Siliciclastic rocks

Siliciclastic rocks, which include siltstone, sandstone, and conglomerate, are second only to mudrocks in abundance among the world’s sedimentary rock types. They form major reservoirs for water, oil, and natural gas and are the repositories of much of the world’s diamonds, gold, and other precious minerals. In their composition and sedimentary structures, siliciclastic rocks reveal much about paleogeography and, consequently, Earth history.

and Erosion

Weathering and erosion constantly strip the earth’s surface of its rocky exterior. Rocks experience fluctuating temperatures, humidity, and freeze-thaw cycles that physically disaggregate (break up) mineral grains. Simultaneously, these mineral grains are exposed to water, oxygen, carbon dioxide, and dissolved acids or bases that chemically attack them. As a result, a hard, rocky surface is eventually transformed into a collection of mineral grains, clay minerals, and ions dissolved in water.

Weathering and erosion are slow but continuous agents of destruction. Because mountains and other high regions are constantly being uplifted, weathering and erosion provide a more or less steady supply of sediment to streams and rivers and, from there, into the ocean for deposition. Deposited sediment is eventually compacted and cemented into rocks. The rocks that are formed by this process are known collectively as siliciclastic rocks because they are made mostly of particles of silicate minerals.

Siliciclastic Rock Composition

Siliciclastic rocks differ in the sizes of their grains and their composition. Three main size classes are recognized: silt, sand, and gravel. Silt includes particles between 0.004 and 0.0625 millimeter (1/256 to 1/16 millimeter) in diameter; sand is composed of particles between 0.0625 (1/16) and 2 millimeters in diameter; and gravel generally includes particles larger than 2 millimeters. Most sedimentologists refer to sand and sandstone when discussing siliciclastic rocks.

Siliciclastic rocks are composed of three broad classes of material: framework grains, matrix, and cement. Framework grains are particles of minerals or small fragments of other rocks that usually make up the bulk of a siliciclastic rock. Matrix is extremely fine-grained material such as clay that is deposited at the same time as framework grains. Cement is any material precipitated within the spaces, or pores, between grains in a sediment or rock. Framework grains and matrix are primary deposits, whereas cement is a secondary deposit because it is precipitated in pores after the primary material.

Quartz and Feldspar Minerals

The main minerals that compose siliciclastic rocks are quartz, various types of feldspar, and micas. The ferromagnesian minerals, such as olivine, pyroxene, and amphibole, are not as common in sedimentary rocks as they are in igneous and metamorphic rocks. Ferromagnesian minerals are more easily dissolved during weathering, and they fracture more readily during erosion, than the more durable quartz and feldspar. Consequently, quartz and feldspar (and, to a lesser extent, the micas) are concentrated in sediments because the other minerals are selectively removed beforehand. Carbonate rocks dissolve very easily and seldom form much residue to contribute to sand.

Quartz is the most chemically and physically stable mineral that forms siliciclastic rocks. Although quartz is uncommon in some igneous and metamorphic rocks, it is sufficiently abundant in granite and gneiss to contribute a large amount of bulk to almost all sands. Subtle differences may provide useful clues as to the origin of some quartz grains. For example, quartz from plastically deformed metamorphic rocks (schist, gneiss) usually occurs as polycrystalline aggregates or displays a distorted crystal structure, revealed by the petrographic microscope. Quartz from volcanic rocks often possesses planar crystal faces or embayments (deep, rounded indentations) that were formed as the crystal grew in magma. Quartz grains eroded from older sedimentary rocks sometimes retain the previous rock’s quartz cement as a rind, which is called an inherited overgrowth.

Feldspar minerals are also common in most siliciclastic rocks. Two main groups of feldspar are recognized: plagioclase feldspar and the potassium feldspars (microcline, orthoclase, and sanidine). The plagioclase group is actually a collection of many similar minerals with different chemical compositions. Anorthite is plagioclase feldspar composed of calcium, silicon, and oxygen, whereas albite is plagioclase composed of sodium, silicon, and oxygen. Calcium and sodium substitute rather easily for each other, so most plagioclase feldspars have both calcium and sodium. In general, plagioclase with more calcium is more susceptible to chemical weathering than is sodium-rich plagioclase. Consequently, sodium-rich albite is more common than anorthite in most siliciclastic rocks.

Potassium feldspars all have the same chemical composition of potassium, silicon, and oxygen. They differ from one another in their chemical structure. Microcline is the most highly organized, crystallographically, followed by orthoclase and then sanidine. Microcline is formed in igneous and metamorphic rocks that crystallize very slowly, permitting the greatest amount of crystallographic ordering. At the other extreme, sanidine is formed in volcanic rocks where little time is available for ions to get into their “proper places.” During chemical weathering of detrital potassium feldspars, sanidine dissolves much more readily than does microcline or orthoclase. Microcline is usually about equal in abundance to orthoclase in siliciclastic rocks, whereas sanidine is usually rare.

Micas and Accessory Minerals

The mica minerals biotite and muscovite are common in some siliciclastic rocks and rare in others. In general, muscovite (white mica) is more durable and, consequently, more abundant than biotite (brown mica). Chlorite is a mineral with a sheetlike crystal structure similar to the micas. It is common in a few sandstones but generally less abundant than the micas. A large variety of grains is often present in siliciclastic rocks as “accessory minerals.” Altogether they seldom constitute more than 1 percent of any siliciclastic rock. Sometimes these accessory minerals are called heavy minerals because most of them have a much greater density than does quartz or feldspar. This group includes zircon, tourmaline, rutile, and garnet; the ferromagnesian minerals pyroxene, amphibole, and olivine; and iron oxides such as hematite, magnetite, chromite, and limonite. If one examines a handful of sand, the accessory minerals are usually the dark grains.

Accessory minerals are important in several ways. First, they may reveal information about the source rock from which the sand was eroded. For example, the accessory mineral chromite is generally formed in basalt, so its presence in a sand indicates that basalt (usually from the sea floor) had previously been uplifted and eroded nearby. Similarly, garnets of particular composition may be representative of particular metamorphic source rocks. Second, accessory minerals may tell geologists how “mature” a sand is—in other words, how great has been its exposure to chemical and physical weathering. Zircon, tourmaline, and rutile are far more durable than the other accessory minerals. If they are the only accessory minerals in a sand, then the sand probably experienced prolonged weathering in the period before its final deposition. Third, accessory minerals may be economically valuable. Diamonds and gold are accessory minerals in a few cases. Much of the United States’ titanium comes from unusual concentrations of rutile and ilmenite in beach sands along the coast of South Carolina. Zircon in sands provides an important source of zirconium, used in high-temperature ceramics.

Lithic Fragments and Matrix

Pieces of rocks (lithic fragments) may form an important part of coarse-grained sandstone and conglomerate. Lithic fragments may be composed of pieces of almost any igneous, metamorphic, or sedimentary rock. Their presence is a powerful indicator of the source rock from which the sand or gravel was weathered and eroded. As one might expect, not all rock fragments are equally durable in the sedimentary environment. Limestone and shale fragments disintegrate very rapidly, whereas granite and gneiss rock fragments are quite robust. Consequently, both the presence and the absence of certain rock fragments may reveal information about the origin of a siliciclastic sediment.

Matrix in siliciclastic rocks is usually clay or fine silt. This substance is the “mud” that often accompanies sand or gravel during deposition. It is commonly deformed between the more rigid framework grains as the sediment is compacted. Some types of matrix were not originally deposited as fine-grained matrix. This “pseudomatrix” is actually squashed fragments of soft grains of, for example, shale or schist. During compaction of the sediment, these fragments are more easily deformed than durable quartz and feldspar, and the fragments superficially resemble original matrix. Considerable skill is required to distinguish between true matrix and pseudomatrix.

Sandstones

Sand-sized siliciclastic rocks are often classified based on their mineral composition. Sandstones may be named with the term “arenite” (after the Latin harena, “sand”). Relative abundances of quartz, feldspar, and rock fragments determine the name. One of the major types of sandstone is quartz arenite, which is composed almost entirely of quartz, with less than 5 percent of other framework minerals and less than 15 percent clay matrix (usually much less). This rock is usually white when fresh. Another type is feldspathic arenite, which is sometimes called arkose. This rock has abundant feldspar and up to 50 percent lithic fragments. The typical composition of arkose is perhaps 30 percent feldspar, 45 percent quartz, and 25 percent lithic fragments. Up to 15 percent detrital clay matrix may also be present. This rock is often pinkish-gray in color. A third type is lithic arenite, a kind of sand composed largely of lithic fragments and up to 50 percent feldspar. The typical composition of lithic arenite might be 60 percent lithic fragments, 30 percent feldspar, and 10 percent quartz. Up to 15 percent matrix may be present as well. Almost all conglomerates are lithic arenites. Lithic arenites are often dark gray and sometimes feature a salt-and-pepper appearance from the lithic fragments. A fourth type is wacke, sometimes called greywacke, which has between 15 and 75 percent clay matrix at the time of deposition. The term “wacke” may be modified by prefixing with the name of the dominant nonmatrix component; examples might include “quartz wacke,” “feldspathic wacke,” or “lithic wacke.” Some wackes may be “formed” diagenetically from lithic arenites by alteration of shale or schist fragments so that they appear similar to originally detrital clay matrix. This rock is usually gray in color (thus the name “greywacke”).

The composition of sandstones holds important clues as to the kinds of rocks from which they were derived. These clues may help petrologists to reconstruct the plate tectonic setting of the sand’s source region even though the source region has long since disappeared because of erosion or tectonic destruction. For example, quartz arenite often reflects sedimentation on or near a stable craton (a piece of the earth’s crust), where physical and chemical weathering are permitted to eliminate unstable minerals for an extended period of time. At the other extreme, a lithic arenite with abundant basaltic rock fragments, calcic plagioclase, and rare quartz may represent detritus shed from an island arc during plate collision. Careful observation of the mineral composition of sandstones has become an important part of reconstructing the earth’s history.

The diagenesis of siliciclastic rocks is a complex collection of processes that include cementation, replacement, recrystallization, and dissolution. Diagenesis includes chemical reactions that begin at the time of deposition and may continue until metamorphism takes place. Some sedimentary petrologists consider low-temperature cementation as part of diagenesis, whereas others consider diagenesis to include only those reactions that occur at temperatures exceeding 100 degrees Celsius. The upper range of diagenesis grades into metamorphism; for that reason, sedimentary petrologists and metamorphic petrologists share some common territory in diagenesis.

Siliciclastic sediments are transformed into rocks by compaction and cementation. Common cements in siliciclastic rocks are quartz, calcite, clays, and iron oxides such as limonite or hematite. It is common for a rock to have several cements, each deposited at different times or under different conditions of pore-water chemistry. Cement is introduced into sediment in the form of elements or compounds dissolved in water. For example, silica may be dissolved from quartz grains or given off during low-grade metamorphism and then may saturate water in the pores of a rock. One means of dissolving silica from quartz grains is called pressure solution. Quartz grains exert very high pressures upon one another at their points of contact. Quartz is more soluble under high pressure; therefore it is more easily dissolved where grains touch each other. Well-compacted sands often display flattened grain-to-grain contacts as a consequence of pressure solution. The quartz lost from these grains is carried away, dissolved in pore water, to be precipitated elsewhere.

As the water moves slowly through the pores of the rock, it may encounter different temperatures, pressures, acidity, or gas concentrations. Changes in these conditions may cause the water to lose some of its ability to dissolve silica, and the silica may therefore precipitate as crystals. Similar factors influence the dissolution and precipitation of other cements in siliciclastic rocks. Cement not only holds sediment grains together, forming a rock, but it also fills the pores between grains, which may inhibit further flow of fluids. As a consequence, the porosity and permeability of most rocks are reduced by cementation. Rocks with abundant cement generally make poor reservoirs for petroleum or water.

In some cases, the pore-water chemistry conducive to precipitation of one cement is capable of dissolving another cement. A common example is the generally inverse relationship between quartz and calcite cement. Pore water that precipitates quartz often dissolves calcite at the same time, or the other way around. Sometimes dissolution occurs without immediate precipitation of another mineral type in the void that is formed. Pores formed by dissolution are called secondary porosity; such porosity may be the main cause of porosity in many sandstones. Minerals commonly dissolved include carbonates such as calcite and aragonite, silicates such as feldspars and ferromagnesian minerals, and, though rarely, accessory minerals such as garnet. In some cases, earlier cements may be dissolved to form secondary porosity.

Recrystallization and Replacement

Recrystallization is a common feature in diagenesis. In recrystallization, one mineral is transformed into another mineral with the same (or nearly the same) chemical composition or into differently sized (usually larger) crystals of the original mineral. The most common example is the transformation of aragonite into calcite in limestones. In siliciclastic rocks, microcrystalline quartz may recrystallize into more coarsely crystalline quartz, or rutile may recrystallize into anatase.

A more prevalent form of diagenesis among ancient sandstones is replacement. In this process, one mineral is replaced volume-for-volume by another mineral. In some cases, it appears that the replacing mineral has combined material from its host grain with dissolved ions from pore water. The most common replacement examples involve clay minerals. For example, kaolinite may replace potassium feldspar, forming a grain of kaolinite that looks superficially like a potassium feldspar grain.

Examining Siliclastic Rocks

The principal means of studying siliciclastic rocks is with the petrographic microscope. This microscope is similar to other microscopes except that light passes through the specimen rather than illuminates its surface. Rocks are normally not transparent, however, and they must be cut into very thin slices in order for light to pass through them. Magnifications of up to four hundred times are routinely used. In thin section, it is possible to view closely individual grains and their relations to other grains, identify the mineral grains in the rock, and view matrices and cements. Thin sections are routinely used to estimate the amount of pore space in rocks, revealing their potential as reservoirs for fluids such as water, oil, and natural gas.

When illuminated by polarized light, different minerals display characteristic interference colors that reveal the identity of a mineral to the experienced observer. Other clues are the existence and pattern of cleavages in the minerals, alteration products resulting from diagenesis, and overall grain shape. In addition, it is possible to view cements that surround grains and partially or completely fill pores. The sequence of cementation can be interpreted by the relative positions of cements filling pores. Knowing the sequence of cements can help geologists understand how deeply buried the rock was when it was cemented and how the chemical composition of the pore water changed through time.

Another means of examining siliciclastic rocks is with the scanning electron microscope (SEM). The SEM permits examination of clastic grains with much greater magnification than with the conventional petrographic microscope. Magnifications of up to ten thousand times are easily accomplished, with details as small as 0.001 micrometer being easily resolved. The SEM is very useful in examining the surface texture of minerals. The presence of pits, grooves, and cracks can reveal clues as to the environment under which the sand grains were deposited. Were the sand grains blown by wind, carried by water, or transported by glaciers? In some cases, this question may be answered by examination under the extreme magnification of the SEM. The SEM is also very useful for studying the cements that bind sandstones.

Electron Microprobe and X-ray Study

When accurate data concerning chemical composition of minerals are needed, the electron microprobe is used. The microprobe is actually an elaborate version of an SEM. It differs in that its beam does not normally scan but instead remains fixed in one spot, and its detectors are carefully calibrated to give accurate readings of the elemental composition of the material being examined. Microprobes collect X-rays and can determine from them the amounts of different elements in almost any mineral. In some instances, the abundance of trace elements in minerals such as zircon, tourmaline, garnet, or rutile may provide clues as to the source region from which the grains were derived, giving an indication of the source for a sedimentary formation. The electron microprobe is a very useful tool in these studies.

There are limitations to the microprobe. It cannot analyze for elements lighter than sodium, so the amount of oxygen or carbon cannot be determined. Furthermore, rock material must be very carefully prepared for readings to be accurate. Also, electron microprobes are rather expensive and require considerable maintenance; however, X-rays are also employed in the study of rocks. X-ray diffraction of powdered or whole specimens can identify minerals and their degree of crystal ordering. X-ray fluorescence identifies the chemical composition of minerals. Both techniques are commonly used in the study of clays in the matrix of siliciclastic rocks. Sample preparation is relatively easy, and beginners can operate most modern X-ray machines. X-ray analysis usually requires destruction of the sample by grinding, so the sizes, shapes, and relationships between grains are lost.

Economic Significance

Sandstone and its cousins siltstone and conglomerate are common sedimentary deposits. These rocks, collectively known as siliciclastic rocks, are important for a number of reasons. For example, they are porous—that is, they contain small spaces between grains that may be filled with valuable fluids. Siliciclastic rocks are the major type of aquifer in most of the world. Also, they form reservoirs for oil and natural gas. Sand and gravel are essential parts of modern construction because they form the “bulk” in concrete. In this role, they represent the most economically significant mineral resource in the United States, ahead of petroleum, coal, and precious metals.

Principal Terms

clastic rock: a sedimentary rock composed of particles without regard to their composition; this term is sometimes used, incorrectly, as a synonym for “detrital”

detrital rock: a sedimentary rock composed mainly of grains of silicate minerals as opposed to grains of calcite or clays

diagenesis: chemical and mineralogical changes that occur in a sediment after deposition and before metamorphism

lithic fragment: a grain composed of a particle of another rock; in other words, a rock fragment

mudrock: a rock composed of abundant clay minerals and extremely fine siliciclastic material

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