1. Earth in Time The Rock Record and Geologic Time
GEOLOGY TODAY
Barbara W. Murck
Brian J. Skinner
Chapter 3
Maroon Bells, Colorado
N. Lindsley-Griffin,

2. Relative Age - Principles
Original horizontality:
Sediments are deposited on a horizontal surface
Sedimentary rocks form in horizontal layers
Sedimentary rocks not horizontal were disturbed after they formed
Folded sedimentary strata, Crete
N. Lindsley-Griffin, 1999

3. Relative Age - Principles
Stratigraphic superposition:
Each layer of sedimentary rocks is deposited over the previous layer
Lower layers are always older than upper layers
Sandstone, limestone, and shale strata, Grand Canyon N.P., AZ
N. Lindsley-Griffin, 1999

4. Relative Age - Principles
Cross-cutting relationships:
A rock unit is always older than any feature which cuts across or disrupts it.
Fractures cutting sandstone layers, Merseyside, UK
N. Lindsley-Griffin, 1999

5. Relative Age - Principles
Cross-cutting relationships:
A rock unit is always older than any feature which cuts across or disrupts it.
Fractures cutting sandstone layers, Merseyside, UK
N. Lindsley-Griffin, 1999

6. Relative Age - Principles
Correlation:
Lateral continuity --
Strata can be traced from one location to another
Physical similarity --
Same characteristics
= same strata
Similar sequence of strata in two sections
= same sequence
Coal and sandstone strata, Badlands N.P., SD
N. Lindsley-Griffin, 1999

7. Relative Age - Principles
Faunal succession:
Each formation contains a unique fossil assemblage
Assemblages succeed one another in orderly, predictable sequence
Same everywhere in the world
In general, simpler organisms precede more complex ones in the same group
Dinosaur tracks
N. Lindsley-Griffin, 1999

8. Relative Age - Principles
Faunal succession used for correlation:
Look for same fossils in same sequence of similar strata.
Fig. 3.6, p. 63
N. Lindsley-Griffin, 1999

9. Absolute Age - Principles
Radioactive Decay - Release of particles from nucleus
ALPHA EMISSION:
Two protons + two neutrons
Mass Number - 2
BETA EMISSION:
Neutron decays to a proton
and gives off an electron
Mass - no change Number + 1
BETA CAPTURE:
Proton captures electron and
becomes a neutron
Mass - no change Number - 1
© Houghton Mifflin 1998; N. Lindsley-Griffin, All rights reserved

10. Absolute Age - Principles
Half-life:
Time needed for the number of parent atoms to be reduced by one-half
At time zero, 100% P.A.
After one half-life, 50% P.A. and 50% D.A.
After two half-lives, 25% P.A. and 75% D.A.
Figure 3.12, p. 70
N. Lindsley-Griffin, 1999

11. Carbon-14 Dating Neutrons in atmosphere change
nitrogen-14 to carbon-14
C-14 incorporated into tissue of
living organisms
Ratio of C-14 to other C-isotopes
remains constant in living tissue
At death, C-14 not replenished,
ratio of C-14 to other C-isotopes
decreases
Amount of C-14 remaining
determines time since death
of organism
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12. The Geologic Column Proterozoic Quaternary Cretaceous
[Fig. 3.8, p. 64]
N. Lindsley-Griffin, 1999

13. Dating the Geologic Time Scale
Sedimentary rocks
not easily dated
radiometrically
Radiometric
ages added to
geologic time
scale by:
1. Cross-cutting
relationships
2. Bracketing
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14. Relative Age - Principles
Unconformities:
Gaps in the rock record - recognized by an erosional surface
Signifies a major break in deposition
Columbia River Gorge, OR-WA
N. Lindsley-Griffin, 1999

15. Relative Age - Principles
Original horizontality
Superposition
Cross-cutting relationships
Horizontal sedimentary rocks
Youngest
sedimentary rock
Dike cuts strata
- dike is younger
Fault cuts dike and strata
- fault is youngest
Oldest
sedimentary rock
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16. Dating the Geologic Time Scale
Radiometric dating and cross-cutting relationships
bracket ages of sedimentary rocks
Radiometric age of
intrusion Y is 350 m.y.
intrusion X is 400 m.y.
Devonian strata (B) must be older than 350 m.y. and younger than 400 m.y.
Mississippian strata are younger than 350 m.y.
Silurian strata are older than both Devonian and 400 m.y.
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17. Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ
N. Lindsley-Griffin, 1998

18. Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ
N. Lindsley-Griffin, 1998

19. Disconformity Angular Unconformity
Time gap without angular relationship
Time gap after folding, faulting
N. Lindsley-Griffin, 1998

20. Disconformity Angular Unconformity
Time gap without angular relationship
Time gap after folding, faulting
N. Lindsley-Griffin, 1998

21. “Baked zone” - soil formed on older lava flow before being covered and cooked by a new lava flow - Disconformity
N. Lindsley-Griffin, 1998

22. “Baked zone” - soil formed on older lava flow before being covered and cooked by a new lava flow - Disconformity
N. Lindsley-Griffin, 1998

23. Nonconformity - sedimentary rocks over crystalline (metamorphic or igneous) rocks
N. Lindsley-Griffin, 1998

24. Nonconformity - sedimentary rocks over crystalline (metamorphic or igneous) rocks
N. Lindsley-Griffin, 1998

25. Nonconformity - sedimentary gravels over garnet schist
Nonconformity - sedimentary gravels over garnet schist. What is the history of this outcrop?
N. Lindsley-Griffin, 1998

26. Nonconformity - sedimentary gravels over garnet schist
Nonconformity - sedimentary gravels over garnet schist. What is the history of this outcrop?
N. Lindsley-Griffin, 1998

27. Cross-cutting relationships: dike cutting gneiss is younger
N. Lindsley-Griffin, 1998

28. Cross-cutting relationships: dike cutting gneiss is younger
N. Lindsley-Griffin, 1998

29. Cross cutting relationships: what is the sequence of events in this folded marble with two dikes?
N. Lindsley-Griffin, 1998

30. Cross cutting relationships: what is the sequence of events in this folded marble with two dikes?
N. Lindsley-Griffin, 1998

31. Grand Canyon Formations
Superposition
Disconformity
Original
horizontality
Angular
unconformity
Cross-cutting
relationships
Nonconformity
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32. Grand Canyon Formations
Sequence of events based on geology of the Inner Gorge
1.Deposition of shale in
marine environment
2. Metamorphism of shale
into Vishnu schist
3. Intrusion by
Zoroaster granite
4. Erosion to sea level,
unconformity surface
5. Deposition of Grand
Canyon Supergroup
7. Erosion of fault blocks,
deposition of Tapeats
sandstone
6. Faulting
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33. Geologic Time - Summary
Relative Age -- whether a particular rock or feature is older or younger than another
Absolute Age -- the age of a rock in years (m.y. = millions of years)
Geologic Column -- shows succession of all known strata, in chronological order, based on fossils and other relative ages
N. Lindsley-Griffin, 1999