G EOLOGY 12
A
C HAPTER 6 S EDIMENTARY R OCKS
Q UESTIONS F OR R EVIEW
1. On what basis are clastic sediments subdivided and named?
• Clastic sediments are subdivided and named primarily on the basis of GRAIN SIZE .
• The coarsest materials are further divided by the
ROUNDEDNESS
or
ANGULARITY
of the fragments.
• None of the names are based on mineral composition.
2. What is meant by saying that a sediment is well sorted? Well rounded?
• A
WELL
-
SORTED
sediment has fragments that span a narrow range of grain sizes.
• A
WELL
-
ROUNDED
sediment has fragments that have smoothly rounded contours.
3. Name and describe two kinds of chemical sediments.
(A
MONG THE POSSIBILITIES ARE THE FOLLOWING
:)
• L IMESTONE --a sediment rich in carbonate, especially calcite (calcium carbonate)
• D
OLOMITE
-- a sediment rich in the carbonate mineral dolomite (calcium magnesium carbonate)
• R
OCK SALT
--an evaporite sediment composed primarily of halite (sodium chloride)
4. Discuss how compaction and cementation contribute to lithification of sediments.
• C
OMPACTION
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.
• C
EMENTATION
adds a mineral glue causing the sediment particles to stick together to make a solid rock.
5. Explain how the porosity and permeability of sediments are modified as they are lithified.
• Both compaction and cementation tend to decrease porosity and permeability.
• Compaction packs the grains closer together, and may also reorient platy grains so that they stack more tightly.
• Cementation fills pores and plugs the connections (cracks, grain boundaries) between them.
• 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.

6. Describe graded bedding and cross-bedding, and briefly explain one way in which each can arise.
• G
RADED BEDDING
is a gradual, systematic change in grain size vertically within a sedimentary bed.
• S UBMARINE 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.
• C
ROSS
-
BEDS
are a set of parallel, sloping beds deposited as wind or water flows in a consistent direction over a sloping surface.
• Cross-beds may be formed in sediments on the bottom of a stream channel or in sand dunes.
7. Mud cracks form most readily in finer-grained sediments. Why?
• F
INE
-
GRAINED SEDIMENTS
are generally richer in water so there is more potential for shrinkage as they dry out.
• Also, fine-grained sediments commonly include abundant clay minerals, and many clays shrink when dehydrated.
8. Give two examples of trace fossils.
• Possible examples include
ANIMAL TRACKS
,
WORM BURROWS
, or
CASTS
of organisms that are no longer present in the rock.
9. Compare the kinds of sediment you would expect in a beach environment and in a tidal flat.
• 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.
• 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.
10. How are sediments modified as they are transported farther and farther from their source regions?
• 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.
• 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

11. What is a facies? How does the existence of sedimentary facies complicate the identification or mapping of individual rock units?
• A
FACIES
is a set of conditions that leads to the deposition of a particular type of sediment.
• The fact that deposition can vary laterally means that several different rock types
(lithofacies) may occur within a area at one time.
• Where rocks are poorly exposed, it may be difficult to realize that rocks of different facies belong to the same rock unit.
12. What is a transgression? Describe how this might be recognized in the rock record.
• 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.)
13. Cite and briefly describe two possible paleocurrent indicators.
• The orientation of
ASYMMETRIC RIPPLE MARKS
is one example: the shallowly sloping side was up-current, the steeply sloping side down-current.
• 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