Historical Geology

• 2 Types of dating:

  1. Relative dating
  2. Absolute Dating
• Relative dating

  • Deals with the sequence and chronology of the layers with respect to each other
  • Does not entail numerical ages
• Absolute dating

  • Determines the numerical ages of minerals, rocks, and fossils
• Radioactive

  • unstable nuclei that dissipates energy in form of radiation
• Isotopes

  • variants of the same atoms but with with different mass numbers
• Half-life

  • the length of time required for one-half of the nuclei of a radioactive isotope to decay
• Oldest rock in northwestern canada: Acasta gneisses
• Oldest rock in Greenland: Isua Supracrustal rocks
• Oldest rocks in minnesota river valley and northern michigan
• Oldest rocks on earth

  • Acasta gneisses in northwestern Canada (4.03 Ga)
  • Isua Supracrustal rocks in Greenland (3.7 to 3.8 Ga)
  • Rocks found in the minnesota river valley and northern michigan (3.5-3.7 billion years)
  • Western Australia (3.4-3.6 billion years ago)
  • Faux-Amphibolites
• Oldest materials in West Central Australia: Zircon grains found in sedimentary rocks
• Oldest Materials on Earth

  • Zircon grains found in sedimentary rocks in west central Australia = 4.4 b.y
  • 70 well-dated meteorites using different dating methods (e.g Rb-Sr,Sm-Nd, Ar-Ar) = 4.4-4.6 b.y
  • Iron meteorite (canyon Diablo meteorite) = 4.54 b.y.)
• Principle of Uniformitarianism - the present is the key to the pas
• Steno’s Laws

  1. Law of Superposition
  2. Law of original horizontality
  3. Law of lateral Continuity
  4. Law of cross cutting relationships
• According to law of superposition:

  Layer A - sandstone and shale
• According to law of superposition:

  Layer B - shale and siltstone
• According to law of superposition:

  Layer C - sandstone
• According to law of superposition:

  Layer D - sandstone and limestone
• According to law of superposition:

  Layer E - limestone
• Law of original horizontality

  • Most layers are deposited horizontally
• Law of lateral Continuity

  • Sediments would spread out until they thin out at the edge of the deposition basin, stop at a depositional barrier
• Law of cross cutting relationships

  • When a fault or intrusion cuts through a another rock, the fault or intrusion is younger that the rocks which it cuts
• Principles of inclusions - the rock mass containing the inclusion is younger than the rock that provided the inclusion
• Unconformities - Any significant break in time within a stratigraphic column
• Types of unconformities:

  1. Angular unconformity
  2. Disconformity
  3. Paraconformity
  4. Nonconformity
• Angular unconformity

  • develop an angle with in respect to the more horizontal strata
• Angular unconformity

  • Tilted or folded sediments that are overlain by younger, more flat-lying strata
• Deposition

  
• Deformation

  
• Erosion

  
• Renewed deposition

  
• Disconformity

  • Strata on either side of the unconformity are essentially parallel with a distinctly recognizable surface
  • Has an erosional plane
• Nondeposition

  • the time when the rate of deposition is equal to the rate of erosion
• Paraconformity - created by nondeposition

  • Beds above and below are parallel and the unconformity is identified by some evidence such as lack of certain diagnostic zone fossils in some horizon
• Nonconformity

  • Older metamorphic or igneous rocks are overlain by younger sedimentary strata
  • Sedimentary on top is eroded, metamorphic rock is exposed, new sedimentary is added
• Disconformity

  
• Angular conformity

  
• Nonconformity

  
• Principle of Faunal succession

  • Fossil organisms succeed one another in a definite and terminable order. Thus, any time period can be recognized by its fossil content
• Lower Cambrian