Geologic Time Physical Geology Chapter 8

1
Geologic TimePhysical Geology Chapter 8
Tim Horner, CSUS Geology Department
2
Dating rocksRelative dating Puts events in
order Doesnt 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
3
Figure 8.1
4
Figure 8.2
Step 1) Rocks are deposited in the ocean
5
Figure 8.3

• Step 2) Sequence of rocks is deposited
• (oldest first)
• Lutgrad Fm
• Birkland Fm
• Tarburg Fm
• Leet Junction Fm

6
Figure 8.4
Step 3) Sequence is intruded, with contact
metamorphism at edges of pluton
7
Figure 8.5
Step 4) Sequence is tilted, top of new surface is
eroded
8
Figure 8.1
9
Figure 8.6
Step 5) Area subsided, was covered by ocean,
Larsonton Fm was deposited
10
Figure 8.8
Step 6) Larsonton Fm was intruded by dike
11
Figure 8.9
Step 7) Erode material (expose dike)
12
Figure 8.10
Step 8) Subsidence, deposit more marine sediments
(Foster City Fm) above unconformity
unconformity
13
Figure 8.11
Step 9) Raise entire sequence above sea level,
begin downcutting by modern stream
14
Figure 8.1
Final product
15
Figure 8.14a-d
Formation of angular unconformity
16
Figure 8.14e
Angular unconformity
17
Figure 8.14f
Angular unconformity
18
Figure 8.12
Non-conformity
19
Figure 8.15
Non-conformity
20
Figure 8.13
Disconformity
21
Figure 8.16
Lithologic correlation- tracing beds
22
Figure 8.17
Lithologic correlation- similar rock sequences
(Gondwana Sequence)
23
Figure 8.18
Correlation by fossils
24
Table 8.2
Geologic time scale
25
Figure 8.19
26
Table 8.3
Commonly used radiogenic isotopes
27
Figure 8.22
Radiogenic dating parent and daughter isotopes
28
Figure 8.21
29
Figure 8.23
Radiogenic dating brackets events
30
Figure 8.24
Geologic time scale, with absolute dates
31
Figure 8.25
Absolute time scale
32
Figure 8.20
Numeric (absolute) dating Uses radiogenic
isotopes Decay rates are known, constant 3
decay methods Alpha decay Beta
decay Electron capture