Sedimentary processes, rocks, and mineral deposits
Sedimentary processes are key geological phenomena that involve the breakdown, transportation, and deposition of materials, culminating in the formation of sedimentary rocks and mineral deposits. These processes begin with weathering, which can be both physical and chemical, resulting in the generation of different sediment types. Sediments are then transported by various agents such as water, wind, and glaciers, with their characteristics influenced by the energy of the transporting medium. Over time, sediments settle in specific environments—marine, transitional, or terrestrial—where they can become lithified into sedimentary rocks through diagenesis, including compaction and cementation.
Sedimentary rocks are classified into clastic and nonclastic types, with clastic rocks arising from the accumulation of weathered fragments and nonclastic rocks forming from chemical precipitation or biological processes. These rocks play significant economic roles; for instance, sedimentary environments are key sources of metallic and nonmetallic minerals, evaporites, and fossil fuels. The study of sedimentary processes, rocks, and mineral deposits reveals the intricate relationships between geological formations and the resources that are vital for various human needs, highlighting both their geological significance and economic importance.
Sedimentary processes, rocks, and mineral deposits
Sedimentary processes occur only at the Earth’s surface, because they are driven by various components of the hydrologic and biologic cycles. Sedimentary processes involve the breakdown, movement, and ultimate deposition of broken rock fragments and chemicals in solution. These processes create many of the important resources used by humans.
Background
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.
Transportation
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 close to its source area.
Transport of material also tends to make grains more rounded in shape. Rounding occurs as particles bump into one another or into objects in the environment, causing abrasion of their edges. The longer and farther a grain is moved, the smaller and rounder it becomes. Transportation tends to winnow material either by sorting or by continued weathering of the particles. Some minerals are more resistant to breakdown than other minerals. Quartz is more resistant, while minerals such as feldspar, mica, pyroxene, and amphibole are less resistant. The longer and farther the sediments are transported, the more likely the weaker minerals are to disappear, which would increase the percentage of quartz present. Therefore, rocks composed of nearly all quartz can be interpreted as having traveled a great distance from their source of weathering. These rocks also exhibit good sorting and are well rounded.
Most sediments are carried by running water. Running water begins as rainfall, flows into increasingly larger streams, and eventually runs into the oceans. Running water is present nearly everywhere on the Earth, including in arid regions (where, although unusual, it is particularly effective as an agent of erosion because of the lack of vegetation for ground cover and because much of the rain in arid lands occurs in the form of downpours).
Waves and currents along shorelines also move large amounts of sediment, as can some currents out in deeper waters. Energy is usually greater nearer a shoreline, so, if grains accumulate, beaches form. Depending on the energy level, the beaches can be composed of sand, pebbles, cobbles, or even boulders. The energy usually decreases offshore, so finer-grained silt and clay (mud) often accumulate in the deeper, quieter water.
Groundwater can carry dissolved minerals. Sometimes when the rock is dissolved, minerals of economic importance, such as copper, aluminum, or iron, can be left behind. In some limestone areas, groundwater can dissolve enough limestone to form caves. If conditions are right in a cave, the groundwater can redeposit some minerals in the forms of dripstone and flowstone, features admired by tourists.
Wind can move sediment grains as well, but neither in the quantity nor in sizes as large as does water. Fine sand is usually the upper size limit that wind can transport. Grains carried by wind do not have the cushioning effect that water provides, so grains are rapidly abraded as they hit each other during transport. Quartz sand grains become frosted in this manner.
Sediments are also transported by glaciers. Movement of grains in glacial ice will break the grains, making them smaller, but they remain fairly angular because they do not tumble around next to each other as they would in water or wind and, thus, do not undergo much rounding. Because ice will not let particles readily settle, as water or wind will, the particles do not become sorted at all. Glacial deposits are probably the most poorly sorted of all sediments. However, some sediments do not end their journeys under a glacier; they may move away from the ice in glacial meltwater. When this happens, the sediments are transported by running water and take on the characteristics of any sediment carried in running water.
Deposition (Sedimentation)
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.
Diagenesis
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.
Sedimentary Rocks and Minerals
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 contains fairly large, angular granules of pink feldspar. Sedimentary rocks composed of silt are called siltstone, and shale is composed of grains of clay that are so small they cannot be seen, even with a microscope. (Because the grains are so small, shale can easily be misidentified as a nonclastic rock.)
The nonclastic rocks are dominated by limestone and dolostone (or dolomite). Limestone is calcium carbonate that formed in warm, shallow seas. Most limestone originated as accumulations of shells either in reefs or in beds on the seafloor. Some limestone can also form by chemical precipitation. Dolostone, a close relative of limestone, is thought to form diagenetically from limestone deposits when magnesium ions in the environment replace some of the calcium in the limestone. Microscopic shell accumulations from calcareous algae have created thick layers of chalk, while similar accumulations from siliceous plankton create diatomaceous earth and chert (flint). Plants that die in swamps and do not completely rot away can evolve into peat and eventually into coal, and microscopic algae and bacteria are the source of petroleum deposits. Although petroleum is not a rock, it is a resource that originates in a sedimentary setting.
Some resource minerals form under evaporative conditions. Ions in solution travel with running water and eventually end up in the oceans or in lakes. In shallow embayments where seawater can flush into the bay and not be diluted with fresh stream water, salts can accumulate if the water evaporates. Likewise, in arid regions where streams enter landlocked lakes (playa lakes), evaporative conditions cause salts to accumulate. There are a great variety of salts (gypsum, halite, magnesium sulfate, and potassium salts, to name a few), and each one forms in turn as the chemical concentrations change as more and more of the water evaporates.
Economic Importance of Selected Sedimentary Rocks and Minerals
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 by quarrying; the rock can be either cut into large blocks or blasted into fragments. The blocks, used for building stone, are taken to mills and cut to order for particular buildings; decorative carvings can also be made. Limestone that has been blasted is usually ground into lime for either agricultural purposes or the manufacture of cement. Larger blocks may be used as riprap along shorelines or rivers to protect those areas from excessive erosion.
Pure quartz sandstone, which usually originates from beach deposits, is quarried for use in glassmaking and fiber-optic cables. Coarser sand and gravel, which originates from glacial deposits or channel deposits in rapidly moving rivers, is quarried for construction purposes. Clays of high purity, often formed in coal swamps or in areas of prolonged weathering, are used for both craft and industrial ceramics. Phosphatic rocks, usually marine shales and limestones that have been chemically enriched in phosphate in deep marine environments, are an ingredient in agricultural fertilizers.
Evaporite deposits have created vast and varied salt resources. Halite is used both as table salt and as an ice melter for road clearance. Gypsum is used in the making of plaster as well as the writing utensils used on modern “chalk” boards, contrary to popular belief that these writing implements are really chalk. Potassium salts can be used as table salt and in fertilizer. Epsom salts (magnesium sulfate) have health benefits, and borates (such as borax and boron) have uses that range from manufacturing of enamel to additives to soap and gasoline.
The fossil fuels, the major energy resources in use in the twentieth century and early twenty-first century, all have biogenic origins in depositional environments. Coal forms from vegetation that grew in ancient swamps. Coal is obtained from both strip mining and underground mining. Petroleum products are buried microscopic planktonic life-forms that lived in seas of the past. Although these organics collect on small scales, the sedimentary rocks with which they are associated permit the migration and eventual accumulation of great enough volumes of oil and gas that they can be extracted for human use.
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