Sedimentary Rocks

Sedimentary RocksSedimentary rocks account for about 5 to 10 percent of the Earth’s outer 10 miles of crust. Because they are concentrated at the Earth’s surface, the importance of this group is much greater than this percentage implies.

Sedimentary rocks contain much of the basic information needed to reconstruct Earth history. In addition, this group is associated with many important energy and mineral resources.

Examples of Sedimentary

Rocks

Sandstone Shale Limestone

Bituminous Coal

Rock Salt Breccia

Origins of Sedimentary RockWeathering begins the process. It involves the physical disintegration and chemical deposition of pre-existing rocks (igneous, metamorphic and sedimentary), which are the raw materials for sedimentary rocks.

Origins of Sedimentary RockSoluble materials are carried away by runoff and water. Solid particles are moved down slope by gravity (called mass wasting). Transportation by wind, water, glacial ice, etc., moves the materials from the sites where they originated to the sites where they accumulate.

Origins of Sedimentary RockDeposition of solid particles occurs when wind and water currents slow down and as glacial ice melts. The word sedimentary is derived from the Latin sedimentum, which means “to settle.” Examples include the mud on the floor of a lake, a gravel bar in a stream bed, particles in a desert sand dune – even household dust in a corner of a room.

Origins of Sedimentary RockAs deposition continues, older sediments are buried beneath younger layers and gradually converted to sedimentary rock (lithified) by compaction and cementation. This and other changes are referred to as diagenesis, a collective term for all of the changes (short of metamorphism) that take place in texture, composition, and other physical properties after sediments are deposited.

Classification of Sedimentary RocksThree main categories of sedimentary rocks are recognized.

-Sediment has two principal sources: First, it may be an accumulation of material that originates and is transported as solid particles derived from both mechanical and chemical weathering. These deposits are termed detrital, and the rocks they form are called detrital sedimentary rocks.

-The second major source of sediment is soluble material produced largely by chemical weathering. When these ions in solution are precipitated either by inorganic or biologic processes, the material is called chemical sediment, which makes chemical sedimentary rocks.

-The third category is organic sedimentary rocks. The primary example is coal, which is made up of organic carbon from the remains of plants that died and accumulated on the floor of a swamp.

Detrital Sedimentary RocksThough a wide variety of minerals and rock fragments (clasts) may be found in detrital rocks, clay minerals and quartz are the most common.

Sandstone Shale Breccia

Remember that clay materials are the most abundant product of the chemical weathering of silicate materials, especially the feldspars. Clays are fine-grained materials with sheetlike crystalline structures similar to the micas. The other common mineral, quartz, is abundant because it is extremely durable and very resistant to chemical weathering.

Detrital Sedimentary Rock Examples:

Detrital Sedimentary RocksOther common solid particles found in detrital rocks include pieces of feldspars and micas. Because chemical weathering rapidly transforms these minerals, their presence in sedimentary rocks indicates that erosion and deposition were fast enough to preserve some of the primary minerals from the source rock before they could be decomposed. - No matter what minerals come together to form a detrital rock, particle size is the primary basis for classifying detrital sedimentary rocks.

Detrital Sedimentary RocksThe sizes of grains within detrital rocks may provide useful information about environments of deposition. Currents of water or air sort the particles by size – the stronger the current, the larger the particle size carried. Gravels are moved by swiftly flowing rivers as well as by landslides and glaciers. Less energy is required to transport sand; thus it is found on windblown dunes and beaches. Even less energy is required to move clay, so it settles very slowly in water environments.

Detrital Sedimentary RocksAfter determining particle size, the next way to classify a detrital sedimentary rock is by describing how well the particle sizes are sorted within a rock. Then, classification depends on a particle’s shape.

Detrital Sedimentary RocksBy studying the degree of sorting, we can learn much about the depositing current. Deposits of windblown sand are usually better sorted than deposits sorted by wave activity. Particles washed by waves are commonly better sorted than materials deposited by streams.

By studying particle shape, we can see if grains have rounded edges or sharp ones, which help us learn about the distance and area a rock was formed in. For example, when streams, winds, or waves move sand, the quartz grains lose their sharp edges and corners and become more rounded as they collide with other particles during transport.

Detrital Sedimentary RocksCommon detrital rocks, in order of increasing particle size, are shale, sandstone, and conglomerate (breccia).

SandstoneShale Breccia

Shale consists of silt- and clay-size particles. These fine-grained detritals account for more than half of all sedimentary rocks. The tiny grains in shale tell us deposition occurs as the result of gradual settling from quiet, nonturbulent currents such as lakes, river floodplains, lagoons, or deep-ocean basins. True shale has the ability to break into thin layers along well-developed, closely spaced planes (fissility). If the rock breaks into chunks (lacks fissility), it is called mudstone or siltstone.

Detrital Sedimentary RocksCommon detrital rocks, in order of increasing particle size, are shale, sandstone, and conglomerate (breccia).

SandstoneShale Breccia

Sandstone rocks have more sand-sized grains than any other. These detritals account for 20 percent of all sedimentary rocks. Owing to its durability, quartz is the most common mineral in most sandstones. If is made of mostly quartz, it may be called quartz sandstone. If it is made up of 25 percent or more of feldspar, it is called arkose sandstone. If more than 15 percent of the rock’s composition is silt and clay particles along with large particles (called matrix), the rock is called graywacke.

Sandstone Types

Quartz Sandstone

Arkose Sandstone

Graywacke

Detrital Sedimentary RocksCommon detrital rocks, in order of increasing particle size, are shale, sandstone, and conglomerate (breccia).

SandstoneShale Breccia

Conglomerates (like breccia) consist largely of gravel and are made of poorly sorted particles. If the large particles are more angular, the rock is called breccia. These large, angular particles indicate the material did not travel far from its source area before being deposited. The particle size of conglomerates tells us the strength of the current that transported them, while the degree of rounding tells us how far the particles traveled. The fragments in a rock help us identify the source rock that formed them.

Chemical Sedimentary Rocks

Limestone Rock Salt

In contrast to detrital rocks, which form from solid materials, chemical sediments come from ions that are carried in solution to lakes and seas. This material does not remain dissolved in water indefinitely, however. Some of it precipitates to form chemical sediments, which later become chemical sedimentary rocks such as limestone, chert, and rock salt.

Chert

Precipitation of material occurs in two ways: Inorganic (like evaporation and chemical activity) and organic (biochemical life processes from water-dwelling organisms).

Chemical Sedimentary Rock Examples:

Chemical Sedimentary Rocks

Representing about 10 percent of the total volume of all sedimentary rocks, limestone is the most abundant chemical sedimentary rock. It is composed chiefly of the mineral calcite (CaCO3) and forms both inorganically and organically.

Limestone Rock SaltChert

Organically, limestone is found in forms of coral reefs, coquina (a rock composed of shell fragments), and chalk. Inorganic limestone includes travertine (commonly seen in caves), oolitic limestone (a rock made up of spherical grains), and dolostone (calcium and magnesium-rich rocks).

Limestone Examples

Travertine

Coquina

Chalk

Coral Reefs

Chemical Sedimentary Rocks

Chert is a name used for a number of very compact and hard rocks made of microcrystalline quartz (SiO2). One well-known form is flint, whose dark color results from the organic matter it contains. Jasper, a red variety, gets its color from iron oxide. The banded form is usually referred to as agate.

Limestone Rock SaltChert

Like glass, most chert has a conchodial fracture. Its hardness, ease of chipping, and ability to hold a sharp edge made chert a favorite of Native Americans for making points for spears and arrows.

Chert Examples

Jasper

Flint

Agate

Chemical Sedimentary Rocks

Very often, evaporation is the mechanism triggering deposition of chemical precipitates. Minerals commonly created in this fashion include halite (the chief component of rock salt), and gypsum.

Limestone Rock SaltChert

When a body of seawater evaporates, less soluble minerals precipitate first, and more soluble minerals precipitate later as salinity increases. Gypsum precipitates when 80 percent of seawater has evaporated, while halite settles out when 90 percent of the water has been removed. During the last stages, potassium (potash) and magnesium salts precipitate out.

Examples of Evaporites

Rock Salt

Bonneville Salt Flats Gypsum

Sylvite: Mined for

potassium (potash) fertilizer

Coal – An Organic Sedimentary Rock

Bituminous Coal

Coal is quite different from other sedimentary rocks. Unlike limestone and chert, which are calcite- or silica-rich, coal is made mostly of organic matter.

Coal forms as decaying plant matter accumulates in swamp environments. There are different stages of coal, and it isn’t until the material is compacted to a specific amount that it becomes a sedimentary rock called lignite and bituminous coal.

When the bituminous coal is heated or put under further stress, it eventually creates anthracite coal, which is technically a metamorphic rock.

Organic Sedimentary Rock

Formation of CoalPartial decomposition of plants in an oxygen-poor swamp creates a layer of peat, a soft brown material in which plant structures are still easily recognized.

With shallow burial, peat slowly changes to lignite, a soft brown coal. Burial increases the temperature of the sediments as well as the pressure on them, eventually creating a harder, more compacted sedimentary rock called bituminous coal.

More compaction, and thus a higher concentration of fixed carbon, creates anthracite – a very hard, shiny, black metamorphic rock.

Classification of Sedimentary Rocks

Sedimentary environments are those places where sediment accumulates. They are grouped into continental, marine, and transitional (marine) environments.

Sediments and Earth History

Because sediment contains clues about the environment in which it was deposited, sedimentary rocks are important in the interpretation of Earth’s history.

Sediments and Earth History

Sedimentary Structures

Ripple marks are produced by currents of water or wind. Mud cracks form when wet mud or clay dries out and shrinks. Fossils, the remains or traces of prehistoric life, are important inclusions in sedimentary rock. They are important tools for interpreting the geologic past.

Sedimentary Rocks and Resources

- Producing one ton of steel requires about 1/3 ton of limestone and between 2 and 20 pounds of fluorite.

- Sedimentary rocks provide humans with nonmetallic and energy resources, as well as help make metallic resources for every day use.

Sedimentary Rocks and Energy UseCoal, petroleum, and natural gas are the primary fuels of our modern industrial economy. About 85 percent of the energy consumed in the United States today comes from these basic fossil fuels.

Today, coal provides 22 percent of the energy needs used in the United States.

Coal Fields of the United States

The Carbon Cycle