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Geologic Time

Geologic Time. Correlation and Dating of the Rock Record. Time. Relative Order of deposition of a body of rock based on position Absolute A number representing the time a body of rock was deposited. Relative Time. Determining of sequence of events Which came first?.

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Geologic Time

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  1. Geologic Time Correlation and Dating of the Rock Record

  2. Time • Relative • 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? D

  5. Relative Time • Tools • Smith • Fossil Succession • Steno • 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 - 1858).

  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 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. DisconformityBig Brook

  13. Angular Unconformity

  14. NonconformityBrahma Schist underlies Tapeats Sandstone http://www.physci.mc.maricopa.edu/Geology/FieldTrips/ColoradoRiver/ColoradoRiver_2006_Summer/ColoradoRiver_2006_Summer_Images_1024/DSC00512a_1024c.jpg

  15. NonconformityBrahma Schist underlies Tapeats Sandstone http://www.physci.mc.maricopa.edu/Geology/FieldTrips/ColoradoRiver/ColoradoRiver_2006_Summer/ColoradoRiver_2006_Summer_Images_1024/DSC00512a_1024c.jpg

  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 • 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 • 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 • 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) 1726-1797. • 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 24-40 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 1899 - 1901, 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 = 90-100 million years.

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

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