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.
¨ 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.
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 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.
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 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
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).
Table 5.2 Classification of Stratigraphic Units
Figure 5.16 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 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
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
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.
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 / geologic column / range zonebiostratigraphic 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
SUGGESTED MEDIA
Videos
1. Earth’s Structures, Earth Revealed #8, Annenberg/CPB