Geologic TimePhysical Geology Chapter 8

Tim Horner, CSUS Geology Department

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

Figure 8.1

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 8.1

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

Figure 8.19

Table 8.3

Commonly used radiogenic isotopes:

Figure 8.22

Radiogenic dating: parent and daughter isotopes

Figure 8.21

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 decayBeta decayElectron capture