1. Correlation and Dating of the Rock Record
Geologic Time
Correlation and Dating of the Rock Record

2. Time Relative Absolute
Order of deposition of a body of rock based on position
Absolute
A number representing the time a body of rock was deposited

3. Relative Time
Determining of sequence of events
Which came first?

4. In what order did these events occur in American History?

5. Relative Time Tools Smith Steno Lyell Fossil Succession Superposition
Original horizontality
Lyell
Cross-cutting relationships
Intrusions
Inclusions

6. Principle of fossil succession
Fossils occur in a consistent vertical order in sedimentary rocks all over the world. (William"Strata Bill" Smith, late 1700's, England).
This principle is valid and does not depend on any pre-existing ideas of evolution. (In fact, Charles Darwin's ideas on evolution did not appear until 50 years later ).

7. Principle of fossil succession
Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.
"Fossil species succeed one another in a definite and recognizable order"
Fossils at the base of a thick sequence of sediments (so older, by previous principles) are less like present-day species than those near the top

8. Principle of fossil succession
* Fossils unlike present-day species, but like each other, are found in widely separated sites
* A fossil species which is observed to occur above (and so younger than, by previous principles) a second fossil species in one locality will always occur above that second species, wherever found.

9. Principle of fossil succession

10. Unconformities
Unconformities are buried surfaces of erosion or non-deposition

11. Unconformities 1.Angular unconformities 2.Nonconformity
Implies tectonic deformation and erosion of underlying strata.
2.Nonconformity
Sedimentary strata overlying igneous or metamorphic rocks (in an erosional - not intrusive- contact)
3.Disconformity
An irregular surface of erosion between two units of parallel strata

12. Disconformity Big Brook

13. Angular Unconformity

14. Nonconformity Brahma Schist underlies Tapeats Sandstone

15. Nonconformity Brahma Schist underlies Tapeats Sandstone

16. Siccar Point

17. Relative Dating Principle of Cross-cutting relationships
Principle of Intrusions
Principle of Inclusions

18. Principle of Cross-cutting relationships

19. Faulting

20. Principle of Inclusions
In which picture is the Granite older?

21. Principle of Intrusions

22. Intrusions vs Unconformities
A xenolith is a fragment of country rocks which has been broken off during an intrusion, and has become surrounded by magma. The xenolith is older than the igneous rock which contains it.
Through erosion and resedimentation, younger rocks will often have pieces of the older rock included (sedimentary).

23. Principle of Inclusions

24. Principle of Inclusions

25. Principle of Inclusions

26. History of Geologic Time
Geologic Systems
Body of rock that contains fossils of diverse animal life
Corresponds to geologic period
Sedgewick
Named Cambrian
Murchison
Named Silurian

27. Stratigraphy
Study of stratified rocks, especially their geometric relations, compositions, origins, and age relations
Stratigraphic units
Strata
Distinguished by some physical, chemical, or paleontological property
Units of time based on ages of strata
Geologic Systems
Correlation
Demonstrate correspondence between geographically separated parts of a stratigraphic unit
Lithologic
Temporal

28. Units of Time Time-rock unit Time unit Boundary stratotype
Chronostratigraphic unit
All the strata in the world deposited during a particular interval of time
Erathem, System, Series, Stage
Time unit
Geochronologic unit
Interval during which a time-rock unit is formed
Eras, Period, Epoch, Age
Boundary stratotype
Boundary between two systems, series or stages, formally defined at a single locality

29. Geologic Time Scale Chronologic units - Time/Age Eons (largest): Era
Periods
Epochs
Ages

30. Geologic Time Scale Geochronologic Units = Place Eon (largest) = Eon
Era = Era
System = Period
Series = Epoch
Stage = Age

31. Biostratigraphy Biostratigraphic unit Stratigraphic range
Defined and characterized by their fossil content
Stratigraphic range
Total vertical interval through which that species occurs in strata, from lowermost to uppermost occurrence

32. Biostratigraphy Index fossil
Abundant enough in the stratigraphic record to be found easily
Easily distinguished from other taxa
Geographically widespread and thus can be used to correlate rocks over a large area
Occurs in many kinds of sedimentary rocks and therefore can be found in many places
Has a narrow stratigraphic range, which allows for precise correlation if its mere presence is used to define a zone

33. Magnetic Stratigraphy
Use of magnetic properties of a rock to characterize and correlate rock units
Magnetic field
Reversals in polarity of field are recorded in rocks when they crystallize or settle from water

34. Magnetic Stratigraphy
Chron
Polarity time-rock unit
Period of normal or reversed polarity
Normal interval
Same as today
Black
Reversed interval
Opposite to today
White

35. Lithostratigraphy
Subdivision of the stratigraphic record on the basis of physical or chemical characteristics of rock
Lithostratigraphic units
Formation
Local three-dimensional bodies of rock
Group
Member
Stratigraphic section
Local outcrop of a formation that displays a continuous vertical sequence
Type section
Locality where the unit is well exposed, that defines the unit

36. Lithologic Correlation
Cross-sections of strata
Establish geometric relationships
Interpret mode of origin

37. Lithologic Correlation
Grand Canyon
McKee
Used Trilobite biostratigraphy to determine age relationships
Eastern portion of units is younger than western

38. Facies Transgression Facies Facies changes
Landward migration of shoreline
Grand Canyon
Cambrian transgression
Facies
Set of characteristics of a body of rock that presents a particular environment
Facies changes
Later changes in the characteristics of ancient strata

39. Absolute Age 4.6 billion years old Early estimates Salts in the ocean
90 million years old
Accumulation of sediment
100 m.y. or less
Gaps in stratigraphic record
Unconformities represent large breaks in accumulation
Didn’t include metamorphosed sedimentary rocks
Earth’s temperature
Kelvin
20-40 million years old

40. Absolute Ages How old is the Earth?
4.6 billion years (4,600,000,000 years)
Radiometric dating (Uranium, Thorium). Mass spectrometer.

41. Early Attempts
1654 Archbishop Usher (Ireland), genealogy in Bible Earth was created October 22, 4004 BC,
9:00 am was added later
Earth was 6000 years old.
Led to the Doctrine of Catastrophism:
Earth was shaped by series of giant disasters.
Many processes fit into a short time scale.

42. Early Attempts 1770's, 1780's "Revolution"
James Hutton, Father of Geology (Scotland)
Published Theory of the Earth in 1785.

43. Hutton
Hadrian's Wall built by Romans, after 1500 years no change. Suspected that Earth was much older.
Slow processes shape earth.
Mountains arise continuously as a balance against erosion and weathering

44. Hutton Doctrine of Uniformitarianism: "Present is key to the past".
The physical and chemical laws that govern nature are uniform
Unconformity at Siccar Point, Scotland
"No vestige of a beginning, no prospect of an end"

45. Charles Lyell
Charles Lyell 1800's compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene.
Estimated 80 million years for the Cenozoic alone.

46. Various Geologists
Thickness of total sedimentary record divided by average sedimentation rates (in mm/yr).
In 1860, calculated to be about 3 million years old.
In 1910, calculated to be about 1.6 billion years old.

47. Lord Kelvin
In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation.
Age of Earth was calculated to be about million years.

48. Lord Kelvin
Problem: Earth has an internal heat source (radioactive decay)
Discovery of radioactivity by Henri Becquerel in 1896.

49. John Joly
In , John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea.
Evaporative concentration of salts.
Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year)
Age of Earth = million years.

50. John Joly
Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.

51. von Helmholtz and Newcomb
The German physicist Hermann von Helmholtz and the American astronomer Simon Newcomb joined in by independently calculating the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born.
100 million years, consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. They knew of no other ways for it to produce its energy.

52. Rutherford and Boltwood
In 1905, they used radioactive decay to measure the age of rocks and minerals.
Uranium decay produces He, leading to a date of 500 million years.

53. Rutherford and Boltwood
In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. Published the age of a sample of urananite based on Uranium-Lead dating.
Date was 1.64 billion years.

54. Age of Earth So far, oldest dated Earth rocks are 3.96 billion years.
Canadian Shield. (NW Territories near Great Slave Lake, 3.96 byr).
Detrital Zircons in sedimentary rocks are byr
Older rocks include meteorites and moon rocks with dates on the order of 4.6 billion years.

55. Geologic Time Scale. The age for the base of each division is in accordance with recommendations of the International Commission on Stratigraphy for the year 2000.

56. Geologic Time Scale

57. The standard geologic time scale for the Paleozoic and other eras developed without benefit of a grand plan. Instead, it developed by the compilation of “type sections” for each of the systems.

58. Ordovician

59. Silurian Murchison

60. Silurian Murchison

61. Jurassic - Jura Mountains

62. Cretaceous
Kreta = Chalk
White Cliffs of Dover

63. Absolute Age Radioactive decay Becquerel, 1895
Uranium undergoes spontaneous decay
Atoms release subatomic particles and energy
Change to another element
Parent isotope decays/daughter isotope produced

64. Principles of Radiometric Dating
Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.
Radioactive parent elements decay to stable daughter elements.

65. What is an Isotope?
Nuclide of an element with different masses

66. Absolute Age Three modes of decay Loss of alpha particle
Convert parent into element that has nucleus containing two fewer protons
Loss of beta particle
Convert parent into element whose nucleus contains one more proton by losing an electron
Capture of beta particle
Convert parent into element whose nucleus has one less proton

67. Absolute Age Radiometric dating Half-life
Radioactive isotopes decay at constant geometric rate
After a certain amount of time, half of the parent present will survive and half will decay to daughter
Half-life
Interval of time for half of parent to decay

68. Absolute Age Useful isotopes Uranium 238 and thorium 232
Zircon grains
Uranium 238 and lead 206
Fission track dating
Rubidium-Strontium
Potassium-Argon, Argon-Argon
Radiocarbon dating
Produced in upper atmosphere
Half life = 5730 years
Maximum age for dating: 70,000 years
Bone, teeth, wood

69. Absolute Age Fission-Track Dating
Measure decay of uranium 238 by counting number of tracks
Tracks formed by subatomic particles that fly apart upon decay

70. Radioactive parent isotopes and their stable daughter products
Each radioactive isotope has its own unique half-life.
A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

71. Radioactive parent isotopes and their stable daughter products
Radioactive Parent Stable Daughter Half Life
Potassium 40 Argon billion yrs
Rubidium 87 Strontium billion yrs
Thorium 232 Lead billion years
Uranium 235 Lead million years
Uranium 238 Lead billion years
Carbon 14 Nitrogen years

72. Radioactive Decay

73. Radioactive Decay
Radioactive decay occurs by releasing particles and energy.
Alpha particles
Beta particles
Neutrons
Gamma rays (high energy X-rays) are also produced.

74. Radioactive Decay Alpha particles (He) large, easily stopped by paper
charge = +2
mass = 4

75. Radioactive Decay Beta particles
penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.
charge = -1
mass = negligible

76. Radioactive decay series of uranium-238 (238U) to lead-206 (206Pb).

77. Datable Minerals
Most minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time the magma cooled.
Potassium 40 is found in:
potassium feldspar (orthoclase)
muscovite
amphibole
glauconite (greensand; found in some sedimentary rocks; rare)

78. Datable Rocks
Radioactive elements tend to become concentrated in the residual melt that forms during the crystallization of igneous rocks. More common in SIALIC rocks (granite, granite pegmatite) and continental crust.

79. Datable Rocks
Radioactive isotopes don't tell much about the age of sedimentary rocks (or fossils). The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.

80. Carbon 14

81. How does 14Carbon dating work?
Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive 14C, which combines with oxygen to form radioactive CO2.

82. How does 14Carbon dating work?
Living things are in equilibrium with the atmosphere, and the radioactive CO2 is absorbed and used by plants. The radioactive CO2 gets into the food chain and the carbon cycle.

83. How does 14Carbon dating work?
All living things contain a constant ratio of 14C to 12C (1 in a trillion).
At death, 14C exchange ceases and any 14C in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new 14C.

84. How does 14Carbon dating work?
The change in the 14C to 12C ratio is the basis for dating.
The half-life is so short (5730 years) that this method can only be used on materials less than 50,000 years old.
Assumes that the rate of 14C production (and hence the amount of cosmic rays striking the Earth) has been constant.

85. Deviation of carbon-14 ages to true ages from the present back to about 5000 B.C. Data are obtained from analysis of bristle cone pines from the western United States. Calculations of carbon-14 are based on half-life of 5730 years. (Adapted from Ralph, E. K., Michael, H. N., and Han, M. C Radiocarbon dates and reality. MASCA Newsletter 9:1.)

86. Absolute Age Best candidates for most radiometric dating are igneous
Not necessarily useful for sediments
Error in age estimate can be sizable

87. Absolute Age Absolute ages change
Error increases in older rocks
Techniques change
Biostratigraphic correlations are usually more accurate
Radiometric dates used when fossils not present

88. How old is the Old Red Sandstone?
Older than 425 myr
Younger than 370 myr
Between 425 and 370 myr
Have no idea