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Geologic TimePhysical Geology Chapter 8
Tim Horner, CSUS Geology Department
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Dating rocks:
Relative dating:
Puts events in orderDoesn’t necessarily give an exact age
Absolute (Numerical) dating:
Used principles of radioactive decayAssigns a numerical age to a rockOnly works for certain rock types
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Figure 8.1
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Figure 8.2
Step 1) Rocks are deposited in the ocean
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Figure 8.3
Step 2) Sequence of rocks is deposited (oldest first):
• Lutgrad Fm• Birkland Fm• Tarburg Fm• Leet Junction Fm
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Figure 8.4
Step 3) Sequence is intruded, with contact metamorphism at edges of pluton
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Figure 8.5
Step 4) Sequence is tilted, top of new surface is eroded
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Figure 8.1
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Figure 8.6
Step 5) Area subsided, was covered by ocean, Larsonton Fm was deposited
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Figure 8.8
Step 6) Larsonton Fm was intruded by dike
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Figure 8.9
Step 7) Erode material (expose dike)
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Figure 8.10
Step 8) Subsidence, deposit more marine sediments (Foster City Fm) above unconformity
unconformity
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Figure 8.11
Step 9) Raise entire sequence above sea level, begin downcutting by modern stream
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Figure 8.1
Final product:
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Figure 8.14a-d
Formation of angular unconformity
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Figure 8.14e
Angular unconformity
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Figure 8.14f
Angular unconformity
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Figure 8.12
Non-conformity
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Figure 8.15
Non-conformity
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Figure 8.13
Disconformity
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Figure 8.16
Lithologic correlation- tracing beds
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Figure 8.17
Lithologic correlation- similar rock sequences (Gondwana Sequence)
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Figure 8.18
Correlation by fossils
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Table 8.2
Geologic time scale
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Figure 8.19
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Table 8.3
Commonly used radiogenic isotopes:
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Figure 8.22
Radiogenic dating: parent and daughter isotopes
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Figure 8.21
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Figure 8.23
Radiogenic dating brackets events
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Figure 8.24
Geologic time scale, with absolute dates
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Figure 8.25
Absolute time scale
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Figure 8.20
Numeric (absolute) dating
Uses radiogenic isotopes
Decay rates are known,constant
3 decay methods:
Alpha decayBeta decayElectron capture