2Sedimentary 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.
3Examples of Sedimentary Rocks SandstoneShaleLimestoneBituminous CoalRock SaltBreccia
4Origins of Sedimentary Rock Weathering 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.
5Origins of Sedimentary Rock Soluble 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.
6Origins of Sedimentary Rock Deposition 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.
7Origins of Sedimentary Rock As 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.
8Classification of Sedimentary Rocks Three 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.
9Detrital Sedimentary Rocks Though a wide variety of minerals and rock fragments (clasts) may be found in detrital rocks, clay minerals and quartz are the most common.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:SandstoneShaleBreccia
10Detrital Sedimentary Rocks Other 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.
11Detrital Sedimentary Rocks The 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.
12Detrital Sedimentary Rocks After 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.
13Detrital Sedimentary Rocks By 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.
14Detrital Sedimentary Rocks Common detrital rocks, in order of increasing particle size, are shale, sandstone, and conglomerate (breccia).ShaleBrecciaSandstoneShale 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.
15Detrital Sedimentary Rocks Common detrital rocks, in order of increasing particle size, are shale, sandstone, and conglomerate (breccia).ShaleSandstoneBrecciaSandstone 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.
17Detrital Sedimentary Rocks Common detrital rocks, in order of increasing particle size, are shale, sandstone, and conglomerate (breccia).ShaleBrecciaSandstoneConglomerates (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.
18Chemical Sedimentary Rocks 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.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:LimestoneChertRock Salt
19Chemical Sedimentary Rocks LimestoneChertRock SaltRepresenting 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.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).
21Chemical Sedimentary Rocks LimestoneChertRock SaltChert 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.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.
23Chemical Sedimentary Rocks LimestoneChertRock SaltVery 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.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.
24Examples of Evaporites Sylvite: Mined for potassium (potash) fertilizerRock SaltGypsumBonneville Salt Flats
25Coal – An Organic Sedimentary Rock Organic Sedimentary Rock 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.Organic Sedimentary RockCoal 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.Bituminous CoalWhen the bituminous coal is heated or put under further stress, it eventually creates anthracite coal, which is technically a metamorphic rock.
26Formation 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.
28Sediments and Earth History Sedimentary environments are those places where sediment accumulates. They are grouped into continental, marine, and transitional (marine) environments.
29Sediments 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.
30Sedimentary 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.
31Sedimentary Rocks and Resources - Sedimentary rocks provide humans with nonmetallic and energy resources, as well as help make metallic resources for every day use.- Producing one ton of steel requires about 1/3 ton of limestone and between 2 and 20 pounds of fluorite.
32Sedimentary Rocks and Energy Use Coal Fields of the United States Coal, 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