Sedimentary Rocks and Sediment
by Owen Borville
January 30, 2021
Learning, Geology, Science
Sediment is loose pieces of rock or minerals which can be transported by erosion from wind, water, or glacial ice and deposited. Sedimentary rock is composed of sediment which has been compacted and cemented, or rocks containing minerals which have precipitated chemically from solution, or rocks originating from biological organisms and materials.
Sedimentary Processes
Weathering includes two surface or near-surface processes that work in concert to decompose rocks and both processes occur in place (no movement is involved in weathering). Chemical weathering involves a chemical change in at least some of the minerals within a rock. Mechanical weathering involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within the rock. Mechanical weathering includes processes such as water in cracks freezing and expanding, or changes in temperature that expand and shrink individual minerals enough to break them apart.
Erosion occurs when a particle is loosened chemically or mechanically, and begins to move. Differential weathering and erosion occurs when there are large contrasts in the susceptibility to weathering and erosion within a large body of rock and the more susceptible parts of the rock weather and erode faster than the more resistant portions of the rock. This can be seen in the American western states such as Utah and Arizona where differential erosion has formed unusual features.
Soil is loose, unconsolidated earth and organic materials above bedrock that support plant growth. Soil formation is caused by mechanical and chemical weathering. Soil formation is also influenced by several factors including climate, parent material, the activity of biological organisms, the shape of the landscape including elevation and slope. Different types of soil are formed in different climate regions. Various rock types will affect the type of soil formed. The existence of biological organisms helps develop the soil by decomposition and the production of organic material. Soil is less likely to develop on steep angle slopes.
Soil horizons (or soil profile) are the distinct layers of soil including the top organic layer or the O horizon. Below the O horizon is the A horizon, which is the loamy top soil where most plant humus is derived. Below the A horizon is the B horizon, which is the section of the soil commonly light colored, denser, and less organic than the A horizon above. The B horizon is where leached materials accumulate from the A horizon. During leaching, water dissolves minerals from the A horizon and carries them to the B horizon. Below the B horizon, the C horizon begins with some partially altered rock and mineral material but is unaltered further below. Loam or loamy soil contains a relatively equal percentage of sand, silt, and clay particles.
Mechanical weathering involves the physical breakdown of rocks into fragments without changing the chemical composition of the minerals within it and is caused most commonly by temperature changes and pressure release of overlying rock.
Frost action or freeze-thaw cycle is a mechanical weathering process which occurs in colder temperate regions where water trapped in fractures and between grains of rock repeatedly freezes and thaws during the winter months. The water freezing causes expansion, which eventually breaks rock apart and can produce piles of rock debris known as talus. In some areas this occurs on a daily basis as water freezes at night, and melts in warmer daytime temperatures. Only in the coldest regions does water remain frozen throughout the winter. Frost heaving occurs as a result of the freeze thaw cycle where soil expands and contracts, causing soil surfaces to lift or heave and can damage vegetation. Frost wedging is a process that mechanically breaks apart rock and is caused by expansion of water as it freezes in cracks and crevices.
Rock can also expand from heat. In areas with large daily temperature variation, the rapid increase in temperature causes the rock to expand and adds pressure to the rock. This eventually can cause fracture in the rock. Rock can also be fractured by crystal growth and the biological activity of plants and animals inside rock and soil. Salt weathering is a type of mechanical weathering of rock in which salt crystals derived from evaporated salt solutions from an outside source grow or expand inside rock pores and exert pressure on the rock, causing breakdown.
Pressure release is a mechanical weathering process that occurs as sedimentary layers at the surface are eroded and the underlying rock (usually igneous) is under reduced stress and expands upward, causing the rock to crack into sheets separated by joints parallel to the overlying surface. As this rock is exposed, more erosion and weathering occurs. Pressure release can produce features known as exfoliation domes.
Weathering also occurs when water penetrates fractures in rock and initiates physical or chemical weathering.
Chemical weathering is the process that changes the chemical makeup of a rock or mineral at or near the Earth's surface. Chemical weathering alters the internal structure of minerals by removing or adding elements. Oxygen, water, and acids are the most important initiators of chemical weathering. Biological organisms are also important in chemical weathering.
Common chemical weathering processes include solution, oxidation, and hydrolysis. Solution occurs when rocks or minerals are exposed to water and form a liquid with dissolved minerals. The chemistry of water molecules causes some minerals to break down into liquid form and some minerals are more affected by solution than others. In addition to water, the presence of acid can greatly enhance the solution process or cause solution with a mineral that would normally not react with pure water.
Oxidation occurs when metals in minerals (commonly iron) are exposed to oxygen and form iron oxides such as the mineral hematite, which has a red to brown color. If water is added to the reaction, a hydroxide forms with a yellow to brown color.
Hydrolysis is a chemical reaction during which hydrogen and hydroxyl (OH-) ions from water react with the ions in a mineral. Hydrolysis is responsible for the chemical alteration of feldspar minerals into clay minerals.
A gossan is a yellow or red colored mix of several forms of hydrated iron oxides that form by the oxidation and leaching of metallic sulfide minerals (usually pyrite, FeS2) and represents the upper section of an ore deposit or mineral vein. These reactions produce sulfuric acid which dissolves other metallic minerals and carries them deeper into the rock. Oxidized ore minerals are produced which contain silver, copper, lead, zinc, iron, and nickel. These minerals are produced just above the water table.
Supergene enrichment is a process which occurs below the water table and gossan when mineral saturated water from above reacts chemically with metallic ore minerals. Supergene enrichment occurs as iron sulfide minerals are commonly replaced with higher grade or more enriched metallic sulfide minerals containing copper, zinc, and lead.
Sediment is loose pieces of rock or minerals which can be transported by erosion (wind, water, or glacial ice). Sedimentary rock is composed of sediment which has been compacted and cemented, or rocks containing minerals which have precipitated chemically from solution, or rocks originating from biological organisms and materials. Sedimentary rocks are formed as sediment is transported and deposited by a variety of processes, including erosion, mechanical weathering, and chemical weathering. After deposition, sediment is lithified into rock.
Sedimentary Particle Size:
Gravel: Greater than 2 millimeters
Sand: 1/16 millimeters to 2 millimeters
Silt: 1/256 millimeters to 1/16 millimeters
Clay: Less than 1/256 millimeters
Sediment is classified as detrital sediment or chemical sediment. Detrital sediment is formed from detritus, or weathered and eroded rock. Detrital sediment is also termed clastic. Chemical sediment is formed by chemical reactions from organic or inorganic processes.
Sediment transport can be initiated by wind, water, or ice (glaciers). Abrasion is the wearing away of the Earth’s surface by agents of erosion, including wind, water, or ice. Abrasion causes rounding, which describes the shape of sedimentary particles having round shaped corners instead of angular shaped corners. Rounding occurs as sedimentary particles are transported by water or wind, causing sharp edges to become smooth and round.
Sorting refers to similar sized sedimentary particles being deposited together and is controlled by the size and weight of particles, with the heaviest sediment being deposited first, and finer sediment being transported considerable distances. Sorting is also controlled by the distance of transport as sedimentary particles transported long distances are likely to be well sorted while particles not transported very far are likely to be poorly sorted. Angularity refers to the degree in which rocks or particles have sharp edges. Angularity is caused by the lack of erosion and transport. Subangular describes rock grains which are partially angular but do not have sharp edges.
Allochthonous refers to rocks or sediments that have been transported from their place of origin by erosion and deposited in another location. Allogenic is another term which refers to material which has been transported to its present location. Autochthonous describes a rock or sediment formed at the location in which it is currently found.
Diagenesis is a group of processes that cause physical and chemical changes in sediment after it has been deposited and buried under another layer of sediment. Diagenesis may result in the lithification of sediment, or the conversion of sediment into solid rock. Lithification is the conversion of loose sediment into solid sedimentary rock and includes several processes, including compaction of grains and the filling of spaces between grains with mineral cement. Compaction occurs when the weight of overlying material compresses more deeply buried sediment. Along with cementation, this process converts sediments to solid rock.
Cement is a mineral or minerals that precipitate between grains of sediment and cement or bind the sediment into rock. Cementation is one of the processes that works to turn sediment into sedimentary rock (lithification). Mineral saturated water percolates through sediment with open pore spaces and the spaces are gradually filled by minerals precipitating from the water, which binds the grains together. Quartz, calcite, iron oxides, and clay minerals are the most common cements.
Sedimentary Rock Classification
Conglomerate is a sedimentary rock made of rounded rock fragments, such as pebbles, cobbles, and boulders inside a finer-grained matrix. To be a conglomerate, some of the constituent pebbles must be at least two millimeters (about 1/13th of an inch) across. Conglomerate can be found in stream environments and beaches.
Breccia is a coarse-grained sedimentary rock made up of broken rock fragments with relatively sharp corners. These large, angular fragments are cemented by a fine-grained matrix or mineral cement. Breccia can be found in stream environments and beaches.
Sandstone is sedimentary rock composed of sand-sized grains of quartz, feldspar, rock fragments, and other minerals depending on the environment of formation. Sandstone can form in stream channels, beaches, barrier islands, deltas, and the continental shelf. Arkose is a feldspar-rich sandstone (up to one third feldspar) and has a high quartz content. Arenite is a sandstone that is well-sorted and is a pure or nearly pure chemically cemented sandstone containing less than 10 percent argillaceous matrix.
Wacke is impure sandstone that consists of a mixed variety of unsorted or poorly sorted mineral and rock fragments, an abundant matrix of clay, and fine silt. Wackes contain more than 10 percent argillaceous matrix. Graywacke is a sandstone that is made up mostly of sand-size grains that were rapidly deposited very near the source rock from which they were weathered. Graywacke is deposited in deep ocean water near volcanic mountain ranges, where underwater landslides and density currents called turbidites quickly transport sediment short distances into a subduction zone or ocean trench. Graywacke sandstone contains fewer grains made of quartz and more grains made of feldspar, volcanic rock fragments, silt, and clay in comparison to most sandstone. The volcanic rock fragments give graywacke a green to gray color.
Immature sandstone is characterized by grains of different sizes, angular grains, and more clay content than mature sandstones. Immature sandstones are more likely to be located near the site of origin. Mature sandstone is characterized by abundant quartz grains, similar sized grains, rounded grains, and little clay content. Mature sandstones are likely to be composed of grains that were transported great distances.
Orthoquartzite is a quartzite of sedimentary origin, or a pure quartz sandstone, or a clastic sedimentary rock containing mostly quartz sand and has very little or no fine grained material.
Mudrock or mudstone is a very fine-grained sedimentary rock and can be composed of silt or clay sized particles. Pelite is a term for fine-grained sedimentary rock consisting mostly of clay or silt. Mudstone, shale, siltstone, and claystone are all classified as pelitic. Shale is sedimentary rock derived from mud and is commonly finely laminated (bedded). Shale commonly includes clay minerals mixed with tiny grains of quartz eroded from pre-existing rocks. Shale is fissile, which is the property of breaking along parallel layers. Marl (marlstone) is a mud or mudstone rich in calcium carbonate and also contains other carbonate minerals and clays. Marl is found in marine and lake sediments. Argillite is an unusually hard, fine-grained sedimentary rock, such as shale, mudstone, siltstone, and claystone. Argillite is commonly black.
Chemical and biochemical sedimentary rocks result from inorganic chemical processes or from the chemical processes of biological organisms. Limestone is a sedimentary rock made mostly of the mineral calcite (calcium carbonate). Limestone is usually formed in a marine environment from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation such as hot springs. Chalk and tufa are other forms of calcium carbonate rock. Coquina is a limestone consisting almost completely of mechanically transported fossil debris with little or no matrix, loosely cemented together so that the rock looks very porous. Such rocks form in zones of high energy and typically the shells are clams and snails. Micrite is sediment composed of clay-size crystals of calcium carbonate (CaCO3) produced by organic or inorganic precipitation.
Dolostone (dolomite rock) is a sedimentary carbonate rock with a high percentage of the mineral dolomite. Dolomite is a carbonate mineral similar to limestone except that dolomite includes magnesium in addition to calcium.
Evaporites are sedimentary rocks containing minerals that precipitate as lake or ocean water evaporates, leaving the mineral behind. Halite (NaCl) and gypsum (CaSO4*2H2O) are the most common evaporite minerals. Sylvite (KCl) is a potassium chloride mineral forming isometric crystals and can be white, yellow, or red. Sylvite is found in evaporite deposits around the world.
Chert is a very fine-grained sedimentary rock made of microscopic quartz grains and commonly consists of millions of globular siliceous skeletons of marine plankton called radiolarians. Black colored chert is called flint. Chert can occur in beds or nodules.
Nodules are replacement bodies of solid rock and may also form by the selective precipitation of dissolved minerals that completely replace the original sediment with a mineral of different composition. Nodules contain a different composition than the surrounding sediment. A concretion is a spherical rock that forms around a nucleus rock or material of the same composition. Concretions form by the selective precipitation of dissolved minerals from groundwater and as these minerals precipitate, they fill in pore spaces between grains of sediment thereby cementing them together.
Sedimentary Environments
Facies are defined as all of the geologic characteristics of a particular rock unit. As sedimentary processes vary in different locations, different sedimentary facies can be deposited near or adjacent to each other. Sedimentary facies can be used to identify differences in adjacent rock units.
The rise and fall of sea level can change the facies in a nearshore environment. When sea level rises, land is submerged underwater and this land underneath the newly submerged water is changed by the processes associated with a marine environment. A rising sea level would produce a sedimentary sequence of limestone, mudstone, and sandstone. When sea level falls, land that was previously submerged is now exposed. A falling sea level would produce a sedimentary sequence of sandstone, mudstone, and limestone.
Sedimentary Structures are formed during the deposition of sediment and these structures record the deposition of sediment in rock. Sedimentary structures are useful because they allow for the reconstruction of the environments of deposition of the sediments. Examples include bedding, cross-beds, ripple marks, and mudcracks. Sole marks (tool marks) are sedimentary structures formed by marine currents depositing coarser sediment on top of fine grained sediment. The marks are formed by the coarser grains in the current eroding the finer sediment during the deposition process.
Bedding (or strata) describes parallel layers of deposited sediment which have lithified into rock. Bedding can change in composition from one layer to the next and can also vary in thickness. Bedding planes are the boundaries that separate the beds or strata of sedimentary deposits. Bedrock is the relatively hard, solid rock that commonly underlies softer rock, sediment, or soil.
Graded bedding occurs when sediment is deposited with larger grains at the bottom and progressively smaller grains toward the top of the bedding or layers. Turbidity currents (or density currents) are sediment saturated currents of water in seas or lakes which flow downslope (as a result of higher density and gravity) and deposit at the end of the slope in the form of graded bedding. Turbidity currents can also be generated by storms. Graded bedding can also occur in stream channels.
Cross bedding occurs when bedding or strata is deposited at an angle to the surface of deposition. Cross bedding can occur in deserts, streams, and shallow marine environments. Cross bedding dips in the direction of flow, so older cross beds show the direction of flow from the past.
Ripple marks are sedimentary structures resembling small ridges that often develop in soft beds of sand lying in shallow water. The back and forth motions of waves create symmetrical ripples. Ripple marks are asymmetric when sculpted by the one-way motion of currents, with steeper sides facing in the down-current direction.
Mudcracks are cracks that occur in fine grained sediment during dry conditions and are preserved when sediment fills the cracks. Mudcracks are a very good indicator of past environments and are commonly found in riverbeds, floodplains, natural levees, deltas, and tidal flats.
Fossils are the recognizable remains of past life on Earth. Fossils include bones, shells, leaves, tracks, burrows, or impressions. Fossils only occur in sedimentary rocks because the formation of igneous and metamorphic rocks destroys the fossil structures. Trace fossils or ichnofossils leave a mark or a shape of an organism but do not consist of the body fossil.
Sedimentary rocks have a variety of industrial uses such as in construction materials, ceramics, glass, table salt, and fertilizers.
Hydrocarbons are a very large group of chemical compounds consisting primarily of carbon and hydrogen. The largest source of hydrocarbons is petroleum. Petroleum forms from organic materials inside the Earth over long periods of time and rises upward toward the surface because of its low density. The petroleum rises until it is trapped by an impermeable rock layer, a deformed layer, or a fault. Coral reefs are also sources of petroleum. The permeable rock layer where the petroleum is stored is called a reservoir. Petroleum includes natural gas and oil with oil being denser than natural gas. Natural gas is the least dense form of petroleum and forms at the top of a petroleum reservoir.
Salt domes occur when deeply buried salt rock rises toward the surface because of its lower density and in the process deforms the rock layers above. Salt domes commonly produce petroleum reservoir traps (see Chapter 8 for an illustration). Caprock is a hard erosion resistant rock on top of a softer, less resistant rock. Cap rock also refers to an impermeable rock layer that traps oil, water, or gas.
Oil shale generally refers to any sedimentary rock that contains solid bituminous materials (called kerogen) that are released as petroleum-like liquids when the rock is heated in the chemical process of pyrolysis. Oil shale is formed by the deposition of silt and organic debris on lake beds and sea bottoms. Over time, heat and pressure transform these materials into oil.
Tar sand is a sedimentary material composed primarily of sand, clay, water (in some deposits) and organic constituents known as bitumen. Petroleum from tar sand is very thick with high viscosity and cannot be drilled like other types of petroleum. This petroleum must be mined as in Alberta, Canada, which has one of the largest deposits of tar sands and is currently being mined.
Coal is a readily combustible black or brownish-black rock whose composition, including inherent moisture, consists of more than 50 percent carbonaceous material by weight and more than 70 percent carbonaceous material by volume. Coal is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. Coal is classified into four types: anthracite, bituminous, sub-bituminous, and lignite. Anthracite is the highest grade of coal while lignite is the lowest grade. Currently, the United States contains the largest coal reserves in the world. The majority of this coal is used in power plants to generate electricity.
Uranium is a radioactive element inside the Earth that produces heat and causes convection inside the Earth. Uranium, which is used to produce nuclear energy and electricity, is found in ore minerals such as carnotite and uraninite. Carnotite is a secondary ore mineral of uranium and vanadium with a canary yellow color and can be found in sandstone. Uraninite (UO2) is a black, brown, or steel-gray mineral that is strongly radioactive and is one of the main ores of uranium. Uraninite occurs in veins of lead, tin, and copper minerals. Uraninite also occurs in sandstone deposits, and is a primary constituent of granites and pegmatites.
Banded iron formations (BIF) occur in Proterozoic rocks and are composed of alternating layers of iron-rich material (commonly magnetite) and silica (chert). Each layer is relatively thin, varying in thickness from a millimeter to several centimeters. Banded iron formations occur around the world and are an important source of iron ore.
by Owen Borville
January 30, 2021
Learning, Geology, Science
Sediment is loose pieces of rock or minerals which can be transported by erosion from wind, water, or glacial ice and deposited. Sedimentary rock is composed of sediment which has been compacted and cemented, or rocks containing minerals which have precipitated chemically from solution, or rocks originating from biological organisms and materials.
Sedimentary Processes
Weathering includes two surface or near-surface processes that work in concert to decompose rocks and both processes occur in place (no movement is involved in weathering). Chemical weathering involves a chemical change in at least some of the minerals within a rock. Mechanical weathering involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within the rock. Mechanical weathering includes processes such as water in cracks freezing and expanding, or changes in temperature that expand and shrink individual minerals enough to break them apart.
Erosion occurs when a particle is loosened chemically or mechanically, and begins to move. Differential weathering and erosion occurs when there are large contrasts in the susceptibility to weathering and erosion within a large body of rock and the more susceptible parts of the rock weather and erode faster than the more resistant portions of the rock. This can be seen in the American western states such as Utah and Arizona where differential erosion has formed unusual features.
Soil is loose, unconsolidated earth and organic materials above bedrock that support plant growth. Soil formation is caused by mechanical and chemical weathering. Soil formation is also influenced by several factors including climate, parent material, the activity of biological organisms, the shape of the landscape including elevation and slope. Different types of soil are formed in different climate regions. Various rock types will affect the type of soil formed. The existence of biological organisms helps develop the soil by decomposition and the production of organic material. Soil is less likely to develop on steep angle slopes.
Soil horizons (or soil profile) are the distinct layers of soil including the top organic layer or the O horizon. Below the O horizon is the A horizon, which is the loamy top soil where most plant humus is derived. Below the A horizon is the B horizon, which is the section of the soil commonly light colored, denser, and less organic than the A horizon above. The B horizon is where leached materials accumulate from the A horizon. During leaching, water dissolves minerals from the A horizon and carries them to the B horizon. Below the B horizon, the C horizon begins with some partially altered rock and mineral material but is unaltered further below. Loam or loamy soil contains a relatively equal percentage of sand, silt, and clay particles.
Mechanical weathering involves the physical breakdown of rocks into fragments without changing the chemical composition of the minerals within it and is caused most commonly by temperature changes and pressure release of overlying rock.
Frost action or freeze-thaw cycle is a mechanical weathering process which occurs in colder temperate regions where water trapped in fractures and between grains of rock repeatedly freezes and thaws during the winter months. The water freezing causes expansion, which eventually breaks rock apart and can produce piles of rock debris known as talus. In some areas this occurs on a daily basis as water freezes at night, and melts in warmer daytime temperatures. Only in the coldest regions does water remain frozen throughout the winter. Frost heaving occurs as a result of the freeze thaw cycle where soil expands and contracts, causing soil surfaces to lift or heave and can damage vegetation. Frost wedging is a process that mechanically breaks apart rock and is caused by expansion of water as it freezes in cracks and crevices.
Rock can also expand from heat. In areas with large daily temperature variation, the rapid increase in temperature causes the rock to expand and adds pressure to the rock. This eventually can cause fracture in the rock. Rock can also be fractured by crystal growth and the biological activity of plants and animals inside rock and soil. Salt weathering is a type of mechanical weathering of rock in which salt crystals derived from evaporated salt solutions from an outside source grow or expand inside rock pores and exert pressure on the rock, causing breakdown.
Pressure release is a mechanical weathering process that occurs as sedimentary layers at the surface are eroded and the underlying rock (usually igneous) is under reduced stress and expands upward, causing the rock to crack into sheets separated by joints parallel to the overlying surface. As this rock is exposed, more erosion and weathering occurs. Pressure release can produce features known as exfoliation domes.
Weathering also occurs when water penetrates fractures in rock and initiates physical or chemical weathering.
Chemical weathering is the process that changes the chemical makeup of a rock or mineral at or near the Earth's surface. Chemical weathering alters the internal structure of minerals by removing or adding elements. Oxygen, water, and acids are the most important initiators of chemical weathering. Biological organisms are also important in chemical weathering.
Common chemical weathering processes include solution, oxidation, and hydrolysis. Solution occurs when rocks or minerals are exposed to water and form a liquid with dissolved minerals. The chemistry of water molecules causes some minerals to break down into liquid form and some minerals are more affected by solution than others. In addition to water, the presence of acid can greatly enhance the solution process or cause solution with a mineral that would normally not react with pure water.
Oxidation occurs when metals in minerals (commonly iron) are exposed to oxygen and form iron oxides such as the mineral hematite, which has a red to brown color. If water is added to the reaction, a hydroxide forms with a yellow to brown color.
Hydrolysis is a chemical reaction during which hydrogen and hydroxyl (OH-) ions from water react with the ions in a mineral. Hydrolysis is responsible for the chemical alteration of feldspar minerals into clay minerals.
A gossan is a yellow or red colored mix of several forms of hydrated iron oxides that form by the oxidation and leaching of metallic sulfide minerals (usually pyrite, FeS2) and represents the upper section of an ore deposit or mineral vein. These reactions produce sulfuric acid which dissolves other metallic minerals and carries them deeper into the rock. Oxidized ore minerals are produced which contain silver, copper, lead, zinc, iron, and nickel. These minerals are produced just above the water table.
Supergene enrichment is a process which occurs below the water table and gossan when mineral saturated water from above reacts chemically with metallic ore minerals. Supergene enrichment occurs as iron sulfide minerals are commonly replaced with higher grade or more enriched metallic sulfide minerals containing copper, zinc, and lead.
Sediment is loose pieces of rock or minerals which can be transported by erosion (wind, water, or glacial ice). Sedimentary rock is composed of sediment which has been compacted and cemented, or rocks containing minerals which have precipitated chemically from solution, or rocks originating from biological organisms and materials. Sedimentary rocks are formed as sediment is transported and deposited by a variety of processes, including erosion, mechanical weathering, and chemical weathering. After deposition, sediment is lithified into rock.
Sedimentary Particle Size:
Gravel: Greater than 2 millimeters
Sand: 1/16 millimeters to 2 millimeters
Silt: 1/256 millimeters to 1/16 millimeters
Clay: Less than 1/256 millimeters
Sediment is classified as detrital sediment or chemical sediment. Detrital sediment is formed from detritus, or weathered and eroded rock. Detrital sediment is also termed clastic. Chemical sediment is formed by chemical reactions from organic or inorganic processes.
Sediment transport can be initiated by wind, water, or ice (glaciers). Abrasion is the wearing away of the Earth’s surface by agents of erosion, including wind, water, or ice. Abrasion causes rounding, which describes the shape of sedimentary particles having round shaped corners instead of angular shaped corners. Rounding occurs as sedimentary particles are transported by water or wind, causing sharp edges to become smooth and round.
Sorting refers to similar sized sedimentary particles being deposited together and is controlled by the size and weight of particles, with the heaviest sediment being deposited first, and finer sediment being transported considerable distances. Sorting is also controlled by the distance of transport as sedimentary particles transported long distances are likely to be well sorted while particles not transported very far are likely to be poorly sorted. Angularity refers to the degree in which rocks or particles have sharp edges. Angularity is caused by the lack of erosion and transport. Subangular describes rock grains which are partially angular but do not have sharp edges.
Allochthonous refers to rocks or sediments that have been transported from their place of origin by erosion and deposited in another location. Allogenic is another term which refers to material which has been transported to its present location. Autochthonous describes a rock or sediment formed at the location in which it is currently found.
Diagenesis is a group of processes that cause physical and chemical changes in sediment after it has been deposited and buried under another layer of sediment. Diagenesis may result in the lithification of sediment, or the conversion of sediment into solid rock. Lithification is the conversion of loose sediment into solid sedimentary rock and includes several processes, including compaction of grains and the filling of spaces between grains with mineral cement. Compaction occurs when the weight of overlying material compresses more deeply buried sediment. Along with cementation, this process converts sediments to solid rock.
Cement is a mineral or minerals that precipitate between grains of sediment and cement or bind the sediment into rock. Cementation is one of the processes that works to turn sediment into sedimentary rock (lithification). Mineral saturated water percolates through sediment with open pore spaces and the spaces are gradually filled by minerals precipitating from the water, which binds the grains together. Quartz, calcite, iron oxides, and clay minerals are the most common cements.
Sedimentary Rock Classification
Conglomerate is a sedimentary rock made of rounded rock fragments, such as pebbles, cobbles, and boulders inside a finer-grained matrix. To be a conglomerate, some of the constituent pebbles must be at least two millimeters (about 1/13th of an inch) across. Conglomerate can be found in stream environments and beaches.
Breccia is a coarse-grained sedimentary rock made up of broken rock fragments with relatively sharp corners. These large, angular fragments are cemented by a fine-grained matrix or mineral cement. Breccia can be found in stream environments and beaches.
Sandstone is sedimentary rock composed of sand-sized grains of quartz, feldspar, rock fragments, and other minerals depending on the environment of formation. Sandstone can form in stream channels, beaches, barrier islands, deltas, and the continental shelf. Arkose is a feldspar-rich sandstone (up to one third feldspar) and has a high quartz content. Arenite is a sandstone that is well-sorted and is a pure or nearly pure chemically cemented sandstone containing less than 10 percent argillaceous matrix.
Wacke is impure sandstone that consists of a mixed variety of unsorted or poorly sorted mineral and rock fragments, an abundant matrix of clay, and fine silt. Wackes contain more than 10 percent argillaceous matrix. Graywacke is a sandstone that is made up mostly of sand-size grains that were rapidly deposited very near the source rock from which they were weathered. Graywacke is deposited in deep ocean water near volcanic mountain ranges, where underwater landslides and density currents called turbidites quickly transport sediment short distances into a subduction zone or ocean trench. Graywacke sandstone contains fewer grains made of quartz and more grains made of feldspar, volcanic rock fragments, silt, and clay in comparison to most sandstone. The volcanic rock fragments give graywacke a green to gray color.
Immature sandstone is characterized by grains of different sizes, angular grains, and more clay content than mature sandstones. Immature sandstones are more likely to be located near the site of origin. Mature sandstone is characterized by abundant quartz grains, similar sized grains, rounded grains, and little clay content. Mature sandstones are likely to be composed of grains that were transported great distances.
Orthoquartzite is a quartzite of sedimentary origin, or a pure quartz sandstone, or a clastic sedimentary rock containing mostly quartz sand and has very little or no fine grained material.
Mudrock or mudstone is a very fine-grained sedimentary rock and can be composed of silt or clay sized particles. Pelite is a term for fine-grained sedimentary rock consisting mostly of clay or silt. Mudstone, shale, siltstone, and claystone are all classified as pelitic. Shale is sedimentary rock derived from mud and is commonly finely laminated (bedded). Shale commonly includes clay minerals mixed with tiny grains of quartz eroded from pre-existing rocks. Shale is fissile, which is the property of breaking along parallel layers. Marl (marlstone) is a mud or mudstone rich in calcium carbonate and also contains other carbonate minerals and clays. Marl is found in marine and lake sediments. Argillite is an unusually hard, fine-grained sedimentary rock, such as shale, mudstone, siltstone, and claystone. Argillite is commonly black.
Chemical and biochemical sedimentary rocks result from inorganic chemical processes or from the chemical processes of biological organisms. Limestone is a sedimentary rock made mostly of the mineral calcite (calcium carbonate). Limestone is usually formed in a marine environment from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation such as hot springs. Chalk and tufa are other forms of calcium carbonate rock. Coquina is a limestone consisting almost completely of mechanically transported fossil debris with little or no matrix, loosely cemented together so that the rock looks very porous. Such rocks form in zones of high energy and typically the shells are clams and snails. Micrite is sediment composed of clay-size crystals of calcium carbonate (CaCO3) produced by organic or inorganic precipitation.
Dolostone (dolomite rock) is a sedimentary carbonate rock with a high percentage of the mineral dolomite. Dolomite is a carbonate mineral similar to limestone except that dolomite includes magnesium in addition to calcium.
Evaporites are sedimentary rocks containing minerals that precipitate as lake or ocean water evaporates, leaving the mineral behind. Halite (NaCl) and gypsum (CaSO4*2H2O) are the most common evaporite minerals. Sylvite (KCl) is a potassium chloride mineral forming isometric crystals and can be white, yellow, or red. Sylvite is found in evaporite deposits around the world.
Chert is a very fine-grained sedimentary rock made of microscopic quartz grains and commonly consists of millions of globular siliceous skeletons of marine plankton called radiolarians. Black colored chert is called flint. Chert can occur in beds or nodules.
Nodules are replacement bodies of solid rock and may also form by the selective precipitation of dissolved minerals that completely replace the original sediment with a mineral of different composition. Nodules contain a different composition than the surrounding sediment. A concretion is a spherical rock that forms around a nucleus rock or material of the same composition. Concretions form by the selective precipitation of dissolved minerals from groundwater and as these minerals precipitate, they fill in pore spaces between grains of sediment thereby cementing them together.
Sedimentary Environments
Facies are defined as all of the geologic characteristics of a particular rock unit. As sedimentary processes vary in different locations, different sedimentary facies can be deposited near or adjacent to each other. Sedimentary facies can be used to identify differences in adjacent rock units.
The rise and fall of sea level can change the facies in a nearshore environment. When sea level rises, land is submerged underwater and this land underneath the newly submerged water is changed by the processes associated with a marine environment. A rising sea level would produce a sedimentary sequence of limestone, mudstone, and sandstone. When sea level falls, land that was previously submerged is now exposed. A falling sea level would produce a sedimentary sequence of sandstone, mudstone, and limestone.
Sedimentary Structures are formed during the deposition of sediment and these structures record the deposition of sediment in rock. Sedimentary structures are useful because they allow for the reconstruction of the environments of deposition of the sediments. Examples include bedding, cross-beds, ripple marks, and mudcracks. Sole marks (tool marks) are sedimentary structures formed by marine currents depositing coarser sediment on top of fine grained sediment. The marks are formed by the coarser grains in the current eroding the finer sediment during the deposition process.
Bedding (or strata) describes parallel layers of deposited sediment which have lithified into rock. Bedding can change in composition from one layer to the next and can also vary in thickness. Bedding planes are the boundaries that separate the beds or strata of sedimentary deposits. Bedrock is the relatively hard, solid rock that commonly underlies softer rock, sediment, or soil.
Graded bedding occurs when sediment is deposited with larger grains at the bottom and progressively smaller grains toward the top of the bedding or layers. Turbidity currents (or density currents) are sediment saturated currents of water in seas or lakes which flow downslope (as a result of higher density and gravity) and deposit at the end of the slope in the form of graded bedding. Turbidity currents can also be generated by storms. Graded bedding can also occur in stream channels.
Cross bedding occurs when bedding or strata is deposited at an angle to the surface of deposition. Cross bedding can occur in deserts, streams, and shallow marine environments. Cross bedding dips in the direction of flow, so older cross beds show the direction of flow from the past.
Ripple marks are sedimentary structures resembling small ridges that often develop in soft beds of sand lying in shallow water. The back and forth motions of waves create symmetrical ripples. Ripple marks are asymmetric when sculpted by the one-way motion of currents, with steeper sides facing in the down-current direction.
Mudcracks are cracks that occur in fine grained sediment during dry conditions and are preserved when sediment fills the cracks. Mudcracks are a very good indicator of past environments and are commonly found in riverbeds, floodplains, natural levees, deltas, and tidal flats.
Fossils are the recognizable remains of past life on Earth. Fossils include bones, shells, leaves, tracks, burrows, or impressions. Fossils only occur in sedimentary rocks because the formation of igneous and metamorphic rocks destroys the fossil structures. Trace fossils or ichnofossils leave a mark or a shape of an organism but do not consist of the body fossil.
Sedimentary rocks have a variety of industrial uses such as in construction materials, ceramics, glass, table salt, and fertilizers.
Hydrocarbons are a very large group of chemical compounds consisting primarily of carbon and hydrogen. The largest source of hydrocarbons is petroleum. Petroleum forms from organic materials inside the Earth over long periods of time and rises upward toward the surface because of its low density. The petroleum rises until it is trapped by an impermeable rock layer, a deformed layer, or a fault. Coral reefs are also sources of petroleum. The permeable rock layer where the petroleum is stored is called a reservoir. Petroleum includes natural gas and oil with oil being denser than natural gas. Natural gas is the least dense form of petroleum and forms at the top of a petroleum reservoir.
Salt domes occur when deeply buried salt rock rises toward the surface because of its lower density and in the process deforms the rock layers above. Salt domes commonly produce petroleum reservoir traps (see Chapter 8 for an illustration). Caprock is a hard erosion resistant rock on top of a softer, less resistant rock. Cap rock also refers to an impermeable rock layer that traps oil, water, or gas.
Oil shale generally refers to any sedimentary rock that contains solid bituminous materials (called kerogen) that are released as petroleum-like liquids when the rock is heated in the chemical process of pyrolysis. Oil shale is formed by the deposition of silt and organic debris on lake beds and sea bottoms. Over time, heat and pressure transform these materials into oil.
Tar sand is a sedimentary material composed primarily of sand, clay, water (in some deposits) and organic constituents known as bitumen. Petroleum from tar sand is very thick with high viscosity and cannot be drilled like other types of petroleum. This petroleum must be mined as in Alberta, Canada, which has one of the largest deposits of tar sands and is currently being mined.
Coal is a readily combustible black or brownish-black rock whose composition, including inherent moisture, consists of more than 50 percent carbonaceous material by weight and more than 70 percent carbonaceous material by volume. Coal is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. Coal is classified into four types: anthracite, bituminous, sub-bituminous, and lignite. Anthracite is the highest grade of coal while lignite is the lowest grade. Currently, the United States contains the largest coal reserves in the world. The majority of this coal is used in power plants to generate electricity.
Uranium is a radioactive element inside the Earth that produces heat and causes convection inside the Earth. Uranium, which is used to produce nuclear energy and electricity, is found in ore minerals such as carnotite and uraninite. Carnotite is a secondary ore mineral of uranium and vanadium with a canary yellow color and can be found in sandstone. Uraninite (UO2) is a black, brown, or steel-gray mineral that is strongly radioactive and is one of the main ores of uranium. Uraninite occurs in veins of lead, tin, and copper minerals. Uraninite also occurs in sandstone deposits, and is a primary constituent of granites and pegmatites.
Banded iron formations (BIF) occur in Proterozoic rocks and are composed of alternating layers of iron-rich material (commonly magnetite) and silica (chert). Each layer is relatively thin, varying in thickness from a millimeter to several centimeters. Banded iron formations occur around the world and are an important source of iron ore.
