Section Assessment

Section Summary

I The processes of weathering, erosion, deposition, and lithification form sedimentary rocks.

I Clastic sediments are rock and mineral fragments produced by weathering and erosion. They are classified based on particle size.

I Sediments are lithified into rock by the processes of compaction and cementation.

I Fossils are the remains or other evidence of once-living things that are preserved in sedimentary rocks.

I Sedimentary rocks might contain features such as horizontal bedding, cross-bedding, and ripple marks.

Understand Main Ideas

1. IMMIflf4ldBa Describe how sediments are produced by weathering and erosion.

2. Sequence Use a flowchart to show why sediment deposits tend to form layers.

3. Illustrate the formation of graded bedding.

4. Compare temperature and pressure conditions at Earth's surface and below Earth's surface, and relate them to the process of lithification.

Think Critically

5. Evaluate this statement: It is possible for a layer of rock to show both cross-bedding and graded bedding.

6. Determine whether you are walking upstream or downstream along a dry mountain stream if you notice that the shape of the sediment is getting more angular as you continue walking. Explain.

CES^^Earth Science

7. Imagine you are designing a display for a museum based on a sedimentary rock that contains fossils of corals and other ocean-dwelling animals. Draw a picture of what this environment might have looked like, and write the accompanying description that will be posted next to the display.

140 Chapter 6 • Sedimentary and Metamorphic Rocks llFn Self-Check Quiz glencoe.com

llFn Self-Check Quiz glencoe.com

Different Beach Sediment Sizes

Figure 6.10 Conglomerates and breccias are made of sediments that have not been transported far from their sources.

Infer the circumstances that might cause the types of transport necessary for each to form.

Types of Sedimentary Rocks

N4TTHa Sedimentary rocks are classified by their mode of formation.

Real-World Reading Link If you have ever walked along the beach or along a riverbank, you might have noticed different sizes of sediments. The grain size of the sediment determines what type of sedimentary rock it can become.

Clastic Sedimentary Rocks

The most common sedimentary rocks, clastic sedimentary rocks, are formed from the abundant deposits of loose sediments that accumulate on Earth's surface. The word clastic comes from the Greek word klastos, meaning broken. These rocks are further classified according to the sizes of their particles. As you read about each rock type, refer to Table 6.1 on the next page, which summarizes the classification of sedimentary rocks based on grain size, mode of formation, and mineral content.

Coarse-grained rocks Sedimentary rocks consisting of gravel-sized rock and mineral fragments are classified as coarse-grained rocks, samples of which are shown in Figure 6.10. Conglomerates have rounded, gravel-sized particles. Because of its relatively large mass, gravel is transported by high-energy flows of water, such as those generated by mountain streams, flooding rivers, some ocean waves, and glacial meltwater. During transport, gravel becomes abraded and rounded as the particles scrape against one another. This is why beach and river gravels are often well rounded. Lithification turns these sediments into conglomerates.

In contrast, breccias are composed of angular, gravel-sized particles. The angularity indicates that the sediments from which they formed did not have time to become rounded. This suggests that the particles were transported only a short distance and deposited close to their source. Refer to Table 6.1 to see how these rocks are named.

Figure 6.10 Conglomerates and breccias are made of sediments that have not been transported far from their sources.

Infer the circumstances that might cause the types of transport necessary for each to form.

Breccia

Section 2 • Types of Sedimentary Rocks 141

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Breccia

Section 2 • Types of Sedimentary Rocks 141

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Table 6.1

Classification of Sedimentary Rocks

Interactive Table To explore more about sedimentary rock formation, visit glencoe.com.

Classification

Texture/Grain Size

Composition

Rock Name

Clastic

coarse (> 2 mm)

Fragments of any rock type—quartz, chert 1 rounded and quartzite common J angular

conglomerate breccia

medium (1/16 mm to 2 mm)

quartz and rock fragments quartz, k-spar and rock fragments

sandstone arkose

fine (1/256 mm-1/16 mm)

quartz and clay

siltstone

very fine (< 1/256 mm)

quartz and clay

shale

Biochemical

microcrystalline with conchoidal fracture

calcite (CaCO3)

micrite

abundant fossils in micrite matrix

calcite (CaCO3)

fossiliferous limestone

oolites (small spheres of calcium carbonate)

calcite (CaCO3)

oolitic limestone

shells and shell fragments loosely cemented

calcite (CaCO3)

coquina

microscopic shells and clay

calcite (CaCO3)

chalk

variously sized fragments

highly altered plant remains, some plant fossils

coal

Chemical

fine to coarsely crystalline

calcite (CaCO3)

crystalline limestone

fine to coarsely crystalline

dolomite (Ca,Mg)CO3 (will effervesce if powdered)

dolostone

very finely crystalline

quartz (SiO2)—light colored —dark colored

chert flint

fine to coarsely crystalline

gypsum (CaSO4 • 2H2O)

rock gypsum

fine to coarsely crystalline

halite (NaCl)

Academic vocabulary

Reservoir a subsurface area of rock that has enough porosity to allow for the accumulation of oil, natural gas, or water

The newly discovered reservoir contained large amounts of natural gas and oil

Medium-grained rocks Stream and river channels, beaches, and deserts often contain abundant sand-sized sediments. Sedimentary rocks that contain sand-sized rock and mineral fragments are classified as medium-grained clastic rocks. Refer to Table 6.1 for a listing of rocks with sand-sized particles. Sandstone usually contains several features of interest to scientists. For example, because ripple marks and cross-bedding indicate the direction of current flow, geologists use sandstone layers to map ancient stream and river channels.

Another important feature of sandstone is its relatively high porosity. Porosity is the percentage of open spaces between grains in a rock. Loose sand can have a porosity of up to 40 percent. Some of these open spaces are maintained during the formation of sandstone, often resulting in porosities as high as 30 percent. When pore spaces are connected to one another, fluids can move through sandstone. This feature makes sandstone layers valuable as underground reservoirs of oil, natural gas, and groundwater.

Fine-grained rocks Sedimentary rocks consisting of silt- and clay-sized particles are called fine-grained rocks. Siltstone and shale are fine-grained clastic rocks. These rocks represent environments such as swamps and ponds which have still or slow-moving waters. In the absence of strong currents and wave action, these sediments settle to the bottom where they accumulate in thin horizontal layers. Shale often breaks along thin layers, as shown in Figure 6.11. Unlike sandstone, fine-grained sedimentary rock has low porosity and often forms barriers that hinder the movement of groundwater and oil. Table 6.1 shows how these rocks are named.

^p Reading Check Identify the types of environments in which finegrained rocks form.

Chemical and Biochemical Sedimentary Rocks

The formation of chemical and biochemical rocks involves the processes of evaporation and precipitation of minerals. During weathering, minerals can be dissolved and carried into lakes and oceans. As water evaporates from the lakes and oceans, the dissolved minerals are left behind. In arid regions, high evaporation rates can increase the concentration of dissolved minerals in bodies of water. The Great Salt Lake, shown in Figure 6.12, is an example of a lake that has high concentrations of dissolved minerals.

Chemical sedimentary rocks When the concentration of dissolved minerals in a body of water reaches saturation, crystal grains precipitate out of solution and settle to the bottom. As a result, layers of chemical sedimentary rocks form, which are called evaporites. Evaporites most commonly form in arid regions and in drainage basins on continents that have low water flow. Because little freshwater flows into these areas, the concentration of dissolved minerals remains high. Even as more dissolved minerals are carried into the basins, evaporation continues to remove freshwater and maintain high mineral concentrations. Over time, thick layers of evaporite minerals can accumulate on the basin floor, as illustrated in Figure 6.12.

Global Geology Layers
Figure 6.11 This shale was deposited in thin layers in still waters.

Figure 6.12 The constant evaporation from a body of salt water results in precipitation of large amounts of salt. This process has been occurring in the Great Salt Lake in Utah for approximately 18,000 years.

Figure 6.12 The constant evaporation from a body of salt water results in precipitation of large amounts of salt. This process has been occurring in the Great Salt Lake in Utah for approximately 18,000 years.

Geology SaltLimestone Formed From Evaporation
Figure 6.13 Limestone can contain many different fossil organisms. Geologists can interpret where and when the limestone formed by studying the fossils within the rock.

Biochemical sedimentary rocks Biochemical sedimentary | rocks are formed from the remains of once-living things. The most | abundant of these rocks is limestone, which is composed primarily | of calcite. Some organisms that live in the ocean use the calcium | carbonate that is dissolved in seawater to make their shells. When J

)Al these organisms die, their shells settle to the bottom of the ocean ~ and can form thick layers of carbonate sediment. During burial J and lithification, calcium carbonate precipitates out of the water, f crystallizes between the grains of carbonate sediment, and forms f limestone. |

Limestone is common in shallow water environments, such as s those in the Bahamas, where coral reefs thrive in 15 to 20 m of water just offshore. The skeletal and shell materials that are currently accumulating there will someday become limestone as well. Many types of limestone contain evidence of their biological origin in the form of abundant fossils. As shown in Figure 6.13, these fossils can range from large-shelled organisms to microscopic, unicellular organisms. Not all limestone contains fossils. Some limestone has a crystalline texture, some consists of tiny spheres of carbonate sand, and some is composed of fine-grained carbonate mud. These are listed in Table 6.1.

Other organisms use silica to make their shells. These shells form sediment that is often referred to as siliceous ooze because it is rich in silica. Siliceous ooze becomes lithified into the sedimentary rock chert, which is also listed in Table 6.1.

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Responses

  • brhane
    How do you make a flowchart to show why sediment deposits tend to form layers?
    8 years ago
  • charlene dowdell
    Why sediment deposits tend to be in layers flow chart?
    8 years ago
  • louie
    Are you walking upstream or downstream if the sediments are getting more angular while walking?
    5 years ago

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