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									                                                                 Sedimentary Rocks                       6
Sedimentary Rocks begins with a detailed examination of the various detrital and chemical sedimentary rocks,
including shale, sandstone, conglomerate, limestone, dolostone, chert, and coal, as well as several evaporites.
Having examined the types of rocks, the ways that sediment becomes rock are investigated. Following a look
at the classification of sedimentary rocks and sedimentary structures, the chapter concludes with a discussion
of the mineral and energy resources that are associated with these rocks.

Learning Objectives

After reading, studying, and discussing the chapter, students should be able to:

         Briefly discuss how sediment is turned into sedimentary rock.
         Explain and briefly define the major types of detrital sedimentary rocks.
         Explain and briefly define the major types of chemical sedimentary rocks.
         List and briefly discuss the major sedimentary depositional environments.
         Discuss the single most common characteristic feature of sedimentary rocks.
         Describe the two broad groups of nonmetallic mineral resources.
         List the energy resources that are associated with sedimentary rocks.

Chapter Summary

 Sedimentary rock consists of sediment that, in most cases, has been lithified into solid rock by the processes
of compaction and cementation. Sediment has two principal sources: (1) as detrital material, which originates
and is transported as solid particles from both mechanical and chemical weathering, which, when lithified,
forms detrital sedimentary rocks; and (2) from soluble material produced largely by chemical weathering,
which, when precipitated, forms chemical sedimentary rocks.

 Diagenesis refers to all of the physical, chemical, and biological changes that occur after sediments are
deposited and during and after the time they are turned into sedimentary rock. Burial promotes diagenesis.
Diagenesis includes lithification.

 Lithification refers to the processes by which unconsolidated sediments are transformed into solid
sedimentary rock. Most sedimentary rocks are lithified by means of compaction and/or cementation.
Compaction occurs when the weight of overlying materials compresses the deeper sediments. Cementation,
the most important process by which sediments are converted to sedimentary rocks, occurs when soluble
cementing materials, such as calcite, silica, and iron oxide, are precipitated onto sediment grains, fill open
spaces, and join the particles. Although most sedimentary rocks are lithified by compaction or cementation,
certain chemical rocks, such as the evaporites, initially form as solid masses of intergrown crystals.

 Particle size is the primary basis for distinguishing among various detrital sedimentary rocks. The size of
the particles in a detrital rock indicates the energy of the medium that transported them. For example, gravels
are moved by swiftly flowing rivers, whereas less energy is required to transport sand. Common detrital
sedimentary rocks include shale (silt-and clay-sized particles), sandstone, and conglomerate (rounded gravel-
sized particles) or breccia (angular gravel-sized particles).

46        CHAPTER 6

 Precipitation of chemical sediments occurs in two ways: (1) by inorganic processes, such as evaporation
and chemical activity; or by (2) organic processes of water-dwelling organisms that produce sediments of
biochemical origin. Limestone, the most abundant chemical sedimentary rock, consists of the mineral calcite
(CaCO3) and forms either by inorganic means or as the result of biochemical processes. Inorganic limestones
include travertine, which is commonly seen in caves, and oolitic limestone, consisting of small spherical
grains of calcium carbonate. Other common chemical sedimentary rocks include dolostone (composed of the
calcium-magnesium carbonate mineral dolomite), chert (made of microcrystalline quartz), evaporites (such as
rock salt and rock gypsum), and coal (lignite and bituminous).

 Sedimentary rocks can be divided into two main groups: detrital and chemical. All detrital rocks have a
clastic texture, which consists of discrete fragments and particles that are cemented and compacted together.
The main criterion for subdividing the detrital rocks is particle size. Common detrital rocks include
conglomerate, sandstone, and shale. The primary basis for distinguishing among different rocks in the
chemical group is their mineral composition. Some chemical rocks, such as those deposited when seawater
evaporates, have a nonclastic texture in which the minerals form a pattern of interlocking crystals. However,
in reality, many of the sedimentary rocks classified into the chemical group also contain at least small
quantities of detrital sediment. Common chemical rocks include limestone, rock gypsum, and coal (e.g.,
lignite and bituminous).

 Sedimentary environments are those places where sediment accumulates. They are grouped into
continental, marine, and transitional (shoreline) environments. Each is characterized by certain physical,
chemical, and biological conditions. Because sediment contains clues about the environment in which it was
deposited, sedimentary rocks are important in the interpretation of Earth’s history.

 Layers, called strata, or beds, are probably the single most characteristic feature of sedimentary rocks.
Other features found in some sedimentary rocks, such as ripple marks, mud cracks, cross-bedding, graded
bedding, and fossils, also provide clues to past environments.

 Earth materials that are not used as fuels or processed for the metals they contain are referred to as
nonmetallic resources. Many are sediments or sedimentary rocks. The two broad groups of nonmetallic
resources are building materials and industrial minerals. Limestone, perhaps the most versatile and widely
used rock of all, is found in both groups.

 Coal, petroleum, and natural gas, the fossil fuels of our modern economy, are all associated with
sedimentary rocks. Coal originates from large quantities of plant remains that accumulate in an oxygen-
deficient environment, such as a swamp. More than 70 percent of present-day coal usage is for the generation
of electricity. Air pollution from the sulfur oxide gases that form from burning most types of coal is a major
environmental problem. Through a series of complex reactions in the atmosphere, the sulfur oxides produced
when coal is burned are converted to sulfuric acid, which falls to Earth’s surface in rain or snow.

 Oil and natural gas, which commonly occur together in the pore spaces of some sedimentary rocks, consist
of various hydrocarbon compounds (compounds made of hydrogen and carbon) mixed together. Although
petroleum formation is complex and not completely understood, it is associated with the accumulation of
sediment in ocean areas that are rich in plant and animal remains that become buried and isolated in an
oxygen-deficient environment. As the mobile petroleum and natural gas form, they migrate and accumulate in
adjacent permeable beds such as sandstone. If the upward migration is halted by an impermeable rock layer,
                                                                              Sedimentary Rocks            47

referred to as a cap rock, a geologic environment develops that allows for economically significant amounts
of oil and gas to accumulate underground in what is termed an oil trap, develops. The two basic conditions
common to all oil traps are (1) a porous, permeable reservoir rock that will yield petroleum and/or natural gas
in sufficient quantities, and (2) a cap rock.

Chapter Outline___________________________________________________________________

I. What is a sedimentary rock?                                      1. Detrital rocks - material is solid
    A. Products of mechanical and chemical                             particles
        weathering                                                  2. Chemical rocks - material that was
    B. Account for about 5 percent (by volume)                         once in solution
        of Earth’s outer 16 km (10 miles)
    C. Contain evidence of past environments               IV. Detrital sedimentary rocks
        1. Provide information about sediment                  A. Chief constituents
             transport                                             1. Clay minerals
        2. Often contain fossil                                    2. Quartz
    D. Economic importance                                         3. Others
        1. Coal                                                         a. Feldspars
        2. Petroleum and natural gas                                    b. Micas
        3. Sources of iron, aluminum, and                      B. Particle size is used to distinguish among
             manganese                                             the various types of detrital rocks
                                                               C. Common detrital sedimentary rocks (in
II. Turning sediment into sedimentary rock                         order of increasing particle size)
     A. A great deal of change can occur to                         1. Shale
         sediment after it is deposited                                  a. Thin layers (lamina)
     B. Diagenesis - all chemical, physical, and                         b. Most common sedimentary rock
         biological changes that take place after                   2. Sandstone
         sediments are deposited                                         a. Form in a variety of environments
         1. Occurs within the upper few                                  b. Sorting, shape, and composition
             kilometers of Earth’s crust                                     of the grains can be used to
         2. Includes:                                                        interpret the rock’s history
             a. Recrystallization – development                          c. Quartz is the most predominant
                  of more stable minerals from less                          mineral
                  stable ones                                       3. Conglomerate and breccia
             b. Lithification - unconsolidated                           a. Conglomerate consists largely of
                  sediments are transformed into                             rounded gravels
                  solid sedimentary rocks by                             b. Breccia composed mainly of large
                 1. Compaction                                               angular particles
                 2. Cementation by the materials
                       a. Calcite and/or                   V. Chemical sedimentary rocks
                       b. Silica                               A. Consist of precipitated material that was
             c. Iron oxide                                         once in solution
                                                               B. Precipitation of material occurs in two
III. Types of sedimentary rocks                                    ways
     A. Material originates from mechanical                        1. Inorganic processes
        and/or chemical weathering                                 2. Organic processes (biochemical
     B. Rock types are based on the source of the                      origin)
        material                                               C. Common chemical sedimentary rocks
48            CHAPTER 6

         1.    Limestone                                            a. Pattern of interlocking crystals
                 a. Most abundant chemical rock                     b. May resemble igneous rocks
                 b. Composed chiefly of the
                     mineral calcite                  VII. Sedimentary environments
                 c. Marine biochemical limestones         A. A geographic setting where sediment is
                     1. Coral reefs                           accumulating
                     2. Coquina                           B. Determines the nature of the sediments
                     3. Chalk                                 that accumulate
                 d. Inorganic limestones                  C. Types of sedimentary environments
                     1. Travertine                            1. Continental
                     2. Oolitic limestone                     2. Marine
         2.    Dolostone                                      3. Transitional (shoreline)
         3.    Chert
                 a. Made of microcrystalline          VIII. Sedimentary structures
                     quartz                                A. Provide information useful in the
                 b. Forms                                      interpretation of Earth history
                     1. Flint                              B. Types
                     2. Jasper (banded form called             1. Strata, or beds (most characteristic
                         agate)                                    feature of sedimentary rocks)
         4.    Evaporites                                      2. Bedding planes that separate strata
                 a. Evaporation triggers deposition            3. Cross-bedding
                     of chemical precipitates                  4. Graded beds
                 b. Examples                                   5. Ripple marks
                     1. Rock salt                              6. Mud cracks
                     2. Rock gypsum                            7. Fossils
         5.    Coal
                 a. Different from other rocks –      IX.     Nonmetallic mineral resources from
                     made of organic material                 sedimentary rocks
                 b. Stages in coal formation                A. Use of the word “mineral” is very broad
                     1. Plant material                      B. Two common groups
                     2. Peat                                   1. Building materials
                     3. Lignite                                   a. Natural aggregate (crushed stone,
                     4. Bituminous                                    sand, and gravel)
                                                                  b. Gypsum (plaster and wallboard)
VI. Classification of sedimentary rocks                           c. Clay (tile, bricks, and cement)
    A. Classified according to the type of                     2. Industrial minerals
        material                                                  a. Corundum
    B. Two major groups                                           b. Garnet
        1. Detrital
        2. Chemical                                    X.    Energy resources from sedimentary rocks
    C. Two major textures used in the                       A.   Coal
        classification of sedimentary rocks                    1. Formed mostly from plant material
        1. Clastic                                             2. Along with oil and natural gas, coal
                a. Discrete fragments and
                                                                  is commonly called a fossil fuel
                                                               3. The major fuel used in power plants
              b. All detrital rocks have a clastic
                                                                  to generate electricity
          1. Nonclastic                                        4. Problems with coal use
                                                                               Sedimentary Rocks            49

                Environmental damage from
               a.                                                   4.   A geologic environment that allows
                mining                                                   for economically significant amounts
            b. Air pollution                                             of oil and gas to accumulate
    C. Oil and natural gas                                               underground is termed an oil trap
      1. Oil and natural gas, consisting of                                a. Two basic conditions for an oil
          various hydrocarbon compounds, are                                   trap
          found in similar environments                                       1. Porous, permeable reservoir
      2. Derived from the remains of marine                                       rock
          plants and animals                                                  2. Impermeable cap rock, such
      3. Formation is complex and not                                             as shale
          completely understood                                            b. Cap rock keeps the mobile oil
                                                                               and gas from escaping at the

Answers to the Review Questions

1. In the Earth's crust, igneous rocks exceed sedimentary rocks in volume. Neither rock type is evenly
   distributed. In the interiors of continents, sedimentary rocks occur as thin veneers covering over much
   larger volumes of igneous and metamorphic rocks deeper in the crust. Ocean basin rocks are mainly
   igneous with very thin covers of sediments. Sedimentary strata many kilometers in thickness accumulate
   only in relatively restricted basins along the edges of continents or in deep rift basins where continental
   blocks are splitting apart.

2. Diagenesis refers to the collective chemical, physical, and biological changes that take place following
   deposition and during and after lithification of sediments. Typically, diagenesis occurs within the upper
   few kilometers of the Earth’s crust at temperatures below 200ºC. An example of diagenesis is
   recrystallization, which involves the formation of more stable minerals from less stable ones.

3. Compaction is most important as a diagenetic process in fine-grained sedimentary rocks such as shales or
   mudstones. Sand and other coarse sediments are much less compressible so that compaction is not as
   significant as it is in finer sediments.

4. The three common cementing agents for sedimentary rocks are calcite, silica (quartz), and iron oxide. They
   are relatively easy to distinguish from one another by their physical or chemical properties. Calcite will
   readily effervesce with dilute hydrochloric acid, silica is much harder than the other two, and a distinctive
   orange to dark red color normally identifies iron oxide.

5. The most common minerals in sedimentary rocks are clay minerals and quartz. Clay minerals are the most
   abundant product of the chemical weathering of silicate minerals, especially feldspar. Quartz is abundant
   because it is extremely durable and very resistant to chemical weathering.

6. Particle size is the primary basis for distinguishing among various detrital sedimentary rocks.

7. Shale is made of very thin laminae and often is weak because it is poorly cemented and not well lithified.
   As a consequence, shale usually crumbles quite easily.
50         CHAPTER 6

 8. Conglomerate consists largely of rounded gravel particles. If the large particles are angular rather than
    rounded, the rock is called breccia. The degree of rounding of the large particles is related to the distance
    that they have been transported.

 9. The two different categories of chemical sedimentary rocks are defined on the basis of the mechanism of
    formation of the material. Inorganic processes, such as evaporation and chemical activity, can produce
    chemical sediments. On the other hand, organic (life) processes of water-dwelling organisms also form
    chemical sediments, said to be of biochemical origin.

10. Evaporite deposits are chemical sedimentary rocks formed as water becomes chemically saturated and
    salt deposition begins. Common evaporites include rock salt and rock gypsum.

11. Bituminous coal has higher carbon content, generates more heat, and is more compact than lignite.
    Anthracite coal is a metamorphic rock with a higher carbon content than bituminous coal. It is a very
    hard, shiny black, clean burning fuel.

12. (a) gypsum; (b) mudstone; (c) graywacke; (d) limestone; (e) chert; (f) oolitic limestone

13. Some chemical sedimentary rocks are clastic, for example coquina. The non-clastic chemical rocks are
    either made of large crystals that formed as water evaporated (rock salt and rock gypsum) or shell
    fragments that were subsequently obliterated or obscured as the particles recrystallized when they were
    consolidated into limestone or chert.

14. Rocks that display a clastic texture consist of discrete fragments and particles that are cemented or
    compacted together. Non-clastic rocks consist of intergrown crystals. A clastic texture is common to all
    detrital sedimentary rocks.

15. The single most characteristic feature of sedimentary rocks are layers, called strata, or beds.

16. Cross-bedding, most characteristic of sand dunes, is when a bed of sedimentary rock contains layers
    within it that are inclined to the horizontal. Graded bedding is a special type of bedding where the
    particles within a single sedimentary layer gradually change from coarse at the bottom to fine at the top.

17. The two broad groups of nonmetallic resources are building materials (limestone, aggregate, gypsum for
    plaster and wallboard, and clay for tile and bricks) and industrial minerals (fluorite, limestone, corundum,
    garnet, and sylvite).

18. Coal mining is hazardous work with its associated roof falls, gas explosions, and working with heavy
    equipment. Furthermore, air pollution is a major problem associated with the burning of coal because of
    the release of carbon dioxide, sulfur, and other materials during combustion.

19. Oil traps are confined porous and permeable zones, usually in sedimentary rocks that trap and retain
    natural gas and petroleum. All have two conditions in common; a porous, permeable reservoir rock, and a
    cap rock that is virtually impermeable to oil and gas.
                                                               Sedimentary Rocks       51

PowerPoint slides for each chapter of Essentials of Geology accompany the DIGIT disc (ISBN
013-008171-X). There are instructions in the CD's ReadMe file for embedding QuickTime for
use in the PowerPoint slides.

For additional resources, visit       the   Essentials   of   Geology   Home    Page    at
52       CHAPTER 6


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