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Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones make up about 20 to 25 percent of all sedimentary rocks.[1]

Most sandstone is composed of quartz or feldspar (both silicates) because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in the Goldich dissolution series.[2] Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.

Rock formations that are primarily composed of sandstone usually allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs.[3][4]

Quartz-bearing sandstone can be changed into quartzite through metamorphism, usually related to tectonic compression within orogenic belts.[5][6]

Origins

Sand from Coral Pink Sand Dunes State Park, Utah. These are grains of quartz with a hematite coating providing the orange colour. Scale bar is 1.0 mm.

Sandstones are clastic in origin (as opposed to either organic, like chalk and coal, or chemical, like gypsum and jasper).[7] The silicate sand grains from which they form are the product of physical and chemical weathering of bedrock.[8] Weathering and erosion are most rapid in areas of high relief, such as volcanic arcs, areas of continental rifting, and orogenic belts.[9]

Red sandstone interior of Lower Antelope Canyon, Arizona, worn smooth by erosion from flash flooding over thousands of years

Eroded sand is transported by rivers or by the wind from its source areas to depositional environments where tectonics has created accommodation space for sediments to accumulate. Forearc basins tend to accumulate sand rich in lithic grains and plagioclase. Intracontinental basi

Most sandstone is composed of quartz or feldspar (both silicates) because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in the Goldich dissolution series.[2] Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.

Rock formations that are primarily composed of sandstone usually allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs.[3][4]

Quartz-bearing sandstone can be changed into quartzite through metamorphism, usually related to tectonic compression within orogenic belts.[5][6]

Sandstones are clastic in origin (as opposed to either organic, like chalk and coal, or chemical, like gypsum and jasper).[7] The silicate sand grains from which they form are the product of physical and chemical weathering of bedrock.[8] Weathering and erosion are most rapid in areas of high relief, such as volcanic arcs, areas of continental rifting, and orogenic belts.[9]

Red sandstone interior of Lower Antelope Canyon, Arizona, worn smooth by erosion from flash flooding over thousands of years

Eroded sand is transported by rivers or by the wind from its source areas to depositional environments where tectonics has created accommodation space for sediments to accumulate. Forearc basins tend to accumulate sand rich in lithic grains and plagioclase. Intracontinental basins and grabens along continental margins are also common environments for deposition of sand.[10]

As sediments continue to accumulate in the depositional environment, older sand is buried by younger sediments, and it undergoes diagenesis. This mostly consists of compaction and lithification of the sand.[11][12]<

Eroded sand is transported by rivers or by the wind from its source areas to depositional environments where tectonics has created accommodation space for sediments to accumulate. Forearc basins tend to accumulate sand rich in lithic grains and plagioclase. Intracontinental basins and grabens along continental margins are also common environments for deposition of sand.[10]

As sediments continue to accumulate in the depositional environment, older sand is buried by younger sediments, and it undergoes diagenesis. This mostly consists of compaction and lithification of the sand.[11][12] Early stages of diagenesis, described as eogenesis, take place at shallow depths (a few tens of meters) and is characterized by bioturbation and mineralogical changes in the sands, with only slight compaction.[13] The red hematite that gives red bed sandstones their color is likely formed during eogenesis.[14][15] Deeper burial is accompanied by mesogenesis, during which most of the compaction and lithification takes place.[12]

Compaction takes place as the sand comes under increasing pressure from overlying sediments. Sediment grains move into more compact arrangements, ductile grains (such as mica grains) are deformed, and pore space is reduced. In addition to this physical compaction, chemical compaction may take place via press

As sediments continue to accumulate in the depositional environment, older sand is buried by younger sediments, and it undergoes diagenesis. This mostly consists of compaction and lithification of the sand.[11][12] Early stages of diagenesis, described as eogenesis, take place at shallow depths (a few tens of meters) and is characterized by bioturbation and mineralogical changes in the sands, with only slight compaction.[13] The red hematite that gives red bed sandstones their color is likely formed during eogenesis.[14][15] Deeper burial is accompanied by mesogenesis, during which most of the compaction and lithification takes place.[12]

Compaction takes place as the sand comes under increasing pressure from overlying sediments. Sediment grains move into more compact arrangements, ductile grains (such as mica grains) are deformed, and pore space is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact.[12]

Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces.[12]

Mechanical compaction takes place primarily at depths above 1,000 meters (3,300 ft). Chemical compaction continues to depths of 2,000 meters (6,600 ft), and most cementation takes place at depths of 2,000–5,000 meters (6,600–16,400 ft).[16]

Unroofing of buried sandstone is accompanied by telogenesis, the third and final stage of diagenesis.[13] As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the sandstone, such as dissolution of some of the cement to produce secondary porosity.[12]

Framework grains are sand-sized (0.0625-to-2-millimetre (0.00246 to 0.07874 in) diameter) detrital fragments that make up the bulk of a sandstone.[17][18] These grains can be classified into several different categories based on their mineral composition:

  • Quartz framework grains are the dominant minerals in most clastic sedimentary rocks; this is because they have exceptional physical properties, such as hardness and chemical stability.[1] These physical properties allow the quartz grains to survive multiple recycling events, while also allowing the grains to display some degree of rounding.[1] Quartz grains evolve from plutonic rock, which are felsic in origin and also from older sandstones that have been recycled.
  • Feldspathic framework grains are commonly the second most abundant mineral in sandstones.[1] Feldspar can be divided into two smaller subdivisions: alkali feldspars and plagioclase feldspars. The different types of feldspar can be distinguished under a petrographic microscope.[1] Below is a description of the different types of feldspar.
  • Alkali feldspar is a group of minerals in which the chemical composition of the mineral can range from KAlSi3O8 to NaAlSi3O8, this represents a complete solid solution.[1]
  • Plagioclase feldspar is a complex group of solid solution minerals that range in composition from NaAlSi3O8 to CaAl2Si2O8.Matrix is very fine material, which is present within interstitial pore space between the framework grains.[1] The nature of the matrix within the interstitial pore space results in a twofold classification:

    • Arenites are texturally clean sandstones that are free of or have very little matrix.[19]
    • Wackes are texturally dirty sandstones that have a significant amount of matrix.[18]

    Cement

    Cement is what binds the siliciclastic framework grains together. Cement is a secondary mineral that forms after deposition and during burial of the sandstone.[1] These cementing materials may be either silicate minerals or non-silicate minerals, such as calcite.[1]

    • Silica cement can consist of either quartz or opal minerals. Quartz is the most common silicate mineral that acts as cement. In sandstone where there is silica cement present, the quartz grains are attached to cement, which creates a rim around the quartz grain called overgrowth. The overgrowth retains the same crystallographic continuity of quartz framework grain that is being cemented. Opal cement is found in sandstones that are rich in volcanogenic materials, and very rarely is in other sandstones.[1]
    • Calcite cement is the most common carbonate cement. Calcite cement is an assortment of smaller calcite crystals. The cement adheres to the framework grains, cementing the framework grains together.[1]
    • Other minerals that act as cements include: hematite, limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite minerals.[1]

    Pore space

    Pore space includes the open spaces within a rock or a soil.[21] The pore space in a rock has a direct relationship to the porosity and permeability of the rock. The porosity and permeability are directly influenced by the way the sand grains are packed together.[1]

    • Porosity is the percentage of bulk volume that is inhabited by interstices within a given rock.[21] Porosity is directly influenced by the packing of even-sized spherical grains, rearranged from loosely packed to tightest packed in sandstones.[1]
    • Permeability is the rate in which water or other fluids flow through the rock. For groundwater, work permeability may be measured in gallons per day through a one square foot cross section under a unit hydraulic gradient.[21]

    Types of sandstone

    Schematic QFL diagram

    Cement is what binds the siliciclastic framework grains together. Cement is a secondary mineral that forms after deposition and during burial of the sandstone.[1] These cementing materials may be either silicate minerals or non-silicate minerals, such as calcite.[1]

    • Silica cement can consist of either quartz or opal minerals. Quartz is the most common silicate mineral that acts as cement. In sandstone where there is silica cement present, the quartz grains are attached to cement, which creates a rim around the quartz grain called overgrowth. The overgrowth retains the same crystallographic continuity of quartz framework grain that is being cemented. Opal cement is found in sandstones that are rich in volcanogenic materials, and very rarely is in other sandstones.[1]
    • Calcite cement is the most co

      Pore space includes the open spaces within a rock or a soil.[21] The pore space in a rock has a direct relationship to the porosity and permeability of the rock. The porosity and permeability are directly influenced by the way the sand grains are packed together.[1]

      • Porosity is the percentage of bulk volume that is inhabited by interstices within a given rock.[21] Porosity is directly influenced by the packing of even-sized spherical grains, rearranged from loosely packed to tightest packed in sandstones.[1]
      • Permeability is the rate in which water or other fluids flow through the rock. For groundwater, work permeability may be measured in gallons per day through a one square foot cross section under a unit hydraulic gradient.[21]

      Types of sandstone

      Schematic QFL diagram showing tecton

      All sandstones are composed of the same general minerals. These minerals make up the framework components of the sandstones. Such components are quartz, feldspars,[22] and lithic fragments. Matrix may also be present in the interstitial spaces between the framework grains.[1] Below is a list of several major groups of sandstones. These groups are divided based on mineralogy and texture. Even though sandstones have very simple compositions which are based on framework grains, geologists have not been able to agree on a specific, right way, to classify sandstones.[1] Sandstone classifications are typically done by point-counting a thin section using a method like the Gazzi-Dickinson Method. The composition of a sandstone can have important information regarding the genesis of the sediment when used with a triangular Quartz, Feldspar, Lithic fragment (QFL diagrams). Many geologists, however, do not agree on how to separate the triangle parts into the single components so that the framework grains can be plotted.[1] Therefore, there have been many published ways to classify sandstones, all of which are similar in their general format.

      Visual aids are diagrams that allow geologists to interpret different characteristics about a sandstone. The following QFL chart and the sandstone provenance model correspond with each other therefore, when the QFL chart is plotted those points can then be plotted on the sandstone provenance model. The stage of textural maturity chart illustrates the different stages that a sandstone goes through.

      • A QFL chart is a representation of the framework grains and matrix that is present in a sandstone. This chart is similar to those used in igneous petrology. When plotted correctly, this model of analysis creates for a meaningful quantitative classification of sandstones.[23]
      • A sandstone provenance chart allows geologists to visually interpret the different types of places from which sandstones can originate.
      • A stage of textural maturity is a chart that shows the different stages of sandstones. This chart shows the difference between immature, submature, mature, and supermature sandstones. As the sandstone becomes more mature, grains become more rounded, and there is less clay in the matrix of the rock.[1]

      Dott's classification scheme

      Dott's (1964) sandstone classification scheme is one of many such schemes used by geologists for classifying sandstones. Dott's scheme is a modification of Gilbert's classification of silicate sandstones, and it incorporates R.L. Folk's dual textural and compositional maturity concepts into one classification system.[24] The philosophy behind combining Gilbert's and R. L. Folk's schemes is that it is better able to "portray the continuous nature of textural variation from mudstone to arenite and from stable to unstable grain composition".[24] Dott's classification scheme is based on the mineralogy of framework grains, and on the type of matrix present in between the framework grains.

      In this specific classification scheme, Dott has set the boundary between arenite and wackes at 15% matrix. In addition, Dott also breaks up the different types of framework grains that can be present in a sandstone into three major categories: quartz, feldspar, and lithic grains.[1]

      • Arenites are types of sandstone that

        Visual aids are diagrams that allow geologists to interpret different characteristics about a sandstone. The following QFL chart and the sandstone provenance model correspond with each other therefore, when the QFL chart is plotted those points can then be plotted on the sandstone provenance model. The stage of textural maturity chart illustrates the different stages that a sandstone goes through.

        Dott's (1964) sandstone classification scheme is one of many such schemes used by geologists for classifying sandstones. Dott's scheme is a modification of Gilbert's classification of silicate sandstones, and it incorporates R.L. Folk's dual textural and compositional maturity concepts into one classification system.[24] The philosophy behind combining Gilbert's and R. L. Folk's schemes is that it is better able to "portray the continuous nature of textural variation from mudstone to arenite and from stable to unstable grain composition".[24] Dott's classification scheme is based on the mineralogy of framework grains, and on the type of matrix present in between the framework grains.

        In this specific classification scheme, Dott has set the boundary between arenite and wackes at 15% matrix. In addition, Dott also breaks up the different types of framework grains that can be present in a sandstone into three major categories: quartz, feldspar, and lithic grains.[1]

        • Arenites are types of sandstone that have less than 15% clay matrix in between the framework grains.
          • Quartz arenites are sandstones that contain more than 90% of siliceous grains. Grains can include quartz or chert rock fragments.[1] Quartz arenites are texturally mature to supermature sandstones. These pure quartz sands result from extensive weathering that occurred before and during transport. This weathering removed everything but quartz grains, the most stable mineral. They are commonly affiliated with rocks that are deposited in a stable cratonic environment, such as aeolian beaches or shelf environments.In this specific classification scheme, Dott has set the boundary between arenite and wackes at 15% matrix. In addition, Dott also breaks up the different types of framework grains that can be present in a sandstone into three major categories: quartz, feldspar, and lithic grains.[1]

            Sandstone has been used since prehistoric times for construction, decorative art works and housewares, and continues to be used. It has been widely employed around the world in constructing temples, homes, and other buildings.[25]

            Although its resistance to weathering varies, sandstone is easy to work. That makes it a common building and paving material, including in asphalt concrete. However, some types that have been used in the past, such as the Collyhurst sandstone used in North West England, have had poor long-term weather resistance, necessitating repair and replacement in older buildings.[26] Because of the hardness of individual grains, uniformity of grain size and friability of their structure, some types of sandstone are excellent materials from which to make grindstones, for sharpening blades and other implements. Non-friable sandstone can be used to make grindstones for grinding grain, e.g., gritstone.

            A type of pure quartz sandstone, orthoquartzite, with more of 90–95 percent of quartz,[27] has been proposed for nomination to the Global Heritage Stone Resource.[28] In some regions of Argentina, the orthoquartzite-stoned facade is one of the main features of the Mar del Plata style bungalows.[28]

            See also

            Notes

            1. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. pp. 119–135. ISBN 0131547283.