Recall the rock cycle, which shows the origin of sedimentary rocks. Weathering begins the process. Next, gravity and erosional agents remove the products of weathering and carry them to a new location where they are deposited. Usually, the particles are broken down further during this transport phase. Following deposition, this sediment may become lithified, or “turned to rock.”

The word sedimentary indicates the nature of these rocks, for it is derived from the Latin sedimentum, which means “settling,” a reference to a solid material settling out of a fluid. Most sediment is deposited in this fashion. Weathered sediment is constantly being swept from bedrock and carried away by running water, waves, glacial ice, or wind. Eventually, the material is deposited in lakes, river valleys, seas, and countless other places. The particles in a desert sand dune, the mud on the floor of a swamp, the gravels in a streambed, and even household dust are examples of sediment deposition.

The weathering of bedrock and the transport and deposition of this weathered rock material are continuous processes. Therefore, sediment is found almost everywhere. As piles of sediment accumulate, the materials near the bottom are compacted by the weight of the overlying layers. Over long periods, these sediments are cemented together by mineral matter deposited from water into the spaces between particles. This forms solid sedimentary rock.

Geologists estimate that sedimentary rocks account for only about 5 percent (by volume) of Earth’s outer 16 kilometers (~10 miles). However, the importance of this group of rocks is far greater than this percentage implies. If you sampled the rocks exposed at Earth’s surface, you would find that the great majority of them are sedimentary (Figure 15). Indeed, about 75 percent of all rock outcrops (visible exposure of rock) on the continents are sedimentary. Therefore, we can think of sedimentary rocks as comprising a relatively thin and somewhat discontinuous layer in the uppermost portion of the crust. This makes sense because sediment accumulates at the surface.

It is from sedimentary rocks that geologists reconstruct many details of Earth’s history. Because sediments are deposited in a variety of different settings at the surface, the rock layers that they eventually form hold many clues to past surface environments. They may also exhibit characteristics that allow geologists to decipher information about the method and distance of sediment transport. Furthermore, sedimentary rocks contain fossils, which are vital evidence in the study of the geologic past.

Finally, many sedimentary rocks are important economically. Bituminous coal, which is burned to generate electrical energy, is classified as a sedimentary rock. Other major energy resources, such as petroleum and natural gas, occur in pores within sedimentary rocks. Other sedimentary rocks are major sources of iron, aluminum, and manganese, other minerals for fertilizer, plus numerous materials essential to the construction industry, such as sand and gravel.

Types of Sedimentary Rocks

Materials that accumulate as sediment have two principal sources. First, sediments may originate as solid particles from weathered rocks, such as the igneous rocks described earlier. These particles are called detritus, and the sedimentary rocks they form are called detrital sedimentary rocks.

Detrital Sedimentary Rocks

Though a wide variety of minerals and rock fragments may be found in detrital rocks, clay minerals and quartz dominate. As you learned earlier, clay minerals are the most abundant product of the chemical weathering of silicate minerals, especially the feldspars. Quartz, on the other hand, is abundant because it is extremely durable and very resistant to chemical weathering. Thus, when igneous rocks containing quartz, such as granite, are weathered, individual quartz grains are set free and persist in the environment for long time periods.

Particle size is the primary basis for distinguishing among detrital sedimentary rocks. Figure 16 presents the size categories for particles making up detrital rocks. When a rock consists of a significant amount of rounded gravel-size particles, it is called conglomerate. As Figure 16 shows, gravel-size particles can range in size from as large as boulders to particles as small as peas. If the gravel-size particles are angular rather than rounded, the rock is called breccia. Angular fragments indicate that the particles were not transported very far from their source prior to deposition and so have not had corners and rough edges abraded. When sand-size grains rather than gravel-size particles prevail, the rock is called sandstone.

Shale, which is typically composed of clay minerals, is a name commonly applied to all very fine-grained sedimentary rocks (refer to Figure 16). However, in the strict use of the word, a rock can be classified as shale only if it exhibits the ability to split into thin layers. If a fine-grained rock breaks into chunks or blocks, the name mudstone is applied. Siltstone, another fine-grained rock, is composed of slightly larger silt-size grains intermixed with clay-sized sediment.

Particle size also provides useful information about the environment in which the sediment was deposited. Currents of water or air sort the particles by size. The stronger the current, the larger the particle size carried. Gravels, for example, are moved by swiftly flowing rivers, rockslides, and glaciers. Less energy is required to transport sand; thus, it is common in windblown dunes, river deposits, and beaches. Because silts and clays settle very slowly, accumulations of these materials are generally associated with the quiet waters of a lake, lagoon, swamp, or marine environment.

Although detrital sedimentary rocks are largely classified by particle size, in certain cases, the mineral composition is also part of the rock name. For example, most sandstone are rich in quartz, so they are referred to as quartz sandstone. When sandstone contains more than 25 percent of the mineral feldspar, it is called arkose. In addition, rocks consisting of detrital sediments are rarely composed of grains of just one size. Consequently, a rock containing quantities of both sand and silt can be correctly classified as sandy siltstone or silty sandstone, depending on which particle size dominates.

Chemical, Biochemical, and Organic Sedimentary Rocks

In contrast to detrital rocks, which are composed of the solid products of weathering, chemical sedimentary rocks and biochemical sedimentary rocks are derived from material (ions) carried in solution to lakes and seas or dissolved in groundwater (Figure 17). This material does not remain dissolved in water indefinitely. Under certain conditions and due to physical processes, the dissolved matter precipitates to form chemical sediments. An example of chemical sediments resulting from physical processes is the salt left behind as a body of saltwater evaporates.

Precipitation may also occur indirectly through life processes of water-dwelling organisms that form materials called biochemical sediments. Many aquatic animals and plants extract the mineral matter dissolved in the water to form their shells and other hard parts. After the organisms die, their shells or hard parts may accumulate on the floor of a lake or an ocean.

Limestone, an abundant sedimentary rock, is composed chiefly of the mineral calcite (CaCO3).  Nearly 90 percent of limestone is formed from biochemical sediments secreted by marine organisms, and the remaining amount consists of chemical sediments that precipitated directly from seawater.

One easily identified biochemical limestone is coquina, a coarse rock composed of loosely cemented shells and shell fragments (Figure 18). Coquina has uses as an aggregate in concrete, a source of lime for agriculture, and for centuries was used extensively as a building material in coastal areas. Another less obvious but familiar example of a biochemical limestone is chalk, a soft, porous rock made up almost entirely of the hard parts of microscopic organisms that are no larger than the head of a pin. Among the most famous chalk deposits are the White Cliffs of Dover exposed along the southeast coast of England (Figure 19). Chalk is used in a variety of materials, such as cement, cosmetics, ceramics, and as a filler in paper and paints.

Inorganic limestone forms when chemical changes or high water temperatures cause calcium carbonate (calcite) to precipitate. Travertine, the type of limestone that decorates caverns, is one example. Groundwater is the source of travertine that is deposited in caves. As water drops reach the air in a cavern, some of the carbon dioxide dissolved in the water escapes, causing calcium carbonate to precipitate.

Dissolved silica (SiO2) precipitates to form varieties of microcrystalline rocks—rocks consisting of very fine-grained quartz crystals (Figure 20). Sedimentary rocks composed of microcrystalline quartz include chert (light color), flint (dark), jasper (red), and petrified wood (multicolored). These chemical sedimentary rocks may have either an inorganic or biochemical origin, but the mode of origin is usually difficult to determine.

Very often, evaporation causes minerals to precipitate from water. Such minerals include halite, the chief component of rock salt, and gypsum, the main ingredient of rock gypsum. Both materials have significant commercial importance. Halite is familiar as the common salt used in cooking and seasoning foods. Of course, it has many other uses and has been considered important enough that people have sought, traded, and fought over it for much of human history. Gypsum is the basic ingredient in plaster of Paris. This material is used most extensively in the construction industry for drywall and plaster.

In the geologic past, many areas that are now dry land were covered by shallow arms of the sea that had only narrow connections to the open ocean. Under these conditions, water continually moved into the bay to replace water lost by evaporation. Eventually, the waters of the bay became saturated, and salt deposition began. Today, these arms of the sea are gone, and the remaining deposits are called evaporite deposits.

Evaporite deposits are also found in enclosed basins on land. For example, the Bonneville Salt Flats (Figure 21) formed about  years ago when a drying climate began to evaporate the glacial-fed waters of the expansive Lake Bonneville. As the water in this basin evaporated, materials that were dissolved in the water were left behind, forming a white, salt-rich crust on the ground that grows in thickness to generate a salt flat.

Coal—An Organic Sedimentary Rock

In contrast to sedimentary rocks that are calcite or silica rich, coal consists mostly of organic matter. Because coal is produced by biochemical activity and contains organic matter, it is often classified as a biochemical, or organic, rock. Examining a piece of lignite, also called brown coal, under a magnifying glass reveals plant structures, such as leaves, bark, and wood, that have been chemically altered but remain identifiable (Figure 22). This observation supports the conclusion that coal is the end product of the burial of large amounts of plant material over extended periods.

The initial stage in coal formation is the accumulation of large quantities of plant remains. However, special conditions are required for such accumulations because dead plants normally decompose when exposed to the atmosphere. Ideal environments that allow plant matter to accumulate are swamps. Stagnant swamp water is oxygen deficient, which makes it impossible for plant material to completely decay (oxidize). At various times during Earth history, such environments have been common.

Coal forms in a series of stages. With each successive stage, higher temperatures and pressures drive off impurities and volatiles, as shown in Figure 22. Lignite and bituminous coals are sedimentary rocks, but anthracite is a metamorphic rock. Anthracite forms when sedimentary layers are subjected to the folding and deformation associated with mountain building.

Lithification of Sediment

Lithification refers to the processes by which sediments are transformed into solid sedimentary rocks. One of the most common processes is compaction (Figure 23A). As sediments accumulate through time, the weight of overlying material compresses the deeper sediments. As the grains are pressed closer and closer, pore space is greatly reduced. For example, when clays are buried beneath several thousand meters of material, the volume of the clay may be reduced as much as 40 percent. Compaction is most effective in converting very fine-grained sediments, such as clay-size particles, into sedimentary rocks.

Because sand and coarse sediments (gravel) are not easily compressed, they are generally transformed into sedimentary rocks by the process of cementation (Figure 23B). The cementing materials are carried in solution by water that percolates through the pore spaces between particles. Over time, the cement precipitates onto the sediment grains, fills the open spaces, and acts like glue, joining the particles together. Calcite, silica, and iron oxide are the most common cements. The cementing material is relatively easy to identify. Calcite cement effervesces (fizzes) in contact with dilute hydrochloric acid. Silica is the hardest cement and thus produces the hardest sedimentary rocks. An orange or red color in a sedimentary rock usually means iron oxide is present.

Features of Sedimentary Rocks

Sedimentary rocks are particularly important in the study of Earth history. These rocks form at Earth’s surface, and as layer upon layer of sediment accumulates, each layer records the nature of the environment at the time the sediment was deposited. These layers, called strata or beds, are the single most characteristic feature of sedimentary rocks (refer to Figure 15).

Bed thickness ranges from microscopically thin to tens of meters thick. Separating the strata are bedding planes, flat surfaces along which rocks tend to separate or break. Generally, each bedding plane marks the end of one episode of sedimentation and the beginning of another.

Sedimentary rocks provide geologists with evidence for deciphering past environments. A conglomerate, for example, indicates a high-energy environment, such as a rushing stream, where only the coarse materials can settle out. By contrast, both coal and black shale (with its high carbon content) are associated with a low-energy, organic-rich environment, such as a swamp or lagoon. Other features found in some sedimentary rocks also give clues to past environments (Figure 24).

Fossils, the traces or remains of prehistoric life, are perhaps the most important inclusions found in some sedimentary rocks (Figure 25). Knowing the nature of the life-forms that existed at a particular time may help answer many questions about the environment. Was it land or ocean, lake or swamp? Was the climate hot or cold, rainy or dry? Was the ocean water shallow or deep, turbid or clear? Furthermore, fossils are important time indicators and play a key role in matching up rocks from different places that are the same age. Fossils are important tools used in interpreting the geologic past and will be examined in more detail later this semester.

• Although igneous and metamorphic rocks make up most of Earth’s crust by volume, sediment and sedimentary rocks are concentrated near the surface.

• Detrital sedimentary rocks are made of solid particles, mostly quartz grains and microscopic clay minerals. Common detrital sedimentary rocks include shale (the most abundant sedimentary rock), sandstone, and conglomerate.

• Chemical and biochemical sedimentary rocks are derived from mineral matter (ions) that is carried in solution to lakes and seas. Under certain conditions, ions in solution precipitate (settle out) to form chemical sediments as a result of physical processes, such as evaporation. Precipitation may also occur indirectly through life processes of water-dwelling organisms that form materials called biochemical sediments. Many water-dwelling animals and plants extract dissolved mineral matter to form shells and other hard parts. After the organisms die, their skeletons may accumulate on the floor of a lake or an ocean.

• Limestone, an abundant sedimentary rock, is composed chiefly of the mineral calcite (CaCO3).  Rock gypsum and rock salt are chemical rocks that form as water evaporates.

• Coal forms from the burial of large amounts of plant matter in low-oxygen depositional environments, such as swamps and bogs.

• The transformation of sediment into sedimentary rock is called lithification. The two main processes that contribute to lithification are compaction (a reduction in pore space that results from packing grains more tightly together) and cementation (a reduction in pore space that results from adding new mineral material that acts as a “glue” to bind the grains to each other).