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