Slide 1Slide 2Figure 8.1Figure 8.2Figure 8.3Figure 8.4Figure 8.5Slide 8Figure 8.6Figure 8.8Figure 8.9Figure 8.10Figure 8.11Slide 14Figure 8.14a-dFigure 8.14eFigure 8.14fFigure 8.12Figure 8.15Figure 8.13Figure 8.16Figure 8.17Figure 8.18Table 8.2Figure 8.19Table 8.3Figure 8.22Figure 8.21Figure 8.23Figure 8.24Figure 8.25Figure 8.20Geologic TimePhysical Geology Chapter 8Tim Horner, CSUS Geology DepartmentDating rocks:Relative dating:Puts events in orderDoesn’t necessarily give an exact ageAbsolute (Numerical) dating:Used principles of radioactive decayAssigns a numerical age to a rockOnly works for certain rock typesFigure 8.1Figure 8.2Step 1) Rocks are deposited in the oceanFigure 8.3Step 2) Sequence of rocks is deposited (oldest first):•Lutgrad Fm•Birkland Fm•Tarburg Fm•Leet Junction FmFigure 8.4Step 3) Sequence is intruded, with contact metamorphism at edges of plutonFigure 8.5Step 4) Sequence is tilted, top of new surface is erodedFigure 8.1Figure 8.6Step 5) Area subsided, was covered by ocean, Larsonton Fm was depositedFigure 8.8Step 6) Larsonton Fm was intruded by dikeFigure 8.9Step 7) Erode material (expose dike)Figure 8.10Step 8) Subsidence, deposit more marine sediments (Foster City Fm) above unconformityunconformityFigure 8.11Step 9) Raise entire sequence above sea level, begin downcutting by modern streamFigure 8.1Final product:Figure 8.14a-dFormation of angular unconformityFigure 8.14eAngular unconformityFigure 8.14fAngular unconformityFigure 8.12Non-conformityFigure 8.15Non-conformityFigure 8.13DisconformityFigure 8.16Lithologic correlation- tracing bedsFigure 8.17Lithologic correlation- similar rock sequences (Gondwana Sequence)Figure 8.18Correlation by fossilsTable 8.2Geologic time scaleFigure 8.19Table 8.3Commonly used radiogenic isotopes:Figure 8.22Radiogenic dating: parent and daughter isotopesFigure 8.21Figure 8.23Radiogenic dating brackets eventsFigure 8.24Geologic time scale, with absolute datesFigure 8.25Absolute time scaleFigure 8.20Numeric (absolute) datingUses radiogenic isotopesDecay rates are known,constant3 decay methods:Alpha decayBeta decayElectron