Sedimentation: Rocks Formed by Surface Processes resources (by Author)/… · the clastic particles and dissolved ions that compose sediment. Water, wind, and ice trans- port the

Chapter Summary

• Plate tectonic processes play an important role in producing depressions (basins) inwhich sediments accumulate. Sedimentary basins result from rifting, thermal sag, andflexure of the lithosphere.

• The sedimentary stages of the rock cycle involve the overlapping processes of weathering,erosion, transportation, deposition, burial, and diagenesis. Weathering and erosion producethe clastic particles and dissolved ions that compose sediment. Water, wind, and ice trans-port the sediment downhill to where it is deposited. Burial and diagenesis harden sedimentsinto sedimentary rocks via pressure, heat, and chemical reactions.

• The two major types of sediments are clastic and chemical/biochemical. Clastic sedi-ments are formed from rock particles and mineral fragments. Chemical and biochemicalsediments originate from the ions dissolved in water. Chemical and biochemical reac-tions precipitate these dissolved ions from solution.

• Understanding the characteristics of sediments and modern sedimentary environmentsprovides a basis for reconstructing past environmental conditions using the rock record.Sedimentary structures like bedding, ripple marks, and mud cracks, provide importantclues about the sedimentary environment.

• Diagenesis transforms sediment into sedimentary rock. Burial promotes this transfor-mation by subjecting sediments to increasing heat and pressure. Cementation is espe-cially important in the lithification of clastic sediments.

• The classification of clastic sediments and sedimentary rocks is based primarily on thesize of the grains within the rock. The name of chemical and biochemical sediments andsedimentary rock is based primarily on their composition.

Learning ObjectivesFocus your instruction on clear learning objectives. In this section we provide a sampling of possibleobjectives for this chapter. No class could or should try to accomplish all of these objectives. Chooseobjectives based on your analysis of your class. Refer to Chapter 1: Learning Objectives—How toDefine Your Goals for Your Course in the Instructional Design section of this manual for thoughts andideas about how to go about such an analysis.

Knowledge

• Understand how sediments are produced, transported, deposited, and transformed into rock.

• Know the major types of sediments and sedimentary rocks and how they are classified.

• Know common characteristics of sediments, sedimentary rocks, and sedimentary environ-ments.

• Understand how the accumulation of sediments is linked to plate tectonics.

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Geology Skills/Applications/Attitudes

• Give a description of sedimentary rock(s) and associated sedimentary structuresand fossils, reconstruct characteristics of the source region, agent of transport,distance of transport, and environment of deposition.

General Education Skills

• Write a one-page paper interpreting a local sedimentary rock formation that con-tains some interesting sedimentary structures and fossils. Write the paper asthough it was to be published in a local newspaper for citizens of your communi-ty to read. (writing/critical thinking)

Freshman Survival Skills• Right after the first exam is a make or break time for students who received low

scores. Given proper assistance most students are capable of dramatic improve-ment. This is a good time to make interventions with students who come in forhelp and to emphasize freshman survival skills. For example, repeat the freshmansurvival skills information presented earlier in the course.

• Provide individual help sessions for students after the first exam (see TeachingTips). Encourage students to bring their notes to help sessions.

• Encourage students who want to improve their scores to begin using the StudentStudy Guide for Understanding Earth. Emphasize the importance answering thepreview questions before lecture and working on review questions after lecture.

Sample Lecture Outline Sample lecture outlines highlight the important topics and concepts covered in the text. We sug-gest that you

Chapter 5: Sedimentation—Rocks Formed by Surface ProcessesSedimentary rocks and the rock cycle

Weathering/erosion/transportation/deposition/burial/diagenesis

Classification (based on composition and texture)SiliciclasticChemical and biochemical

Plate Tectonics and sedimentary basinsSubsidence due to rifting, thermal sag, flexure sedimentary basins

Sedimentary EnvironmentsContinentalShorelineMarineClastic vs. chemical and biochemical environments

Sedimentary StructuresStratification/beddingMud cracks, ripples, cross-bedding, graded beddingBioturbationBedding sequences

78 PART II CHAPTER 5

customize it to your own lecture before handing it out to students. At the end of each chapter outline consider adding a selection of review questions that represent a range of thinking levels.

Heat, pressure, chemical alterationCompactionCementation

Types of sedimentary rocksConglomerateSandstones: arkoses, lithic, quartz arenites, graywackeSiltstoneClaystone/ShaleLimestoneDolostoneEvaporitesPhosphoriteIron formationsCoal

Sedimentary rocks are a product ofParent rockClimateAgent and distance of transportEnvironment of deposition

Sedimentation: Rocks Formed by Surface Processes 79

Burial and Diagenesis

Diagenesis: sediments sedimentary rocks

Teaching Tips Cooperative/Collaborative Exercises and In-Class ActivitiesRefer to Chapter 4: Cooperative Learning Teaching Strategies in the Instructional Design sec-tion of this manual for general ideas about conducting cooperative learning exercises in yourclassroom.

Coop Exercise 1: Interpreting Sedimentary RocksThe question below can be the basis for a JIGSAW. Have students team up in groups of twoor three. Post the question on the board for the whole class to see. Assign just one of the four

parts of the question to each student team. Give teams three to five minutes to write down a few sentences for an answer, then review the answers for each part by calling on a member

of a team to read the answer the team formulated. Discuss the answers with the whole class by complimenting the teams for their good answers, correcting any misconceptions, and

reviewing questions students may have.

What geologic history might be inferred from each of the following rocks?

A. feldspar-rich sandstone?B. well-rounded conglomerate?C. sandstone composed almost entirely of quartz?D. limestone with abundant fossils?

Coop Exercise 2: Five-Minute WriteThe Five-Minute Write is done during the last five minutes of lecture. Ask students to puttheir names on a sheet of paper and then address the three questions per the overhead; seeadjacent sample. Start the next lecture by discussing the answers to some of the questions stu-dents had about the previous lecture.

Coop Exercise 3: Essay on SandIn the spaces provided below (1–5), indicate five erroneous statements in the following para-graph, An Essay on Sand.

1. _______________________________________________________________________

2. _______________________________________________________________________

3. _______________________________________________________________________

4. _______________________________________________________________________

5. _______________________________________________________________________

An Essay on SandNearly all naturally occurring sand on the continents is composed entirely of quartz grainsbecause feldspar, which is the most voluminous mineral group in crustal rocks, is convertedentirely to clays by chemical weathering, even in arid to semiarid environments. Sandsdeposited by rivers are the best sorted and best rounded of all sands because little abrasion orwinnowing occurs during eolian and beach transport. In general, coarser grained river sandsare better rounded than finer grained river sands because more abrasion occurs during bed-load transport than during suspension transport. Sandstone beds are generally interpreted asterrestrial (river, dune, etc.), strand line (beach, tidal bar, etc.), or shallow-marine (shelf, etc.)deposits because not even turbidity currents can transport sediment as coarse as sand to thedeep ocean floor. Moreover, the white sands of atolls and small islets built on coral reefs arecomposed chiefly of grains of calcium carbonate, whereas the black sands of beaches on vol-

80 PART II CHAPTER 5

Five-Minute Write

1. What questions do you have aboutthis lecture?

2. What did you findmost interestingabout this lecture?

3. How was this lecture relevant to you?

canic islands like Hawaii are composed largely of detritus eroded from lavas like basalt.Buried layers of sand make very poor aquifers, even though they are porous, because theirpore spaces are not interconnected and their permeability is typically too low to allow groundwater to flow through them at a suitable rate. On the other hand, quartz is readily soluble dur-ing diagenesis, so that tightly cemented sandstone beds commonly develop large caverns thatserve as vast subterranean reservoirs of ground water. Durable sand eroded from bedrock,however, is used by streams as a major abrasive tool for eroding their beds.

Answers1. By no means is all sand composed entirely of quartz, and feldspar is a common con-

stituent of many sands, especially in arid to semiarid climates where breakdown offeldspar to clays is quite incomplete during weathering and erosion.

2. Eolian and beach sands are commonly better sorted and rounded than river sands becausewinnowing is quite effective and abrasion intense in dunes and on beaches.

3. Turbidity currents do transport immense volumes of sand from shelves and deltas to thedeep seafloor, typically via submarine canyons cutting into continental slopes.

4. Sand layers are commonly excellent aquifers because their pore space forms a superblyinterconnected network between the sand grains.

5. Quartz dissolves slowly and with difficulty, and caverns form much more readily in lime-stone.

Visual Summary of Sedimentary Rocks

Homework Exercise 1: Common Sedimentary EnvironmentsUsing Figures 5.1 and 5.9 as a guide, fill in the blanks in the table below with the clastic orchemical sediment (e.g., sand, silt, mud, salts, carbonate, peat) that best matches the follow-ing environment of deposition.

Sedimentation: Rocks Formed by Surface Processes 81

Environment of Deposition Fill in the Blank

Alpine or glacial river channel

Dunes in a desert

Flood plain along a broad meander bend

River delta along a marine shoreline

Continental shelf

Deep sea adjacent to a continental shelf

Shoreline beach dunes

Tidal flats

Organic reef

The following exercises from the Student Study Guide for Understanding Earth (available in the e-Book) provide a good visual summary. You might assign all three exercises for credit. Exercise 1 deals with sedimentary environments, Exercise 2 will help students learn to classify sedimentary rocks according to grain size, and Exercise 3 links charac-teristics of sediments with sedimentary rocks.

Homework Exercise 2: Grain-sizes for Clastic Sedimentary RocksUsing the terms for sediments and rocks listed in Table 5.3, fill in the blanks with the appropriate name of the sediment and rock that matches the typical particle size of the Common Object that is given in the table below. Hint: A few answers have been filled in as a reference.

Grain Size Common Object Sediment Rock Type

Coarse Football to bus boulder gravel

Plum or lime conglomerate

Pea or bean

Coarse-ground pepper or salt sandstone

Fine-ground pepper or salt

Fine Talcum powder or baby powder

Pass around examples of the major types of sedimentary rocks during lecture.

Topics for Class Discussion

• Why is quartz a common mineral in sandstones?

• Why is shale the most common sedimentary rock?

• If plate tectonics were to grind to a halt on this planet, what predictions(hypothesis) would you advance concerning the future formation of sedimen-tary rock? Defend your reasoning.

82 PART II CHAPTER 5

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Composed largely of rock fragments

Precipitated in the environment of deposition

Important source of coal

Often formed by diagenesis chemical dolostone and phosphorite

Formed from abundant skeleton fragments of marine or lake organisms, such as coral, seashells, foraminifera

Produced by physical weathering

Produced from rapidly eroding granitic and gneissic terrains in an arid or semi-arid climate

• Socratic Questions for the Rock Cycle and Sedimentation

1. What forces of nature weather a mountain?

2. Once a particle (be it a tiny sliver or a boulder) is weathered off a moun-tain, what sort of vehicles are available to move it down the mountainslope?

3. Where (what sort of environments) may those particles be transported to?

4. As hundreds, thousands, even millions of years pass and those sedi-ments keep accumulating into layer, upon layer, upon layer what willhappen to the layers at the bottom of this “sediment dogpile?”

5. When enough pressure builds up what effect will that have on the lay-ers at the bottom?

6. With all that pressure and heat acting on them, how do you think the lay-ers of sediment may be transformed? What will it become? If the sedimentis sand what kind of sedimentary rock will it become? If the sediment is mud what kind of sed rock will it become? If it is gravel or even boulders?

Homework Exercise 3: Clastic and Chemical Sediments andSedimentary Rocks Given the descriptive statements on the left side, fill in the blanks with the appropriate sedi-ment type and rock type in the table below. A list of common sediment types are providedbelow for your reference. Use the following terms for sediment type and sedimentary rock tofill in the table below.

Sediment types Sedimentary rocksbiochemical arkose dolostone limestone sandstoneclastic chert evaporite peat shalechemical conglomerate graywacke phosphorite siltstone

In Part II, Chapter 5: Sedimentation—Rocks Formed by Surface Processes

Before Lecture: Preview Questions and Brief Answers

During Lecture: Note-Taking Tip(s)

After Lecture:

Check Your Notes

Intensive Study Session

Memory and Learning Tip–Chunking

Exam Prep:

Chapter Summary

Practice Exercises: Common Sedimentary EnvironmentsGrain Sizes for Clastic Sedimentary RocksClastic and Chemical Sediments and Sedimentary Rocks

Review questions

Sample Exercises

Exercise: Essay on Sand—homework exercise

Exercise: Sands of the World

Exercise: Interpretation of a Sandstone—a homework exercise

Sedimentation: Rocks Formed by Surface Processes 83

Teaching ResourcesStudent Study Guide Highlights (part of the Understanding Earth e-Book)

In Part I, chapters provide strategies for learning geology. Ideally, students would read thesechapters early in the course.

Chapter 1: Brief Preview of the Student Study Guide for Understanding Earth

Chapter 2: Meet the Authors

Chapter 3: How to Be Successful in Geology