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Geologic Time Physical Geology Chapter 8
Tim Horner, CSUS Geology Department
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Dating rocks: Relative dating:
Puts events in order Doesn’t necessarily give an exact age Absolute (Numerical) dating: Used principles of radioactive decay Assigns a numerical age to a rock Only works for certain rock types
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Figure 8.1
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Step 1) Rocks are deposited in the ocean
Figure 8.2 Step 1) Rocks are deposited in the ocean
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Step 2) Sequence of rocks is deposited (oldest first): Lutgrad Fm
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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Step 4) Sequence is tilted, top of new surface is eroded
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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Step 6) Larsonton Fm was intruded by dike
Figure 8.8 Step 6) Larsonton Fm was intruded by dike
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Step 7) Erode material (expose dike)
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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Formation of angular unconformity
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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Lithologic correlation- tracing beds
Figure 8.16 Lithologic correlation- tracing beds
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Lithologic correlation- similar rock sequences (Gondwana Sequence)
Figure 8.17 Lithologic correlation- similar rock sequences (Gondwana Sequence)
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Correlation by fossils
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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Commonly used radiogenic isotopes:
Table 8.3 Commonly used radiogenic isotopes:
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Radiogenic dating: parent and daughter isotopes
Figure 8.22 Radiogenic dating: parent and daughter isotopes
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Figure 8.21
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Radiogenic dating brackets events
Figure 8.23 Radiogenic dating brackets events
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Geologic time scale, with absolute dates
Figure 8.24 Geologic time scale, with absolute dates
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Figure 8.25 Absolute time scale
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Numeric (absolute) dating
Figure 8.20 Numeric (absolute) dating Uses radiogenic isotopes Decay rates are known, constant 3 decay methods: Alpha decay Beta decay Electron capture
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