Stratigraphy and Geologic Time. How old are the rocks?

Transcription

1 Stratigraphy and Geologic Time How old are the rocks? 1

2 Questions What is the difference between Absolute and Relative dating? How old is the Earth? How can we use fossils to date rocks? How are the absolute ages for rocks determined? How can we combine fossils, absolute dates and rules of stratigraphy to determine the ages of rocks? What are Geologic Systems [How do geologists tell time?] How are Sea Level changes recorded in the rocks and how can they help us in determining the relative ages of rocks? 2

3 Time Relative Order of deposition of a body of rock Absolute A number representing the time a body of rock was deposited 3

4 Geologic Time and Clocks Lifespan of a human ~ 100 years Human civilization ~10,000 years Modern humans Stone tools Age of oceanic crust Precambrian Explosion Oldest Rocks Age of the Earth Age of the Solar System Age of the Universe ~100,000 years ~2,000,000 years ~100,000,000 years ~540,000,000 years ~3.8 Billion years ~4.56 Billion years ~4.6 Billion years ~14 Billion years

5 Age of the Earth: Originally Based on Mythology Buddhist Tradition: Infinite Age (Cyclic) Han Chinese Tradition: 23 Million Year Cycle

6 Archbishop James Ussher (1654) ( ) 4004 BC October 23 9:00 AM

7 Most scientific attempts are based on principle that: Requires: 1. Natural Process 2. Occurs at a Constant Rate 3. Leaves a Geologic Record Age (Time) = Amount of Change Rate of Change

8 William Thomson, Lord Kelvin ( ) (1869) Million yrs Cooling of Molten Ball

9 John Joly (1899) Million yrs Saltiness of the Oceans ( )

10 Box Models Salt in rivers input Accumulating Salt in the Ocean Output? What is missing?

11 John Phillips (late 1800 s) About Million yrs Accumulation of Sedimentary Rocks

12 Marie and Pierre Curie The Discovery of Radioactivity (1896) Antoine Henri Becquerel

13 Bertram Boltwood Authur Holmes : Dated first rocks: 250 million to 1.3 billion years 1921: Earth about 4 billion years old!

14 Radioactive Decay Parent Isotope --> Daughter Isotope + Decay Particle + Energy

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16 Beta Decay Daughter Isotope Atomic Number = +1 Atomic Weight = +0 Carbon-14 --> Nitrogen-14 + Beta Particle + Energy

17 Decay of Uranium-238 to Lead 206 Alpha Decay Beta Decay

18 Half Life Time it takes for half of the parent isotope to decay into daughter isotope Daughter Isotopes Parent Isotopes

19 Radioactive Isotopes Used for Absolute Dating parent daughter half life (years) 235 U 207 Pb 4.5 Ga 238 U 206 Pb 710 Ma 40 K 40 Ar 1.25 Ga 87 Rb 87 Sr 47.0 Ga 14 C 14 N 5,730 years

20 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 20

21 Dating & Radioactive Decay

22 Note: the Residence time (mean or average life of a particle in the reservoir) L R = 1/k Radioactive Decay N = N 0 e -kt Where N is the amount of the radioactive element in the rock now; N 0 is the amount originally in the rock, e ~ (natural logarithm); k is the decay constant of the radioactive element, and t is time. Half-life when N/N 0 = 0.5

23 Stratigraphy is like reconstructing a crime scene. What happened where and when. Only then can you tell a story about the sequence of evolution, or of the movement of continents. 23

24 Box Models Salt in rivers Input 10 kg/year Assume Steady State Input = output Accumulating Salt in the Ocean Reservoir Size 100 kg What is the average stay of salt in this Ocean? Output 10 kg/year

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26 How much is a gigaton (Gt)? One billion metric tons (10 15 g) It is about 2750 Empire State Buildings. Global anthropogenic C emissions are about 6 Gt. 26

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

30 Absolute Age Absolute ages change Error increases in older rocks Biostratigraphic correlations are frequently more accurate Radiometric dates used when fossils not present

31 Mass Spectrometer

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33 The Basics of Relative Dating 33

34 Principles (Laws) of Stratigraphy Principle of Original Horizontality Superposition Lateral Continuity Cross Cutting Relationships Inclusions Faunal Succession Walther s Law

35 Nicholas Steno

36 1. Principle of Original Horizontality

37 2. Principle of Superposition Youngest Strata Oldest Strata

38 3. Principle of Lateral Continuity

39 4. Principle of Cross Cutting Relationships

40 5. Principle of Inclusions

41 William Strata Smith

42 William Smith: father of stratigraphy

43 "... each stratum contained organized fossils peculiar to itself, and might, in cases otherwise doubtful, be recognized and discriminated from others like it, but in a different part of the series, by examination of them."

44 6. Principle of Faunal Succession Sketch by Baron Cuvier ( ) First to suggest that species became extinct

45 Biostratigraphy Biostratigraphic unit Defined and characterized by their fossil content Stratigraphic range Total vertical interval through which a species occurs in strata, from lowermost to uppermost occurrence 45

46 Biostratigraphy Zone (biozone) Body of rock whose lower and upper boundaries are based on the ranges of one or more taxa in the stratigraphic record Named for the taxon that occurs within it 46

47 Biostratigraphy Index fossil Abundant enough to be found easily Easily distinguished from other taxa Geographically widespread Occurs in many kinds of sedimentary rocks Has a narrow stratigraphic range, which allows for precise correlation 47

48 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 48

49 Stratigraphy Study of stratified (layered) rocks, especially their origins and age relations Stratigraphic units Strata Distinguished by some physical, chemical, or paleontological property Geologic Systems Correlation Demonstrate correspondence between geographically separated parts of a stratigraphic unit Lithologic Temporal 49

50 Units of Time Time-rock unit 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 50

51 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

52 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 52

53 Lithostratigraphy Dividing the rock record on the basis rock type and characteristics Lithostratigraphic units Formation Local three-dimensional bodies of rock Group Member Stratigraphic section Local outcrop drawn as a vertical sequence Type section Locality that defines the unit 53

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

55 Lithologic Correlation Grand Canyon McKee Used Trilobite biostratigraphy to determine age relationships Eastern portion of units is younger than western 55

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57 Transgression Facies 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

58 Isotope Stratigraphy Strontium 86 and strontium 87 Change through time in seawater due to different rates of weathering and seafloor spreading Organisms incorporate Sr instead of Ca into their skeletons in the same ratio as the seawater in which they lived Changes in ratio used to determine time

59 Event Stratigraphy Marker bed Bed of sediment Same age throughout Ash fall Bishop Tuff

60 Event Stratigraphy Cretaceous volcanic eruption Deposited ash between marine sediments 60

61 Event Stratigraphy Evaporites Distinct patterns and geochemistry of layers Useful for correlation over wide regions 61

62 Facies Boundaries Correlating sections within a basin Point of maximum transgression 62

63 Seismic Stratigraphy Interpretation of seismic reflections generated when artificially produced seismic waves bounce off physical discontinuities within buried sediments 63

64 Seismic Stratigraphy Creates an image of the subsurface Discontinuities and unconformities can be identified 64

65 Eustatic Changes Global curve of Cenozoic sea-level changes Extended to rest of Phanerozoic Eustatic change Global change in sea level 65

66 Eustatic Changes Events on land can affect global change on a local level Uplift can mask eustatic change Sediment accumulation influences local response 66

67 Sequence Stratigraphy Sequences Large bodies of marine sediment deposited on continents when the ocean rose in relation to continental surfaces and formed extensive epicontinental seas Sediment geometry is useful for reading sea-level change 67

68 Blue = No deposition

69 Vail Curve of Sea Level History Transgression Regression

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