1. “Clocks in Rocks” (Isotope Dating; Absolute Age Dating)
Geochronology
Relative Dating
&
“Clocks in Rocks” (Isotope Dating; Absolute Age Dating)

2. Geochronology Relative Dating
Usually the term “geochronology” refers to the “precise” numerical age dating, instead of “relative” dating
Absolute age dating using isotope systems
This methodology is usually what the term geochronology refers to

3. Relative Dating
Ordering a sequence of geologic events from oldest to youngest without necessarily having any (or, only very broad) numerical ages

4. Stratigraphy Principle of: Original horizontality Superposition
Stratigraphy: ordered sequences of sedimentary beds
“Stratigraphic Succession”
Oldest on bottom, youngest on top
(Unless some tectonic forces overturns the beds)
Principle of:
Original horizontality
Superposition
Marble Canyon (Grand Canyon)

5. Unconformities: Gaps in the geologic record
The geologic record ain’t perfect!

6. How a gap (or unconformity) might occur

10. Types of unconformities

11. Let’s get the names of unconformity types out there…
Disconformity
Upper sedimentary sequence overlies erosional surface on an undeformed and still-horizontal lower sedimentary sequence (see last several slides!)
Associated with sea level drops or broad tectonic uplifts
Nonconformity
Upper sedimentary beds overlie igneous or metamorphic basement rock
Angular unconformity
Upper sedimentary beds overlie beds that have been folded and then eroded to a nearly level plane.

12. Death Valley

13. Angular Unconformity

14. Sketching Exercise
Draw a sequence of many horizontal sedimentary layers
Shade or pattern them in differently
Apply compressive tectonic forces to fold them
Apply erosion to make a more-or-less smooth erosional surface
Draw several more new sedimentary layers that are now deposited on top
What kind of unconformity have you made?

19. Cross-cutting relationships

26. This is a nice detailed example to study

27. Combining All Relative Dating Info (a lot of it is form the fossil record – Gel 203) into a Geologic Time Scale Arrangement

28. It’d be nice to put actual age dates on these divisions:

29. A “gazillion” renditions exist

31. Poster in my office

32. Fossils can also record “ordered” time and provide correlations across regions
Principle of Faunal Succession
Sedimentary strata in an outcrop contain fossils in a definite sequence
The same sequence at another location can then be correlated
This same idea applies to sedimentary layers, too (“sequence stratigraphy”)
Field of paleontology

39. Question
Can all rock types potentially contain fossils? (igneous, sedimentary, metamorphic)

40. Isotope Age Dating
Putting numerical values (and error bars) on ages of rocks, fossils, and even earthquakes!

41. Isotope Age Dating Concept
ISOTOPES OF AN ELEMENT:
Differing number of neutrons in an element define its isotopes
In an unstable isotope neutrons will “decay” at a rate (dependent on the element)

43. Half-Life
How long it takes for a sample of an unstable isotope (parent) to “decay” to half the initial amount (thus producing as many “daughters”)
Isotopic “clock” is set when minerals in an igneous rock crystallize, or re-crystallize in a metamorphic rock

44. Common Isotope Systems Used in Geologic Dating

46. It’s as “easy as”… Knowing the relevant half life
Determining the amounts of “parent” and “daughter” in your sample, such as a rock sample, or sample of organic matter.
This is done with a mass spectrometer
Plugging in the numbers and letting your calculator have a little fun

47. Two Examples Carbon Dating C14N14 Uranium-Lead System U238  Pb206
(can be used to determine the age of an earthquake on a fault.
In this case we know the amount of beginning carbon in organic matter (from atmospheric conditions) we can just look at the amount of remaining carbon
Uranium-Lead System U238  Pb206
Here we don’t know the initial amount of Uranium
A mass spectrometer can determine amounts of U and Pb per, say, million atoms of Zircon

48. Application to Paleoseismology

49. Carbon Dating Half-life = 5730 years

50. Suppose a sample of organic matter in the fault trace had 85% of carbon (relative to atmospheric left)
How old would we date the earthquake?

51. Example 2: Handout
In the case of unknown starting amount of Uranium, we can use the relative amounts of Parent P (Uranium, U) and Daughter D (Lead, Pb)
Slight Variation of the formula used in Carbon Dating:
Parent (Uranium) half life is 4.5 billion years = 4.5 x 109 years
This implies that the decay rate constant r = 1.54 x (year-1)
(can you calculate that?)

52. 232 atoms of U and 64 atoms of Pb per million zircon minerals in the granitic intrusion
Substitute values into this formula. Make sure you correctly identify
P (parent) and,
D (daughter)

54. Sequence Stratigraphy
Seismic Reflection Surveys: Detecting and characterizing stratigraphic structures that are not exposed

55. Land Seismic Reflection Survey

56. Marine Survey

57. Detecting stratigraphic sequences that are not exposed
Raw data shows boundaries
where the energy from the
seismic survey was reflected
Then the magic/interpretation
happens…

58. Detecting stratigraphic sequences that are not exposed

59. “Seismic Stratigraphers” make $$$
in the energy exploration industry
Conventionally in oil and natural gas
More recently applied to deep sedimentary
basins for geothermal exploration
More in CH 19 of PGT in Week 10

60. end