Malcolm Rivera
Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organisms that accumulate on the Earth's surface. They are classified into three groups: Clastic, Biologic, and Chemical. Clastic sedimentary rocks are made up of pieces of pre-existing rocks loosened by weathering and transported by wind, water, ice, or mass movement. Biologic sedimentary rocks form when large numbers of living things die and are compressed, such as coal and limestone. Chemical sedimentary rocks, such as halite (rock salt) and gypsum, form when mineral constituents in solution become supersaturated and inorganically precipitate.
| Characteristics | Values |
|---|---|
| Formation | Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organisms that accumulate on the Earth's surface. |
| Common Types | Siltstone, sandstone, conglomerate, limestone, and shale. |
| Composition | Sedimentary rocks are made up of particles of minerals or organic matter. |
| Classification | Clastic, Biologic, and Chemical. |
| Cementation | The process by which sediments become lithified or consolidated into hard, compact rocks, through the deposition or precipitation of minerals. |
| Compaction | The process of consolidating fine-grained sediments into rock. |
| Lithification | The conversion of loose sediment into solid sedimentary rock. |
| Biologic Sedimentary Rocks | Coal, limestone, and coquina (rocks made of marine organisms or their remains). |
| Chemical Sedimentary Rocks | Halite (rock salt), sylvite, baryte, gypsum, and oolitic limestone. |
| Organic-Rich Sedimentary Rocks | Coal, oil shale, and source rocks for oil and natural gas. |
| Siliceous Sedimentary Rocks | Chert, opal, chalcedony, and other microcrystalline forms. |
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What You'll Learn
- Clastic sedimentary rocks are made of pieces of pre-existing rocks
- Biologic sedimentary rocks are formed from once-living organisms
- Iron-rich sedimentary rocks are composed of >15% iron
- Phosphatic sedimentary rocks contain over 6.5% phosphorus
- Organic-rich sedimentary rocks contain >3% total organic carbon
Clastic sedimentary rocks are made of pieces of pre-existing rocks
Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organisms that accumulate on the Earth's surface. They often exhibit distinct layering or bedding and create picturesque views of the desert southwest. Sedimentary rocks are classified into three main groups: clastic, biologic, and chemical.
Clastic sedimentary rocks are a type of sedimentary rock primarily composed of broken pieces or clasts of older, weathered, and eroded rocks. These rocks are formed through the process of lithification, which involves physical, chemical, and mineralogical changes. Clastic sedimentary rocks can be further classified based on grain size, clast composition, and texture. The grain size can vary from microscopic clay to large boulders, with corresponding names for each size range.
The process of lithification in clastic sedimentary rocks begins with the compaction of sediments. As new sediments are deposited over time, the weight of the overlying layers increases temperature and pressure, squeezing out water and reducing porosity. This is followed by the diagenesis stage, which includes cementation, where coarse clastic sediments become lithified into hard, compact rocks through the deposition of minerals in the remaining pore spaces.
Clastic sedimentary rocks can be found in various locations, including Arches National Park and Grand Canyon National Park in the United States. Examples of clastic sedimentary rocks include sedimentary breccias, which are composed of angular to subangular clasts of other sedimentary rocks, and siliciclastic rocks, which are non-carbonate rocks composed mainly of silicon in the form of quartz or silicates.
Overall, clastic sedimentary rocks play a significant role in shaping the Earth's geology and contribute to the diverse landscapes we observe today.
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Biologic sedimentary rocks are formed from once-living organisms
Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organisms that accumulate on the Earth's surface. They often exhibit distinct layering or bedding, as seen in the mesas and arches of the desert southwest. Common sedimentary rocks include siltstone, sandstone, conglomerate, limestone, and shale. These rocks typically originate from sediments transported by rivers and deposited in lakes and oceans.
Sedimentary rocks are classified into three main groups: clastic, biologic, and chemical. Biologic sedimentary rocks, the focus of this discussion, are formed from the remains of once-living organisms. This process begins with the death of a large number of living organisms, which then pile up and undergo compression and cementation to form rock. The composition of biologic sedimentary rocks is primarily determined by the organic material they contain, rather than the size of the sediment.
Organic-rich sedimentary rocks, such as coal, oil shale, and source rocks for oil and natural gas, contain significant amounts of organic material, typically exceeding 3% total organic carbon. Coal, for example, is formed from vegetation that has removed carbon from the atmosphere, combining it with other elements to build its tissue. Over time, this vegetation becomes compressed by overlying sediments and undergoes chemical transformations.
Another example of a biologic sedimentary rock is limestone, which is predominantly composed of calcium carbonate. This mineral is abundant in ocean water and is absorbed by organisms such as corals, mollusks, and foraminifera to form their calcareous skeletons. When these organisms die and their remains accumulate, they contribute to the formation of limestone through the process of cementation.
Chert is another type of biologic sedimentary rock that forms from the accumulation of siliceous skeletons of microscopic organisms like radiolaria and diatoms. Additionally, coquina, found in national parks like Biscayne Bay in Florida and Guadalupe Mountains in Texas, is a biologic sedimentary rock composed primarily of marine fragments, including molluscs, brachiopods, foraminifera, and invertebrates.
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Iron-rich sedimentary rocks are composed of >15% iron
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They are formed from deposits that accumulate on the Earth's surface. The nature of a sedimentary rock depends on the sediment supply and the sedimentary depositional environment in which it is formed. Sedimentation is any process that causes particles made of minerals or organic matter to settle in place.
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Phosphatic sedimentary rocks contain over 6.5% phosphorus
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They are formed from deposits that accumulate on the Earth's surface and often have distinctive layering or bedding. Common sedimentary rocks include siltstone, sandstone, conglomerate, limestone, and shale.
Sedimentary rocks are formed when sediments are deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. This sediment is often formed when weathering and erosion break down rocks into loose material in a source area. The type of sediment transported depends on the geology of the hinterland (the source area of the sediment). However, some sedimentary rocks, such as evaporites, are composed of material that forms at the place of deposition.
Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include most types of limestone, which are formed from the calcareous skeletons of organisms such as corals, mollusks, and foraminifera. Coal is another example of a biochemical sedimentary rock, formed from vegetation that has removed carbon from the atmosphere and combined it with other elements to build tissue.
Organic-rich sedimentary rocks have significant amounts of organic material, generally in excess of 3% total organic carbon. Common examples include coal, oil shale, and source rocks for oil and natural gas. Siliceous sedimentary rocks are almost entirely composed of silica (SiO2), typically as chert, opal, chalcedony, or other microcrystalline forms. Iron-rich sedimentary rocks are composed of >15% iron; the most common forms are banded iron formations and ironstones.
Phosphatic sedimentary rocks are a type of organic-rich sedimentary rock composed of phosphate minerals and containing more than 6.5% phosphorus. Examples include deposits of phosphate nodules, bone beds, and phosphatic mudrocks. Phosphatic sedimentary rocks are a vital resource for the world's food supply and security, as they are the primary raw material for phosphoric acid and fertilizers used in agriculture. They are also used in food preservatives, baking flour, pharmaceuticals, anticorrosion agents, cosmetics, fungicides, insecticides, detergents, ceramics, water treatment, and metallurgy.
Phosphorite, phosphate rock, or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite can vary greatly, from 4% to 20% phosphorus pentoxide (P2O5). Marketed phosphate rock is often enriched ("beneficiated") to at least 28%, and sometimes more than 30% P2O5. This occurs through processes such as washing, screening, de-liming, magnetic separation, or flotation. By comparison, the average phosphorus content of sedimentary rocks is less than 0.2%.
Phosphate rocks form either as sedimentary deposits or igneous ores associated with alkaline rocks. Morocco holds the world's largest deposits of sedimentary phosphate rocks, and China, the United States, and Morocco are the world's largest miners of phosphate rock.
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Organic-rich sedimentary rocks contain >3% total organic carbon
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They are formed from deposits that accumulate on the Earth's surface. Common sedimentary rocks include siltstone, sandstone, conglomerate, limestone, and shale.
Organic-rich sedimentary rocks are a specific type of sedimentary rock that contains significant amounts of organic carbon, generally more than 3% total organic carbon. The organic material may be disseminated throughout the rock, giving it a uniform dark colour, and/or it may be present as discrete occurrences of tar, bitumen, asphalt, petroleum, coal or carbonaceous material. The most common types include coal, lignite, oil shale, or black shale.
Organic carbon is derived from the ancient biological deposition of matter, and this organic matter is buried with mineral and rock fragments into sedimentary rocks. The temperature and pressure of the burial conditions will affect the material's diagenetic processes, and determine whether or not the material will be transformed into petroleum.
Coal, for example, is formed from vegetation that has removed carbon from the atmosphere and combined it with other elements to build their tissue. This vegetation gets compressed by overlying sediments and undergoes a chemical transformation. With increased burial and compaction, the percentage of carbon increases, and the peat can progressively transition to lignite, bituminous coal, and eventually anthracite coal.
Organic-rich sedimentary rocks may act as source rocks that generate hydrocarbons that accumulate in other sedimentary "reservoir" rocks. Potential source rocks are any type of sedimentary rock with the ability to dispel available carbon from within it (limestone is a classic example). Reservoir rocks are any sedimentary rock that has high pore-space availability, allowing the hydrocarbons to accumulate within the rock and be stored for long periods.
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