Geologic TimePhysical Geology Chapter 8Tim Horner, CSUS Geology Department

Dating rocks: Relative dating: Puts events in order Doesnt 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

Figure 8.2Step 1) Rocks are deposited in the ocean

Figure 8.3Step 2) Sequence of rocks is deposited (oldest first):

Lutgrad FmBirkland FmTarburg FmLeet Junction Fm

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

Figure 8.5Step 4) Sequence is tilted, top of new surface is eroded

Figure 8.1

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

Figure 8.8Step 6) Larsonton Fm was intruded by dike

Figure 8.9Step 7) Erode material (expose dike)

Figure 8.10Step 8) Subsidence, deposit more marine sediments (Foster City Fm) above unconformityunconformity

Figure 8.11Step 9) Raise entire sequence above sea level, begin downcutting by modern stream

Figure 8.1Final product:

Figure 8.14a-dFormation of angular unconformity

Figure 8.14eAngular unconformity

Figure 8.14fAngular unconformity

Figure 8.12Non-conformity

Figure 8.15Non-conformity

Figure 8.13Disconformity

Figure 8.16Lithologic correlation- tracing beds

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

Figure 8.18Correlation by fossils

Table 8.2Geologic time scale

Figure 8.19

Table 8.3Commonly used radiogenic isotopes:

Figure 8.22Radiogenic dating: parent and daughter isotopes

Figure 8.21

Figure 8.23Radiogenic dating brackets events

Figure 8.24Geologic time scale, with absolute dates

Figure 8.25Absolute time scale

Figure 8.20Numeric (absolute) dating

Uses radiogenic isotopes

Decay rates are known,constant

3 decay methods:

Alpha decayBeta decayElectron capture