Stratigraphy

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Assignments	Chernicoff, Telling Time Geologically, ch. 8. Study the following pages: 144-149, 154. The other pages will be covered in the "Age of the Earth" lesson.
Reminders	Check out the new information posted in the hallway exhibit.... Can you identify the "Bert" of the month?

Lesson Outline

Read the assigned portions of each chapter. Study the illustrations. Focus your attention on the vocabulary terms found in the following notes.

Determining Relative Age

A. Principles of relative dating

1. Uniformitarianism states that the ___________ is the key to the past. See Peters, chapter 5, for a critical discussion of ambiguities glossed over by Chernicoff here.
2. Jean Buridan's system of the earth
   1. How did Buridan explain the origin of...
      1. strata?
      2. caverns?
      3. mountains?
      4. fossils at the tops of high mountains?
   2. Was Buridan's system of the earth uniformitarian or catastrophist?
3. Nicolaus Steno, diagrams of Tuscany
   1. Superposition
      • With aqueous deposition, younger rocks overlie older rocks.
      • Did Jean Buridan's system of the earth suggest a different view? Explain.
      • Does this principle hold for extrusive lava flows?
      • Does this principle hold for intrusive igneous sills?
        • How can you tell the difference between a sill and a buried lava flow?
   2. Original Horizontality
      • Aqueous deposition of sedimentary strata produces essentially horiontal layers.
      • How does crystallization, e.g., within a geode, differ?
      • Does this principle hold for extrusive lava flows?
   3. Lateral Continuity
      • Correlation (see p. 149)
      • Key beds
   4. Restrictions: Steno's principles apply only to undisturbed sedimentary strata.
4. James Hutton
   1. Igneous intrusions
      • Younger plutonic igneous rock intrudes older rocks.
   2. Angular unconformities (Jedburgh and Siccar Point, Scotland)
5. Inclusions
   • Inclusions within igneous rocks, and fragments within sedimentary rocks, consist of rocks older than those within which they occur.
6. Cross-cutting
   • A fault cuts across rocks older than the fault.
7. Truncation
   • At an unconformity, or erosion surface, folds and faults which developed before erosion are cut off or truncated at the base of overlying rock.
8. "Fossils" are the preserved remains, or other evidence, of ancient organisms in rocks.
   1. Scarcity of fossils
   2. Faunal succession
   3. Index fossils

B. Unconformities

• Boundaries known as "Unconformities" separate rocks of very different ages; they occur where rock layers were removed by erosion and/or no rock layers were deposited.
• Types of unconformities:
  1. "Nonconformities" are the obvious boundaries between an unlayered body of plutonic igneous or metamorphic rocks and an overlying layered sequence of sedimentary rocks.
  2. Angular unconformities
  3. Disconformities

Other methods

1. Paired layers of sediment known as "varves" are deposited annually in lakes: a thick, light-colored layer in the summer, and a thin, dark-colored layer in the winter.
   1. Green River, Wyoming

Reading Focus Questions

Be prepared to answer any of these questions should they occur on a quiz or on the final exam.

1. Answer "For Further Thought" question 3 on p. 158.
2. If a horizontal layer of solidified, extrusive lava lies between two horizontal, sedimentary strata, the principle by which the relative ages of each of these layers could be determined is:
   1. original horizontality
   2. superposition
   3. cross-cutting relationships
   4. inclusions
3. The principle of original horizontality most often applies to:
   1. sedimentary rocks
   2. igneous rocks
   3. metamorphic rocks
   4. minerals
   5. all of the above
4. According to the principle of cross-cutting relationships, if a dike is found cutting across sedimentary strata:
   1. the dike is younger than the strata
   2. the dike is older than the strata
   3. the dike and the strata are the same age
   4. one needs more information to determine the relative ages of the dike and the strata
5. According to the principle of inclusions, a particle of sand within a sandstone:
   1. is younger than the sandstone
   2. is older than the sandstone
   3. is the same age as the sandstone
   4. one needs more information to determine the relative ages of the sand and the sandstone
6. According to the principle of inclusions, if a rock is found embedded within an igneous pluton:
   1. the rock is younger than the pluton
   2. the rock is older than the pluton
   3. the rock is the same age as the pluton
   4. one needs more information to determine the relative age of the rock and the pluton
7. Which type of rock is more likely to contain fossils?
   1. sedimentary
   2. igneous
   3. metamorphic
   4. mineral
   5. all of the above are equally likely to contain fossils
8. Of the following organisms, the best index fossils would be:
   1. cockroaches, because they have existed in widespread geographic areas almost since the first apearance of insects.
   2. bacteria, because they have existed in widespread geographic areas since life first began.
   3. a specific type of bryozoa that existed for only a short geologic time in very limited geographic areas.
   4. a specific type of trilobite that existed for only a short geologic time in widespread geographic areas.
9. Correlation of rock units is used to determine:
   1. which geographically distant rock units were formed under approximately the same environments.
   2. which geographically distant rock units are the same type of rock.
   3. the ages of similar fossils found in geographically distant rocks.
   4. which geographically distant rock units are approximately the same age.
10. If two distant rock units contained similar features, they could be positively correlated if both contained any of the following EXCEPT:
    1. the same key bed
    2. the same minerals
    3. the same index fossil
    4. the same fossil assemblages
11. All of the following statements about varve chronology are true EXCEPT:
    1. Varves are more apparent in lakes that freeze in winter than in those that do not.
    2. Varve layers are examined in cores extracted from the bottom of a lake.
    3. Summer sediment layers are dark-colored, while winter sediments are light-colored.
    4. Varves may be used to date geologic events that affected the lake.