EES 106-02

history of geologic time

  • James Hutton and Charles Lyell
    • “present as the key to the past”
  • Nicolaus Steno
    • ordered rocks
  • William Smith
    • correlated with fossils
  • Georges Cuvier
    • extinction

How to tell time

  • Relative
    • order of deposition of a body of rock based on position, sequencing events
  • Absolute
    • a number representing the time a body of rock was deposited, specific age

history of geologic time

  • geologic systems
    • body of rock that contains fossils of diverse animal life
    • corresponds to geologic period

stratigraphy

  • study of stratified rocks, especially their geometric relations, compositions, origins, and age relations
  • stratigraphic units
    • strata
      • distinguished by some physical or chemical proerty or fossil content
      • units of time based on ages of strata
    • geologic systems
      • time scale

using strata as a relative dating tool

  • principle of superposition
    • in a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect
  • principle of original horizontality
    • layers of sediment, when originally deposited, are fairly horizontal
  • principle of lateral continuity
    • sedimentary layers, before erosion, formed fairly continuous sheets over a region

the Grand Canyon is an example of superposition, original horizontality and original continuity

Commonly there are gaps in geologic time because rocks of a certain age could be missing:

  • unconformities: a boundary between two different rock sequen ces representing an interval of time during which new strata were not deposited and/or were eroded
    1. Angular: an unconformity in which the strata below were tilted or folded before the unconformity developed; strata below the unconformity therefore have a different tilt than strata above
    2. Nonconformity: a type of unconformity at which sedimentary rocks overlie basement (older intrusive igneous rocks and/or metamorphic rocks)
    3. Disconformity: an uncomformity parallel to the two sedimentary sequences it separates

principle of cross-cutting relationships: if one geologic feature cuuts across another, the feature that has been cut is older

principle of baked contacts: when an igneous intrusion “bakes” (metamorphoses) surrounding rock, the rock that has been baked must be older than the intrusion

principle of inclusions: if a rock contains fragments of another rock, the fragments must be older than the rock containing them

stratigraphy

  • correlation
    • demonstrate correspondence between geographicall separated parts of a stratigraphic unity
      • lithologic (lithostratigraphy)
        • based on rock type

lithologic correlation: a correlation based on similarities in rock type

facies: the aspects of a rock that reflect the environment of its formation. can be based on the sediments (lithofacies) or fossils (biofacies) present in the rock. Along coastlines, sand close to shore, mud farther away, limestone facies farthest from shore (in sub-tropic to tropic environment)

transgression: the inland migration of shoreline resulting from a rise in sea level

regression: the seaward migration of a shoreline caused by a lowering of sea level

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