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: Used principles of radioactive decay Assigns a numerical age to a rock Only works for certain rock types

Figure 8.1

Step 1) Rocks are deposited in the ocean Figure 8.2 Step 1) Rocks are deposited in the ocean

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

Figure 8.4 Step 3) Sequence is intruded, with contact metamorphism at edges of pluton

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

Figure 8.1

Figure 8.6 Step 5) Area subsided, was covered by ocean, Larsonton Fm was deposited

Step 6) Larsonton Fm was intruded by dike Figure 8.8 Step 6) Larsonton Fm was intruded by dike

Step 7) Erode material (expose 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:

Formation of angular unconformity 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

Lithologic correlation- tracing beds Figure 8.16 Lithologic correlation- tracing beds

Lithologic correlation- similar rock sequences (Gondwana Sequence) Figure 8.17 Lithologic correlation- similar rock sequences (Gondwana Sequence)

Correlation by fossils Figure 8.18 Correlation by fossils

Table 8.2 Geologic time scale

Figure 8.19

Commonly used radiogenic isotopes: Table 8.3 Commonly used radiogenic isotopes:

Radiogenic dating: parent and daughter isotopes Figure 8.22 Radiogenic dating: parent and daughter isotopes

Figure 8.21

Radiogenic dating brackets events Figure 8.23 Radiogenic dating brackets events

Geologic time scale, with absolute dates Figure 8.24 Geologic time scale, with absolute dates

Figure 8.25 Absolute time scale

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