Historical Geology
INTRODUCTION
STRATIGRAPHY
Vertical Stratigraphic Relationships
Lateral Relationships—Facies
Marine Transgressions and Regressions
Extent, Rates, and Causes of Marine Transgressions and Regressions
How Do Fossils Form?
Fossils and Telling Time
CORRELATION
¬Perspective Monument Valley Navajo Tribal Park
ABSOLUTE DATES AND THE RELATIVE GEOLOGIC TIME SCALE
SUMMARY
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 EarthÕs 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.
¬ 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.
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
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.
Figure 5.2 The Principle of Inclusions
Figure 5.3 How to Determine the Relative Ages of Lava Flows, Sills, and
Associated Sedimentary Rocks
Figure 5.4 The Origin of an Unconformity and a Hiatus
Figure 5.5 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 WaltherÕs 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
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.
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 ÒcoprolitesÓ 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 hyenasÕ 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 ichthyosuarsÕ 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
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 WinchesterÕs best-selling 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
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).
Table 5.2 Classification of Stratigraphic Units
Figure 5.16 Graphic Representation of the Lithostratigraphic Units in Capital Reef National Park in Utah
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. (ÒSimplifying the Stratigraphy of Time,Ó 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 potassium-argon 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
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
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.
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 ÒCambrian,Ó ÒOrdovician,Ó and ÒSilurianÓ 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.
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?
|
angular unconformity |
geologic column |
range zone |
|
biostratigraphic unit |
geologic record |
relative geologic time scale |
|
biozone |
guide fossil |
sedimentary facies |
|
body fossil |
lithostratigraphic unit |
stratigraphy |
|
cast |
marine regression |
system |
|
concurrent range zone |
marine transgression |
time-stratigraphic unit |
|
conformable |
mold |
time unit |
|
correlation |
nonconformity |
trace fossil |
|
disconformity |
period |
unconformity |
|
formation |
principle of fossil succession |
Walther's law |
|
fossil |
principle of inclusions |
|
1. EarthÕs 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.
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
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 EducatorÕs Supply
7. Deck of 50 fossil cards, Science Stuff
8. Advanced Fossil Collection, Science Stuff
|
1. a |
5. d |
9. e |
|
2. b |
6. d |
10. c |
|
3. e |
7. c |
|
|
4. c |
8. c |
|
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 Òsystem.Ó 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 rockÕs 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.
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 non-deposition. 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.
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 EarthÕs! 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
Òsystems,Ó with the time representing their formation a Òperiod.Ó 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 1 1 = 0.0105 cm/yr
m 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.
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.
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