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GEOLOGY 12
CHAPTER 6 SEDIMENTARY ROCKS
QUESTIONS FOR REVIEW
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ANSWERS
On what basis are clastic sediments subdivided and named?
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Clastic sediments are subdivided and named primarily on the basis of GRAIN SIZE.
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The coarsest materials are further divided by the ROUNDEDNESS or ANGULARITY of
the fragments.
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None of the names are based on mineral composition.
What is meant by saying that a sediment is well sorted? Well rounded?
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A WELL-SORTED sediment has fragments that span a narrow range of grain sizes.
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A WELL-ROUNDED sediment has fragments that have smoothly rounded contours.
Name and describe two kinds of chemical sediments.
(AMONG THE POSSIBILITIES ARE THE FOLLOWING:)
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LIMESTONE--a sediment rich in carbonate, especially calcite (calcium carbonate)
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DOLOMITE-- a sediment rich in the carbonate mineral dolomite (calcium magnesium
carbonate)
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ROCK SALT--an evaporite sediment composed primarily of halite (sodium chloride)
Discuss how compaction and cementation contribute to lithification of
sediments.
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COMPACTION packs the grains more tightly together and may even cause them to
form a cohesive mass (rock). This is especially true for fine-grained, clay-rich
sediments.
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CEMENTATION adds a mineral glue causing the sediment particles to stick together
to make a solid rock.
Explain how the porosity and permeability of sediments are modified as they are
lithified.
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Both compaction and cementation tend to decrease porosity and permeability.
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Compaction packs the grains closer together, and may also reorient platy grains so
that they stack more tightly.
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Cementation fills pores and plugs the connections (cracks, grain boundaries)
between them.
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Occasionally, however, diagenesis (the process of changing sediments to
sedimentary rock) may involve flow of water through sediment without cementation.
In this case porosity and permeability may be increased as soluble minerals
dissolve.
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Describe graded bedding and cross-bedding, and briefly explain one way in
which each can arise.
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GRADED BEDDING is a gradual, systematic change in grain size vertically within a
sedimentary bed.
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SUBMARINE LANDSLIDES may contribute to the formation of graded bedding by
churning up a variety of sizes of sediment into suspension. The coarsest fragments
settle out first, followed by successively finer material.
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CROSS-BEDS are a set of parallel, sloping beds deposited as wind or water flows in
a consistent direction over a sloping surface.
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Cross-beds may be formed in sediments on the bottom of a stream channel or in
sand dunes.
Mud cracks form most readily in finer-grained sediments. Why?
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FINE-GRAINED SEDIMENTS are generally richer in water so there is more potential for
shrinkage as they dry out.
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Also, fine-grained sediments commonly include abundant clay minerals, and many
clays shrink when dehydrated.
Give two examples of trace fossils.
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Possible examples include ANIMAL TRACKS, WORM BURROWS, or CASTS of organisms
that are no longer present in the rock.
Compare the kinds of sediment you would expect in a beach environment and in
a tidal flat.
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A BEACH is a high-energy environment. The sediments are likely to be relatively
coarse, well rounded by abrasion, and well sorted, with finer materials removed.
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In the quiet setting of a TIDAL FLAT, we would expect more fine-grained, material to
be present, in the silt and clay size fractions.
How are sediments modified as they are transported farther and farther from
their source regions?
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Typical physical changes in sediments would include a decrease in grain size as
fragments are broken up more and more, and an increase in the roundedness of
fragments, as corners and edges are smoothed by abrasion.
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Also, if water is the transporting agent, soluble minerals will be dissolved and less
stable minerals broken down, so that the chemical and mineral maturity of the
sediment increases with increasing transport distance.
Geology 12
Chapter 6
Questions For Review
Page 2
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What is a facies? How does the existence of sedimentary facies complicate the
identification or mapping of individual rock units?
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A FACIES is a set of conditions that leads to the deposition of a particular type of
sediment.
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The fact that deposition can vary laterally means that several different rock types
(lithofacies) may occur within a area at one time.
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Where rocks are poorly exposed, it may be difficult to realize that rocks of different
facies belong to the same rock unit.
What is a transgression? Describe how this might be recognized in the rock
record.
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A TRANSGRESSION is an event in which the sea moves over the land, so that the
shoreline shifts landward. It might be recognized in the rock record if--for example,
sandstone is overlain by shale overlain by limestone.
(Limestone on TOP = TRANSgression. Limestone on the bottom = regression.)
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Cite and briefly describe two possible paleocurrent indicators.
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The orientation of ASYMMETRIC RIPPLE MARKS is one example: the shallowly sloping
side was up-current, the steeply sloping side down-current.
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Another indicator is the orientation of CROSS-BEDS with the flow path indicated by
the down-current slopes.
Geology 12
Chapter 6
Questions For Review
Page 3