EarthAn Introduction to Physical GeologyE.J. Tarbuck F.K. Lutgens D.G. Tasa

Eleventh Edition

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Sedimentary Rocks

This is a mass of chemical sedimentary rock in

Yellowstone National Park. It was formed by the

following process. Rainwater became acidic when

it absorbed carbon dioxide in the air. As the

water seeped beneath the surface, it dissolved

calcite in the limestone bedrock. Eventually,

Yellowstone’s underground plumbing returned

the water, now saturated with calcium

carbonate, to the surface as a hot spring.

When the water emerged, some carbon

dioxide escaped into the air, triggering the

deposition of the rock seen here. (Photo by

Ross Davidson/Alamy Images)

Question 1 Did this rock have a

biochemical origin or an inorganic origin?

Question 2 Is the rock most likely chert or

limestone? Explain.

Question 3 Name the particular variety of

this rock. What figure in this chapter provides

another example?

formed, a bed of bituminous coal may be only 1/10 as thick.

• Formation of anthracite coal. Lignite and bi-tuminous coals are sedimentary rocks. However, when sedimentary layers are subjected to the fold-ing and deformation associated with mountain building, the heat and pressure cause a further loss of volatiles and water, thus increasing the con-centration of fixed carbon. This metamorphoses bituminous coal into anthracite, a very hard, shiny, black metamorphic rock. Although anthracite is a clean-burning fuel, only a relatively small amount

is mined. Anthracite is not widespread and is more difficult and expensive to extract than the rela-tively flat-lying layers of bituminous coal.

5 CONCEP T CHECKS

1. What is the “raw material” for coal? Under what

circumstances does it accumulate?

2. Outline the successive stages in the formation of coal.

EYE on Earth

In 2010, the United States produced nearly 1.1 billion tons of coal. It willlikely remain a major source of energy for many years to come.In 2010, the United States produced nearly 1.1 billion tons of coal. It willlikely remain a major source of energy for many years to come.

Wyoming 442,522 40.8%

West Virginia

Kentucky

Pennsylvania

Montana

Texas

Indiana

Illinois

North Dakota

Ohio

135,577

104,391

58,031

44,732

41,635

35,333

33,159

28,949

27,269

12.5%

9.6%

5.3%

4.1%

3.8%

3.3%

3.1%

2.7%

2.5%

WY

MT ND

TX

IL INOH

PA

WV

KY

CoalCoalA Major Energy SourceA Major Energy SourceGE

O GR

AP

HIC

S

What type of coal is mined?

Renewables

Liquid biofuels

Natural gas

Nuclear

Coal

Oil and otherliquids

(QU

ADR

ILLI

ON

BTU

PER

YEA

R)

HISTORY PROJECTIONS

1980

25%

9%

21%

37%

11%4%

25%

9%

20%

32%

1%7%

1990 2000 2020 2035

125

100

75

50

25

02010

The top 10 coal-producing states in 2010(thousands of tons/percentage of total)

WYOMING

47% 45%

7%0.2%

the United States?Where is coal mined in

as a source of energy? How important is coal

Wyoming is far and away the largest producer of coal in the United States. With nearly 41% of U.S. production, Wyoming is home to the eight largest mines in the country.

Subbituminous coalis 35 to 45% carbon. Large quantitiesoccur in thick beds near the surface,

resulting in low mining costs.Wyoming is the majorsource of this type.

Anthracite coalhas the highest carbon content—86

to 97%. It is relatively rare in theUnited States and is more difficult to

mine. All of the mines are innortheastern Pennsylvania.

Lignite coalhas a low carbon content of 25 to

35% and therefore the lowest energycontent of the four types. Texas and

North Dakota are the leadingproducers.

Bituminous coalCoal classified as bituminous has thewidest range of carbon content—45

to 86%. West Virginia leadsproduction, followed by Kentucky and

Pennsylvania.

In 2010 coal represented 21% of total U.S. energy production. The vast bulk was used togenerate 45% of U.S. electricity. The U.S. Energy Information Administration projectsthat coal will be a very important energy source for decades to come.

Glow Images

Sedimentary Rocks

driven out. Because sands and other coarse sediments are less compressible, compaction is most significant as a lith-ification process in fine-grained sedimentary rocks.

Cementation The most important process by which sediments are converted to sedimentary rock is cementation. It is a change that involves the crystalliza-tion of minerals among the individual sediment grains. Groundwater carries ions in solution. Gradually, the crys-tallization of new minerals from these ions takes place in the pore spaces, cementing the clasts together. Just as the amount of pore space is reduced during compaction, the addition of cement into a sedimentary deposit reduces its porosity as well.

Calcite, silica, and iron oxide are the most common cements. It is often a relatively simple matter to identify the cementing material. Calcite cement will effervesce with dilute hydrochloric acid. Silica is the hardest cement and thus produces the hardest sedimentary rocks. An orange or dark-red color in a sedimentary rock means that iron oxide is present.

Most sedimentary rocks are lithified by means of compaction and cementation. However, some initially form as solid masses of intergrown crystals rather than beginning as accumulations of separate particles that later become solid. Other crystalline sedimentary rocks do not begin that way but are transformed into masses of inter-locking crystals sometime after the sediment is deposited.

For example, with time and burial, loose sediment consisting of delicate calcium carbonate–rich skeletal debris may be recrystallized into a relatively dense crys-talline limestone. Because crystals grow until they fill all the available space, pore spaces are frequently lacking in crystalline sedimentary rocks. Unless the rocks later develop joints and fractures, they will be relatively im-permeable to fluids such as water and oil.

6 CONCEP T CHECKS

1. What is diagenesis?

2. Compaction is most important as a lithification process

with which sediment size?

3. List three common cements. How might each be

identified?

DiagenesisBurial promotes diagenesis because as sediments are buried, they are subjected to increasingly higher temper-atures and pressures. Diagenesis occurs within the upper few kilometers of Earth’s crust at temperatures that are generally less than 150° to 200°C (300° to 400°F). Beyond this somewhat arbitrary threshold, metamor-phism is said to occur.

One example of diagenetic change is recrystalliza-tion, the development of more stable minerals from less stable ones. For example, the mineral aragonite is the less stable form of calcium carbonate (CaCO3). Aragonite is secreted by many marine organisms to form shells and other hard parts, such as the skeletal structures produced by corals. In some environments, large quantities of these solid materials accumulate as sediment. As burial takes place, aragonite recrystallizes to the more stable form of calcium carbonate, calcite, the main constituent in the sedimentary rock limestone.

Another example of diagenesis is provided in the preceding discussion of coal. It involves the chemical alteration of organic matter in an oxygen-poor environ-ment. Instead of completely decaying, as would occur in the presence of oxygen, the organic matter is slowly transformed into solid carbon.

LithificationDiagenesis includes lithification, the processes by which unconsolidated sediments are transformed into solid sed-imentary rocks (lithos 5 stone, fic 5 making). Basic lith-ification processes include compaction and cementation.

Compaction The most common physical diagenetic change is compaction. As sediment accumulates, the weight of overlying material compresses the deeper sedi-ments. The deeper a sediment is buried, the more it is compacted and the firmer it becomes. As the grains are pressed closer and closer, there is considerable reduction in pore space (the open space between particles). For ex-ample, when clays are buried beneath several thousand meters of material, the volume of the clay layer may be re-duced by as much as 40 percent. As pore space decreases, much of the water that was trapped in the sediments is

6 Turning Sediment into Sedimentary Rock: Diagenesis and Lithification

A great deal of change can occur to sediment from the time it is deposited until it becomes a sedi-mentary rock and is subsequently subjected to the temperatures and pressures that convert it to metamorphic rock. The term diagenesis (dia 5 change, genesis 5 origin) is a collective term for all the chemical, physical, and biological changes that take place after sediments are deposited and during and after lithification.

Sedimentary Rocks

that display a clastic texture consist of discrete fragments and particles that are cemented and compacted together. Although cement is present in the spaces between par-ticles, these openings are rarely filled completely. All detrital rocks have a clastic texture. In addition, some chemical sedimentary rocks exhibit this texture. For example, coquina, the limestone composed of shells and shell fragments, is obviously as clastic as a conglomerate or sandstone. The same applies for some varieties of oo-litic limestone.

Some chemical sedimentary rocks have a nonclastic or crystalline texture, in which the miner-als form a pattern of interlocking crystals. The crys-tals may be microscopically small or large enough to

As is the case with many (perhaps most) classifications of natural phenomena, the categories presented in Figure 21 are more rigid than the actual state of nature. In reality, many of the sedimentary rocks classified into the chemi-cal group also contain at least small quantities of detrital sediment. Many limestones, for example, contain varying amounts of mud or sand, giving them a “sandy” or “shaly” quality. Conversely, because practically all detrital rocks are cemented with material that was originally dissolved in water, they too are far from being “pure.”

Texture is a part of sedimentary rock classification. There are two major textures used in the classification of sedimentary rocks: clastic and nonclastic. The term clas-tic is taken from a Greek word meaning “broken.” Rocks

7 Classification of Sedimentary RocksThe classification scheme in FIGURE 21 divides sedimentary rocks into major groups: detrital on the left side and chemical/organic on the right. Further, we can see that the main criterion for subdividing the detrital rocks is particle size, whereas the primary basis for distinguishing among different rocks in the chemical group is their mineral composition.

FIGURE 21 Identification of

sedimentary rocks

The main criterion for nam-

ing detrital rocks is particle

size. The primary basis for

naming chemical and or-

ganic sedimentary rocks is

their composition.

ClasticTexture(particle size)

Sediment Name Rock Name

Coarse(over 2 mm)

Gravel(Rounded particles)

Gravel(Angular particles)

Medium(1/16 to 2 mm)

Sand

Conglomerate

Breccia

Sandstone(Arkose)*

Fine(1/16 to

1/256 mm)

Very ine(less than

1/256 mm)

Mud

Mud

Siltstone

Shale orMudstone

Coquina

Chalk

Chert (light colored)Flint (dark colored)

Jasper (red)Agate (banded)

Rock Gypsum

Rock Salt

Bituminous Coal

Calcite, CaCO3

Quartz, SiO2

GypsumCaSO4•2H2O

Halite, NaCl

Altered plantfragments

Nonclastic:Fine to coarse

crystalline

CrystallineLimestone

Clastic: Visibleshells and shell

fragments looselycemented

Clastic: Various sizeshells and shell

fragments cementedwith calcite cement

Clastic: Microscopicshells and clay

Nonclastic: Very ine crystalline

Nonclastic: Fine tocoarse crystalline

Nonclastic: Fine tocoarse crystalline

Nonclastic:Fine-grained

organic matter

FossiliferousLimestone

Biochemical

Limestone

TextureComposition Rock Name

Detrital Sedimentary Rocks Chemical and Organic Sedimentary Rocks

Travertine

*If abundant feldspar is present the rock is called Arkose.

Sedimentary Rocks

actually have originated as detrital deposits. In these instances, the particles probably consisted of shell fragments and other hard parts rich in calcium car-bonate or silica. The clastic nature of the grains was subsequently obliterated or obscured because the particles recrystallized when they were consolidated into limestone or chert.

Nonclastic rocks consist of intergrown crystals, and some may resemble igneous rocks, which are also crystalline. The two rock types are usually easy to distinguish because the minerals contained in nonclastic sedimentary rocks are quite unlike those found in most igneous rocks. For example, rock salt, rock gypsum, and some forms of limestone consist of

intergrown crystals, but the minerals within these rocks (halite, gypsum, and calcite) are seldom as-sociated with igneous rocks.

be visible without magnification. Common examples of rocks with nonclastic textures are those deposited when saline water evaporates (FIGURE 22). The materi-als that make up many other nonclastic rocks may

Close up

FIGURE 22 Rock salt

Like other evaporites, rock

salt has a nonclastic texture

because it is composed of

intergrown crystals. (Photos

by E. J. Tarbuck)

7 CONCEP T CHECKS

1. What is the primary basis for distinguishing (naming)

different chemical sedimentary rocks? How is the

naming of detrital rocks different?

2. Distinguish between clastic and nonclastic. Which

texture is associated with all detrital rocks?

These wave-eroded cliffs

along the Great Ocean

Road in Victoria, Australia,

are composed of limestone.

(Photo by Emanuele Ciccomartino/

AGE Fotostock)