Sedimentary rock classification

Because sedimentary rocks are formed by several different processes—precipitation, crystallization, and compaction—no single classification scheme is applicable to all of them. Used in various combinations, the main elements of classification are mode of origin, mineralogy, the size of the individual mineral grains that make up the rock, and the origin of these grains.

Because there are several very different processes that lead to the formation of sedimentary rocks, no single classification scheme is suitable to all sedimentary rocks. The main elements of classification, however, are mode of origin, mineralogy, the size of the individual mineral grains making up the rock, and the origin of the individual grains. These elements are used in various combinations to categorize several major groups of sedimentary rocks.

One of the major groups is the evaporites. All natural waters contain some dissolved solids that will precipitate when the water evaporates. The crust that forms in a teakettle that has been used for a long time is an example. Seawater contains about 33 parts per thousand dissolved solids and is the major source of the sedimentary rocks classified as evaporites. When a body of seawater in an area with low rainfall is cut off from the sea, as when a sandbar builds up across the mouth of a bay, the trapped seawater tends to evaporate. During this evaporation, several minerals are precipitated in a predictable order. The first to precipitate is the mineral gypsum, hydrated calcium sulfate. If the water is very hot, the precipitating calcium sulfate will not be hydrated, and the mineral anhydrite will form. Further evaporation will cause the precipitation of halite (sodium chloride, or ordinary table salt), and still further evaporation will lead to the precipitation of a complex series of potassium and magnesium salts.

In some environments, most commonly closed depressions in desert areas (the Dead Sea, for example), fresh water evaporates to produce evaporite minerals that are quite different from the minerals produced by the evaporation of seawater. The natron (hydrated sodium carbonate) that was used by the Egyptians in embalming mummies and the borax that originally made Death Valley, California, famous are freshwater evaporite minerals.

From the standpoint of classification, the rocks of evaporative origin—that is, masses of individual crystals of minerals produced by evaporation of seawater or fresh water—are not usually given distinctive names. A fist-sized piece of evaporative sedimentary rock composed of gypsum is normally called gypsum. When it is necessary to indicate clearly that a rock rather than a mineral is being mentioned, the term “rock gypsum” is used. An exception is a rock composed entirely of crystals of halite, which is almost invariably referred to as rock salt.

A second major group of sedimentary rocks is the clastic rocks. “Clastic” comes from the Greek for “broken”; the individual grains of clastic rocks are the product of the mechanical and chemical breakdown, or weathering, of older rocks. Beach sands are composed of such grains. They are composed of residue from weathering of granites and many other kinds of igneous, sedimentary, and metamorphic rocks. Common soil, or mud, also is a residue of the weathering of rocks. Mud differs from beach sand primarily in its content of fine-grained clay minerals. Clay minerals are similar to the mineral mica, but the individual grains are very small, by definition less than 0.004 millimeter in size. When subject to prolonged attack by water and the atmosphere, many minerals that are common in igneous and metamorphic rocks—principally feldspars—are changed into micalike clay minerals. Between beach sands and the clay-sized component of muds are an intermediate size of mineral grains called silts. Silts range in size, again by definition, from 0.0625 millimeter to 0.004 millimeter. Muds are mixtures of silt-sized and clay-sized mineral grains. Grains larger than 2 millimeters in diameter are classified as granules, pebbles, and boulders with increasing size.

The primary basis for classification of clastic sedimentary rocks is grain size. Coarse-grained rocks composed of pebbles and granules are called conglomerates, rocks composed of sand-sized grains are called sandstones, rocks composed of mud-sized materials are mudstones, and rocks composed only of clay-sized grains are called clay stones. Mudstones and clay stones that split readily along flat planes, the bedding planes, are referred to as shales. Collectively, rocks made of silt and clay-sized particles are called mudrocks.

Sandstones are further classified by mineral composition. Sandstones that contain less than 10 percent of fine material are termed arenites, whereas those with more than 10 percent fine material are called wackes. In most sandstones more than 90 percent of the sand grains are quartz, but some sandstones contain appreciable amounts of feldspar grains, volcanic rock fragments, mica, and micaceous rock fragments. These grains are the basis for further classification. Sandstones that are nearly pure quartz sand (more than 90 percent quartz grains, by one common definition) are called orthoquartzites, or quartz arenites (“arenite” is from the Latin for “sand”). Sandstones containing more than 10 percent feldspar grains and volcanic rock fragments are called arkoses or feldspathic arenites, and sandstones with more than 10 percent mica flakes and rock fragments are called greywackes or lithic arenites. Generally, rocks with abundant feldspar and rock fragments have undergone little weathering and are said to be immature, whereas those made only of very stable minerals, such as quartz, have undergone prolonged weathering and are said to be mature.

Limestones

Limestones are another major group of sedimentary rocks. Limestones are composed predominantly of the mineral calcite (calcium carbonate), although some limestones may include some clastic material, typically quartz sand or clay. A similar group of rocks is the dolomites. Dolomites consist predominantly of the mineral dolomite (calcium-magnesium carbonate) and form almost invariably by chemical alteration of preexisting limestone. Therefore, many workers prefer to lump the limestones and dolomites together in a rock group named carbonates.

The greater part of the calcite in limestones is secreted by marine organisms that make their shells from the mineral; clams and oysters are good examples of such organisms. Once these organisms die, their shells are washed about by waves and currents and are broken and abraded into fragments. The fragments may range in size from pebbles to mud, and most limestones are composed of this biogenic detritus.

Two common components of limestones appear to be of inorganic origin. Oölites are round grains, composed of very small crystals of calcite, that have a superficial resemblance to fish eggs. A shell fragment or quartz grain, the nucleus, at the center of the oölite, is surrounded by a coating of fine-grained calcite crystals layered like tree rings. It appears that oölites grow by inorganic deposition of calcite, directly from seawater, on the surface of the nucleus. Field observation of modern oölites suggests that they form only on sea bottoms that are shallow and are periodically agitated by strong waves or currents. It also appears that mud-sized calcite crystals precipitate directly from seawater in some circumstances.

Finally, small organisms, principally marine worms, ingest calcite mud to extract whatever useful organic matter it may contain and excrete it as fecal pellets. The fecal pellets, held together by mucus from the gut of the organism, survive if the bottom currents are not too strong.

Most limestones consist of aggregates of the materials described above. One classification, originally introduced by Robert L. Folk in 1959, and the most widely used classification, is based on the nature of the aggregates and the material that occurs between the grains and cements them together. Mud-sized calcium carbonate tends to accumulate in quiet waters, and a limestone consisting of only mud-sized material is called micrite. Sand-sized calcite grains are deposited in areas with stronger currents, generated by waves, winds, or tides. After final deposition, open spaces between sand-sized grains are often filled by inorganic calcite cement, called spar. In Folk’s classification, the abbreviated name of the sand-sized material followed by the name of the material between the sand-sized grains is the rock name, with the traditional rock-name ending “-ite.” Typical examples are oösparite, pelmicrite, and biosparite (where “bio-” refers to shell fragments).

Chemical Rocks

A fourth major group of sedimentary rocks is the chemical rocks. These rocks are divided into two subgroups: chemical precipitates and chemical replacements. Chemical precipitates are sediments that accumulate directly on the sea bottom as a result of chemical reactions that do not involve evaporation. Deposits of iron minerals (most typically the iron oxide mineral hematite) and phosphate minerals (most typically calcium phosphate, or the mineral apatite) are the most common, and economically important, examples. The process of formation of these rocks is not well understood, but research suggests that in most cases bacteria are involved in producing the proper chemical environment for formation of these important, although rather rare, deposits.

The second subgroup is the chemical replacements. In some cases, the original sediment is dissolved and a new mineral takes its place. Typical examples are the solution of calcite and its replacement by dolomite and the solution of calcite and its replacement by fine-grained quartz. The replacement of calcite by fine-grained quartz in limestones is especially common. The replacement product, the very fine-grained quartz replacement, is called chert, but most people are more familiar with the popular term “flint.”

Identifying Sedimentary Rocks and Component Minerals

Of the more than five thousand minerals identified, only a few dozen are common in rocks at the earth’s surface; these are easily identified in the field by observation and simple tests of physical properties. Even most fine-grained rocks can be identified in the field with the aid of a twelve-power hand lens. Gypsum, for example, is easily identified by its satiny sheen and the fact that it can be scratched by a fingernail. Calcite and dolomite have very similar appearances and physical properties, but a drop of dilute hydrochloric acid will cause calcite, but not dolomite, to effervesce, or fizz. A pocket-sized dropper bottle of dilute hydrochloric acid is standard equipment for the sedimentary geologist.

More detailed studies are done on samples brought to the laboratory. A very common method is the study of thin sections of rocks. To prepare a thin section, a thin slice about a centimeter thick is cut from the sample. The slice is glued to a glass slide and further thinned. When the thickness has been reduced to about 0.03 millimeter, a thin cover glass is glued to the top of the slice, and the thin section is complete.

Thin sections are studied with a microscope, usually a petrographic microscope that has a polarized light source. The effect of passage of the polarized light through individual mineral crystals can be analyzed and much information about the arrangement of the atoms in the crystals obtained. For example, passage of the light through halite does not affect the planar vibration of the polarized light, but when it passes through quartz, it is forced to vibrate in two planes that are perpendicular to each other and parallel to the two crystallographic axis directions of quartz. Therefore, even microscopic-sized grains of halite and quartz are easily distinguished.

Clay minerals, which are too small to be studied effectively by optical methods, are commonly studied by X-ray diffraction. X-rays have very short wavelengths and can be diffracted by the regularly arranged planes of atoms in a crystal in the same way that light is diffracted by a diffraction grating. The sample to be analyzed is irradiated with X-rays of a single wavelength at steadily varying angles of incidence. The angles at which diffraction occurs represent planes of atoms at different spacings. If the clay mineral kaolin, for example, is present in the sample, a strong diffraction will occur at an angle that corresponds to an atomic plane spacing of 7.2 × 10^-10 meters (usually expressed as 7.2 angstroms), and weaker diffractions will occur at angles corresponding to several other spacings that are characteristic of the mineral.

Scanning electron microscopy is a powerful tool for the study of all types of sedimentary rocks but is especially useful for the very fine-grained varieties. Photographic images at 50,000 magnifications are routinely obtained, revealing remarkable details of individual grains and the openings between grains. In addition, semiquantitative chemical analyses of individual grains for sodium and elements heavier than sodium can be made.

Industrial Applications

Classification of sedimentary rocks has led to the recognition of predictable associations of sedimentary rock types with particular geologic conditions. Sandstones that are derived from and deposited near mountains with granitic cores—the Front Ranges of the Rocky Mountains, for example—most commonly contain more than 10 percent feldspar, mostly of the potassium-rich variety, and are classified as arkoses. Sandstones derived from mountains with metamorphic rock cores, such as the Appalachian Mountains, contain more than 10 percent mica and are classified as greywackes, or lithic arenites. Sandstones deposited far from any mountain chain normally are nearly pure quartz, in some cases more than 99.9 percent quartz. These rocks have generally undergone many cycles of deposition, weathering, erosion, and redeposition, so that only quartz has survived. Such rocks are said to be mature. For quality-control purposes, glass manufacturers prefer sand that is nearly pure quartz, and the sedimentary geologist who specializes in industrial minerals will begin the search for glass sands far from any large mountain chain.

The organic-matter content of muds that lie on the sea bottom for long periods of time tends to be destroyed by scavenging organisms. Sedimentary geologists who specialize in petroleum exploration know that one of the requirements for the generation of petroleum is a mudstone with a high organic-matter content. They look for mudstones that were buried by new mud shortly after deposition—that is, mudstones that had a high rate of sedimentation. As a general rule, such mudstones will be associated with arkoses or greywackes rather than with nearly pure quartz sandstones.

Principal Terms

arkose: a sandstone in which more than 10 percent of the grains are feldspar or feldspathic rock fragments; also called feldspathic arenite

carbonate rock: a sedimentary rock composed of grains of calcite (calcium carbonate) or dolomite (calcium magnesium carbonate)

clastic rock: a sedimentary rock composed of broken fragments of minerals and rocks; typically a sandstone

clay stone: a clastic sedimentary rock composed of clay-sized mineral fragments

greywacke: a sandstone in which more than 10 percent of the grains are mica or micaceous rock fragments; also called lithic arenite

limestone: a carbonate sedimentary rock composed of calcite, commonly in the form of shell fragments or other aggregates of small calcite grains

orthoquartzite: a sandstone in which more than 90 percent of the grains are quartz

sandstone: a clastic sedimentary rock composed of sand-sized mineral or rock fragments

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