R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 5
ROCKS, FOSSILS, AND TIME—MAKING SENSE
OF THE GEOLOGIC RECORD
OUTLINE
INTRODUCTION
STRATIGRAPHY
Vertical Stratigraphic Relationships
Lateral Relationships—Facies
Marine Transgressions and Regressions
Extent, Rates, and Causes of Marine Transgressions and Regressions
FOSSILS AND FOSSILIZATION
How Do Fossils Form?
Fossils and Telling Time
THE RELATIVE GEOLOGIC TIME SCALE
STRATIGRAPHIC TERMINOLOGY
Lithostratigraphic and Biostratigraphic Units
Time Stratigraphic Units and Time Units
CORRELATION
PERSPECTIVE Monument Valley Navajo Tribal Park
ABSOLUTE DATES AND THE RELATIVE GEOLOGIC TIME SCALE
SUMMARY
CHAPTER OBJECTIVES
The following content objectives are presented in Chapter 5:
 To analyze the geologic record, you must first determine the correct vertical sequence
of rocks—that is, from oldest to youngest—even if they have been deformed.
 Although rocks provide our only evidence of prehistoric events, the record is
incomplete at any one locality because discontinuities are common.
 Stratigraphy is a discipline in geology that is concerned with sedimentary rocks most
of which are layered or stratified, but many principles of stratigraphy also apply to
igneous and metamorphic rocks, too.
 Several marine transgressions and regressions occurred during Earth’s history, at
times covering much of the continents and at other times leaving the land above sea
level.
 Fossils, the remains or traces of prehistoric organisms, are preserved in several ways,
and some types of fossils are much more common than most people realize.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
 Distinctive groups of fossils found in sedimentary rocks are useful for determining
the relative ages of rocks in widely separated areas.
 Superposition and the principle of fossil succession were used to piece together a
composite geologic column, which is the basis for the relative geologic time scale.
 Geologists have developed terminology to refer to rocks and to time.
 Several criteria are used to match up (correlate) similar rocks over large regions or to
demonstrate that rocks in different areas are the same age.
 Absolute ages of sedimentary rocks are most often determined by radiometric dating
of associated igneous or metamorphic rocks.
LEARNING OBJECTIVES
To exhibit mastery of this chapter, students should be able to demonstrate comprehension
of the following:
 the nature of vertical and lateral stratigraphic relationships
 the concepts of unconformities and facies
 the causes and consequences of transgressions and regressions
 the process of fossilization and the use of fossils in determining relative ages
 the development of the geologic column and the derivation of the relative time scale
 modern stratigraphic terminology
 the techniques used in lithostratigraphic, biostratigraphic, and time-stratigraphic
correlation
 the methods used to quantify the relative time scale
CHAPTER SUMMARY
1. Stratigraphy is concerned with the composition, origin, age relationships, and
geographic extent of sedimentary rocks. Sedimentary rocks are stratified, with few
exceptions
Figure 5.1
Stratified Sedimentary Rocks
2. In a vertical succession of sedimentary rocks, bedding planes separate individual
strata. The correct order in which the strata were deposited must be determined.
3. In addition to the principle of superposition, geologists can use the principle of
inclusions to determine relative ages of rocks. The principle of inclusions states that
inclusions found in a rock must be older than the rock itself. The geologic record is
an accurate chronicle of ancient events, but the strata record many surfaces known
as unconformities that represent times of nondeposition and/or erosion.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Figure 5.2
Figure 5.3
Figure 5.4
Figure 5.5
The Principle of Inclusions
How to Determine the Relative Ages of Lava Flows, Sills, and
Associated Sedimentary Rocks
The Origin of an Unconformity and a Hiatus
Types of Unconformities
4. Simultaneous deposition in adjacent but different environments yields
sedimentary facies, which are bodies of sediment or sedimentary rock, with
distinctive lithologic and biologic attributes.
Figure 5.6
Lateral Termination of Rock Layers
Figure 5.7
Sedimentary Rocks in the Grand Canyon
5. During a marine transgression a vertical sequence of facies results with offshore
facies superposed over nearshore facies. Just the opposite facies sequence results
from a marine regression.
Figure 5.8 Marine Transgressions and Regressions
6. According to Walther’s law, the facies in a conformable vertical sequence replace
one another laterally.
7. Uplift and subsidence of continents, the amount of water frozen in glaciers, and the
rate of seafloor spreading are responsible for marine transgressions and regressions.
8. Most fossils are found in sedimentary rocks, although they might also be in
volcanic ash and volcanic mudflows, but rarely in other rocks. Geologists use
fossils to determine the relative ages of strata. Fossils also provide useful
information for determining environments of deposition.
Figure 5.9
Relative Ages of Rocks
9. The fossil record is strongly biased toward those organisms that have durable
skeletons and that lived where burial was likely. Body fossils are remains of the
organism itself, while trails, tracks, and burrows may be preserved as trace fossils
Figure 5.10 Body Fossils and Trace Fossils
Figure 5.11 Unaltered Remains
Figure 5.12 Altered Remains of Organisms
Figure 5.13 Origin of a Mold and Cast
Table 5.1
Types of Fossil Preservation
Enrichment Topic 1. How to Achieve Immortality
The author of this article wanted to live forever, and decided that fossilization was his
best choice. He learned that he should die young, while his bones are robust; avoid
predators and scavengers; and choose a time of discovery not too distant from the present
because the more time that passes, the more unfortunate things that can happen.
48
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
The author discussed various possibilities including freezing (which would not make him
a true fossil) and burial in a deep, low-oxygen lake. This is a humorous look at
fossilization that students will enjoy. Earth, April 1998 v.7 n.2 p.48.
Enrichment Topic 2. A Coprolitic History
Although modern research on coprolites continues, the study of fossilized feces can be
traced back to William Buckland. In 1821, Buckland correctly hypothesized that the
white-hued balls of bony material from Kirkdale Cave were fossilized feces of hyenas
(album graecum). However, Buckland did not name the fossilized feces “coprolites” until
1829, when he reported his research on ichthyosaur coprolites (bezoar stones) at the
Geological Society of London. Buckland was not content with hypothesizing. After
suspecting a hyena origin for the album graecum, he researched living hyenas’ feces, and
even had the chemist William Wollaston analyze the material! Buckland also confirmed
that the spiral structure of the bezoar stones most likely represented the internal and
external expression of the extinct ichthyosuars’ digestive system by replicating the spiral
structures with modern dogfish and shark intestines!
10.
The work of William Smith, among others, is the basis for the principle of fossil
succession that holds that fossil assemblages succeed one another through time in a
predictable order.
Figure 5.14 Applying the Principle of Fossil Succession
Enrichment Topic 3. William Smith and the “Map that Changed the World”
William Smith, the English surveyor whose work and observations led to the principle of fossil
succession, was not a successful geologist while he practiced his trade. As a member of the
working class, Smith was excluded from the elite scientific societies of his time, and endured
multiple hardships from others who tried to plagiarize his work. Simon Winchester’s bestselling biography of William Smith, The Map That Changed the World, offers an interesting
glimpse of the participants in the geological discipline during its early years.
11. Superposition and fossil succession were used to piece together a composite
geologic column, which was the basis for the relative time scale.
Figure 5.15 The Geologic Column and the Relative Geologic Time Scale
Enrichment Topic 4. The Devonian Controversy
Martin Rudwick, whom the late Stephen J. Gould called the greatest historian of geology,
documented the controversy and debates surrounding the development of the geologic
time scale and the Devonian Period. His book, The Great Devonian Controversy: The
Shaping of Scientific Knowledge among Gentlemanly Specialists, provides many of the
interesting details about the participants and their disagreements.
12. To bring order to stratigraphic terminology, geologists recognize units based
entirely on rock content (lithostratigraphic and biostratigraphic units) and those
related to time (time-stratigraphic and time units).
49
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Table 5.2
Figure 5.16
Classification of Stratigraphic Units
Graphic Representation of the Lithostratigraphic Units in Capital
Reef National Park in Utah
Enrichment Topic 5. The Debate over Simplifying the Stratigraphy of Time
Researchers in the United Kingdom (Zalasiewica, Smith, Brenchley, and others)
proposed ending the distinction between time-stratigraphic units and time units. They
believe that chronostratigraphy, or time-stratigraphy, should adopt the units of eon, era,
period, epoch, and age. (“Simplifying the Stratigraphy of Time,” Geology, January 2004
v.31n1, p. 1-4).
13. Correlation matches up geologic phenomena in two or more areas.
Lithostratigraphic correlation involves demonstrating the original continuity of a
rock unit over a given area event although it may not now be continuous over this
area.
Figure 5.17 Lithostratigraphic Correlation
14. Correlation of biostratigraphic zones, especially concurrent range zones,
demonstrates that rocks in different areas, even though they may differ in
composition, are of the same relative age. Some physical events of short duration—
such as a distinctive lava flow or an ash fall—also can be used to demonstrate time
equivalence.
Figure 5.18 Comparison of the Geologic Ranges of Three Marine Invertebrate
Animals
Figure 5.19 Time-Stratigraphic Correlation Using Concurrent Range Zones
Figure 5.20 Ash Beds Used in Time-Stratigraphic Correlation.
15.
Glauconite is a mineral in sedimentary rocks that can be dated with the potassiumargon method. The best way to determine absolute ages of sedimentary rocks and
their contained fossils is to obtain absolute ages for associated igneous rocks and
metamorphic rocks.
Figure 5.21 Determining the Absolute Ages of Sedimentary Rocks
LECTURE SUGGESTIONS
Completeness of the Fossil Record
1. It is estimated that there are about 5 million species of plants and animals today.
George Gaylord Simpson estimated that the average life span of a single species
ranges between 0.5 and 5 million years. Using an average life span of 3 million
years, and assuming that the level of diversity since the Cambrian is similar to that
of today, we can calculate the total number of species that have risen since the
Cambrian, approximately 600 million years ago:
(5 x 106) ( 600 x 106) = 1 x 109
3 x 106
50
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Compare these 1 billion species with the approximately 150,000 known fossil
species present in the geological record; only about 0.015% of all possible species
have been recorded in the preserved and identifiable fossil record.
2. Emphasize the selective preservation of organism by creating analogies. The
chances of a living organism being fossilized might be comparable to winning a
lottery. The odds are extremely high against it unless conditions are favorable. For
an organism, the correct conditions of burial are critical. Marine organisms tend to
have greater chances of being buried in sediment than land-dwelling organisms.
Insects comprise about 1,000,000 species, yet there are only about 12,000 insect
species known from the fossil record.
Evolution and Contingency
The late Stephen J. Gould's book, Wonderful Life, focused on two separate but related
stories. Gould documented the phenomenal Cambrian explosion of life as revealed
through the Burgess Shale, one of the most unique fossil locations in the world because
of the preservation of soft-body parts. Gould also presented the story of how scientific
thought evolves, and the time involved before the Burgess Shale and its fauna were fully
appreciated by the scientific community.
In this book Gould explores the concept of contingencies, or the "what if" factor. What
would life be like today if some of the phyla that perished had survived, or if some of the
known survivors perished instead? Would humans be on this Earth had a slightly
different set of organisms survived? Was Pikaia the one-chance shot to evolution of the
vertebrates, or was the eventual rise of the vertebrates inevitable?
Development of the Geologic Time Scale
1.
Have students investigate the origins of the period names for the geologic time
scale. Where do “Cambrian,” “Ordovician,” and “Silurian” originate?
2.
Why is the Carboniferous Period recognized as a single period outside the United
States, but it is divided into the Pennsylvanian and Mississippian periods by US
geologists? Where do these names originate?
3.
Students can access the TimeScale Creator at http://www.tscreator.com to create
portions of the geologic time scale with bio-, magneto-, chemo-, and other events
in Earth History.
51
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CONSIDER THIS
1. (Consider Lecture Suggestion 1.) Is it valid to assume that the level of diversity has
been constant since the Cambrian? If not, how could the formula be rewritten?
2. Does an unconformity encompass the same duration of time everywhere that it
occurs?
3. Are unconformities uniform throughout an area? Could an angular unconformity
present as a disconformity in the same rock sequences? Why or why not?
4. How can geologists in the field determine whether a nonconformity exists, or
whether magma intruded into the sedimentary rocks? What signs should geologists
look for to determine a nonconformity versus an intrusion?
5. Why can fossils be used to demonstrate the age equivalence of geographically
separated and (often) lithologically dissimilar strata? Are all fossils useful for
accomplishing this?
6. Since there is no single region on Earth that has a complete sequence of
sedimentary rocks, how do geologists put together the complete story of Earth
history?
7. When geologists map formations, are they consistent in naming the formation
between geographic and political boundaries?
IMPORTANT TERMS
angular unconformity
biostratigraphic unit
biozone
body fossil
cast
concurrent range zone
conformable
correlation
disconformity
formation
fossil
geologic column
geologic record
guide fossil
lithostratigraphic unit
marine regression
marine transgression
mold
nonconformity
period
principle of fossil succession
principle of inclusions
range zone
relative geologic time scale
sedimentary facies
stratigraphy
system
time-stratigraphic unit
time unit
trace fossil
unconformity
Walther's law
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
SUGGESTED MEDIA
Videos
1. Earth’s Structures, Earth Revealed #8, Annenberg/CPB
2. New Explorers: Mystery of the Andes, A & E Home Videos
3. Sequence Stratigraphy: The Book Cliff of Eastern Utah, Open University
4. The Record of the Rocks, Films for the Humanities and Sciences
5. Physical Geography II: Fossils, Rocks, and Time, Quantum Leap
6. Geologic Time, Tell ME Why Sales Co.
Software
1. An Introduction to Structural Methods, Tasa Graphics Arts, I
2. Explore Fossils, Geological Society of America
3. Explore Cross Sections CD-ROM, Geological Society of America
4. Explore Deep Time, Geological Time and Beyond, Geological Society of America
Slide Sets and Demonstration Aids
1. Introduction to Fossils slide set, Educational Images, Ltd.
2. Fossilization: How Fossils are Formed slide set, Educational Images, Ltd.
3. Fossils and their Living Kin slide set, Educational Images, Ltd.
4. Interpretation of Roadside Geology, slide set, Educational Images, Ltd.
5. Geologic Structures: Unconformities, slide set, Educational Images, Ltd.
6. Assorted Fossil Kits, Earth Science Educator’s Supply
7. Deck of 50 fossil cards, Science Stuff
8. Advanced Fossil Collection, Science Stuff
CHAPTER 5 - ANSWERS TO QUESTONS IN TEXT
Multiple Choice Review Questions
1.
2.
3.
4.
a
b
e
c
5.
6.
7.
8.
d
d
c
c
9. e
10. c
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Short Answer Essay Review Questions
11. Superposition holds that, in an undisturbed sequence of rocks, the oldest are on the
bottom. This is useful for establishing relative ages for a particular area. Fossil
succession states that organisms succeed one another in the rock record in a
specific, non-repeating sequence. These types of data can be used for long distance
or even intercontinental correlation, as long as appropriate fossils are present.
12. The granite may be older than the sandstone. In this case, a nonconformity would
exist between the granite (an igneous rock) and the sandstone (sedimentary rock).
You should look for evidence of an erosion surface at the top of the granite. If the
granite was intruded into the sandstone, then an igneous intrusion would exist, and
the granite would be younger than the sandstone. You should be able to tell if the
granite was intruded as a hot body of magma by investigating whether contact
metamorphism exists between the granite and sandstone.
13.
In the rock record, a marine regression has offshore facies overlain with
progressively nearshore facies. A typical marine regression, from bottom to top of
the column, would include limestone, shale, and sandstone. A regression may also
be represented by an erosional surface (unconformity) if the sea level receded from
the rocks, exposing them to weathering and erosion.
14. A time unit does not involve rocks, but a time-stratigraphic unit is a rock unit that
was formed during a particular interval of geologic time. For the time unit, period,
the corresponding rock that was formed during a period would be a “system.”
Likewise, time units of age, epoch, era, and eon are represented in the rock record
by the rock units stage, series, erathem, and eonothem, respectively.
15. A lithostratigraphic unit is a rock unit based only upon the type of rock, with no
consideration of the time of its origin. Conversely, a time-stratigraphic unit is a
rock unit that was formed during a particular interval of time—this rock unit
considers the time of the rock’s origin.
16. You cannot determine the ages of the columns relative to each other because the
rocks could have been deposited by the same environmental process at different
times. For example, if a portion of the columns represent a marine transgression, A
could be older or younger than B since they are 200 km apart.
17. A good guide fossil is well-suited for time-stratigraphic correlation. Good guide
fossils are widespread, easily identified, and have short geologic ranges.
18. Geologists can investigate processes that occur today in a variety of depositional
settings, such as desert playas, deltas, dune fields, river beds, volcanic ash deposits
and marine environments. Within these modern environments, geologists can
observe the accumulation of sediments, and the burial and incorporation of organic
remains within sediments.
54
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
When a geologist encounters a structure in the rock record that is similar to that
formed in modern dune environments, the geologist can use the principle of
uniformitarianism to interpret the ancient rocks.
19. There can be no worldwide unconformities because it would highly improbable that
all land surfaces (including the seafloor) would be experiencing erosion or nondeposition. There must always be some land surface (or seafloor) that is in a perfect
location to experience deposition.
20. Geologists today can observe modern depositional environments in which
sediments accumulate, and bury and incorporate organic remains within the
sediments. We can use these modern analogs to interpret fossil burial and
preservation in the rock record.
Apply Your Knowledge
1. You should use the principles of superposition (the oldest layer is on the bottom)
and develop a fossil succession for the planet. It is likely that fossils will succeed
each other in a predictable, regular order. However, the fossils of another planet
will probably not duplicate Earth’s! Accurate recording of the fossil types within
each strata should yield some semblance of order. Then, rock strata can be
correlated over the planet by matching up key beds, guide fossils and distinctive
assemblages of fossils within individual strata. Within certain strata, there may be
distinctive features. These can be further defined as “systems,” with the time
representing their formation a “period.” In this way a relative geologic scale for the
planet may be created. In order to determine the absolute ages of the fossil-bearing
rocks, associated igneous or metamorphic rocks can be radiometrically dated. Then,
the principle of cross-cutting relationships can be used to determine maximum or
minimum ages of the fossils.
2. The first rock sequence – limestone, shale, and sandstone – was deposited during a
marine regression. The rocks were then tilted upward to 50o. The second mudstone
was then laid down and the basalt was erupted over it. The basalt weathered for a
time then sandstone was laid down on top. The sandstone incorporated some of the
weathered basalt fragments. If the fossils can be identified, the limestone might be
further dated. Likewise, radiometric dating of the basalt is possible.
3. The distance between the upper and lower ash beds is approximately 21m using the
scale between the columns. Therefore, in 0.2 my, 21m of sediment were deposited.
Rate of sedimentation = 21m x 102cm x
m
1 1
= 0.0105 cm/yr
200,000 yr
This is probably not the average actual rate. It is obvious from the column on the
left that much more deposition took place at another location.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
4. In the image, darker fragments are included in the lighter gray rock. Therefore,
according to the principle of inclusions, the darker fragments must be older.
56