of chemical sediments. Marine environments include shallow-marine environments (from the shore to the edge of the continental shelf), reefs, and deep marine environments. Transitional and terrestrial depositional environments are important for deposition of mainly clastic sediments. Transitional environments include beaches, deltas, barrier islands, lagoons, and tidal marshes. Terrestrial environments include rivers, lakes, alluvial fans, glacial environments, and windy areas such as deserts.
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Diagenesis (Encyclopedia of Global Resources)
Diagenesis is any postdepositional alteration of sediments or sedimentary rocks. Diagenetic processes usually take place after sediments are buried by newer sediments. An important diagenetic process is lithification, the process by which loose sediments are hardened into rock. There are two main processes involved in lithifying sediments: cementation and compaction. Cementation occurs when pore spaces between the grains are large enough for mineral-rich water to seep around the grains, depositing either silica or calcite crystals that grow and eventually coalesce and hold the grains together. Many sediments typically lithify by physical compaction. As more and more sediments collect, the weight of overlying sediments compresses the lower sediments, squeezing out much of the water and pressing the grains close enough together that they become a harder mass of material. Lithification processes can occur below water; an ocean basin does not have to dry up before the sediments it contains can lithify into rock.
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Sedimentary Rocks and Minerals (Encyclopedia of Global Resources)
Sedimentary rocks are the most abundant type of rocks found at the surface of the Earth. All sedimentary rocks and minerals can be put into one of two categories: clastic, composed of rock and mineral fragments that were weathered from preexisting rock materials and lithified in new combinations to create a sedimentary rock, and nonclastic, made of precipitated chemicals or of organically derived material such as shells or plants and animals. Identification of sedimentary rocks begins with the determination of whether a rock is clastic or nonclastic. Once such textural characteristics have been determined, the grain sizes and composition of the rock are used to complete the identification process.
Clastic rocks are fairly easy to identify because individual particles in the rock can usually be seen. The rock is identified mainly on the sizes and shapes of the grains it contains. Rocks made of large particles such as boulders, cobbles, and/or pebbles in a matrix of sand are called conglomerate if the grains are mostly rounded or breccia if the grains are angular. The sandstone family contains many varieties of rocks depending on the composition of grains present. Sandstone feels like sandpaper, and the purer variety can be white, tan, or pink and is composed almost entirely of quartz grains. More commonly sandstone is gray, indicating that it contains particles other than quartz. Arkose is a sandstone that...
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Economic Importance of Selected Sedimentary Rocks and Minerals (Encyclopedia of Global Resources)
Sedimentary resources are classified into four main groups: sedimentary metallic ore deposits, sedimentary nonmetallic deposits, evaporites, and energy resources. The sedimentary ore deposits contain some of the world’s most valuable mineral resources. Many of these deposits were formed in depositional environments where large amounts of dissolved metals collected. For example, theiron ores of the famous Mesabi Range in Minnesota originated when the Earth’s early atmosphere was poor in oxygen. This permitted an abundance of iron in its soluble (ferrous) form to be leached from large areas of the Earth’s surface and transported in solution to vast, shallow marine environments, where it oxidized to its insoluble (ferric) form and precipitated in thin layers.
Although gold originates from igneous and hydrothermal processes, once it weathers out of its original rock setting, it becomes influenced by the sedimentary processes of transportation by running water and ultimate deposition in streambeds. This is called a placer deposit. Placer deposits are not limited to gold. Diamonds, tin, chromite, platinum, and magnetite can undergo similar histories. Placers can sometimes be traced upstream to find the source rock, which can then be mined.
The nonmetallic deposits are also of great economic importance. Limestone is quite extensive and has a variety of uses. It is obtained...
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Further Reading (Encyclopedia of Global Resources)
Boggs, Sam, Jr. Petrology of Sedimentary Rocks. 2d ed. New York: Cambridge University Press, 2009.
_______. Principles of Sedimentology and Stratigraphy. 4th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006.
Chernicoff, Stanley, and Donna Whitney. Geology: An Introduction to Physical Geology. 4th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.
Davis, Richard A. Depositional Systems: An Introduction to Sedimentology and Stratigraphy. 2d ed. Englewood Cliffs, N.J.: Prentice Hall, 1992.
Grotzinger, John P., et al. Understanding Earth. 5th ed. New York: W. H. Freeman, 2007.
Nichols, Gary. Sedimentology and Stratigraphy. 2d ed. Hoboken, N.J.: Wiley-Blackwell, 2009.
Pettijohn, F. J. Sedimentary Rocks. 3d ed. New York: Harper & Row, 1975.
Tennissen, Anthony C. Nature of Earth Materials. 2d ed. Englewood Cliffs, N.J.: Prentice-Hall, 1983.
Tucker, Maurice E. Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks. 3d ed. Malden, Mass.: Blackwell Science, 2001.
U.S. Geological Survey. Sedimentary Rocks. http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/sedimentary_rocks.html
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