Concepts and Principles

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Geologic Time
Concepts and Principles
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I. Relative Time vs. Absolute Time
Relative Age

• _____________________
  
• Does not determine the number years involved but
  is concerned with the sequence of events (rock
  layers, erosion, structures formed, etc.)
• ______________________
• Determines the actual age of the rock in years

Absolute Age
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II. PRINCIPLES USED TO DETERMINE RELATIVE AGE
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A. ____________________________
Principle of Original Horizontality

• 1. Sedimentary rock layers, or strata, were
  originally deposited as relatively horizontal
  sheets of sediment.
• 2. Strata that do not retain their original
  horizontality have been displaced by movements of
  Earths crust.

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Sediment is deposited in essential horizontal
layers

• Therefore, a sequence of sedimentary rock layers
  that is steeply inclined from horizontal must
  have been tilted after deposition and
  lithification

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B. _____________________________
Principle of Lateral Continuity

• 1. Sedimentary rock layers or lava flows extend
  laterally (geographically) in all directions
  until they thin to their termination or ends of
  their basins of deposition.
• 2. Another term for this is that they pinch out.

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Gradual Terminations

• A rock unit becomes progressively thinner until
  it pinches out

• or where it splits into thinner units
• each of which pinches out,
• called intertonging

• where a rock unit changes
• by lateral gradation
• as its composition and/or texture
• becomes increasingly different

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C. ______________________________
Principle of Cross-Cutting Relationships

• Any feature (fracture, fault, intrusive mass)
  that cuts across a body of sediment or rock is
  younger than the body of sediment or rock that it
  cuts across.

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Cross-cutting Relationships

• North shore of Lake Superior, Ontario Canada
• A dark-colored dike has intruded into older light
  colored granite.

• The dike is younger than the granite.

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Cross-cutting Relationships

• Templin Highway, Castaic, California
• A small fault displaces tilted beds.

• The fault is younger than the beds.

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Folds
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D. ___________________________
Principle of Superposition

• In an undisturbed succession of sedimentary rock
  layers,
• the oldest layer is at the bottom
• and the youngest layer is at the top

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Principle of Superposition

• Illustration of the principles of superposition
  and original horizontality

• Superposition The youngest
• rocks are at the top
• of the outcrop
• and the oldest rocks are at the bottom

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Overturned Folds

• Can cause older rocks to be located above
  younger rocks.

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Thrust Faults

• Can push older rocks above younger rocks.

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Chief Mountain, Glacier National Park, Montana

• Erosional remanant of a thrust fault
• Cross section shows older Precambrian rocks
  thrust over younger Cretaceous rocks

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E. ____________________
Principle of Inclusions

• Inclusions or fragments in a rock are older
  than the rock itself

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Principle of Inclusions

• Light-colored granite
• in northern Wisconsin
• showing basalt inclusions (dark)

• Which rock is older?
• Basalt, because the granite includes it

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Age of Lava Flows, Sills

• Determining the relative ages of lava flows,
  sills and associated sedimentary rocks
• uses alteration by heat
• and inclusions

• A lava flow forms in sequence with the
  sedimentary layers.
• Rocks below the lava will have signs of heating
  but not the rocks above.
• The rocks above may have lava inclusions.

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Sill

• A sill will heat the rocks above and below.

• The sill might also have inclusions of the rocks
  above and below,
• but neither of these rocks will have inclusions
  of the sill.

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F. __________________
Unconformities

• 1. Surfaces that represent a gap in the geologic
  record.
• Like pages in a book, they are surfaces on which
  sediment was not deposited for a period of time,
  or surface on which erosion has occurred.
• 3. Unconformities can range in size from bedding
  planes (surfaces between strata) to
  continent-wide surfaces

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The Sequence of Events in the Formation of an
Unconformity
Siccar Point, Scotland
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Huttons UnconformitySiccar Point Scottland
Near Edinburgh
I dont know who this guy is!
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5. Types of Unconformities
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a) _________________
Nonconformities

• An erosional surface that separates older igneous
  or metamorphic (nonsedimentary) rock from younger
  overlying sedimentary strata. Note the absence
  of contact metamorphism in the rocks immediately
  above the igneous or metamorphic rocks.

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Sequence of Events in the Formation of a
Nonconformity
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Nonconformity
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b) ________________________
Angular Unconformities

• An erosional surface that separates tilted or
  folded strata from overlying beds of different
  attitude. This implies that an area has
  undergone uplift, and that the uplift was
  accompanied by either folding or tilting with
  erosion of the strata prior to later subsidence
  and continued deposition.

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The Formation of an Unconformity
An erosional surface buried by younger
sedimentary rocks
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Grand Canyon
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c) __________________
Disconformities

• An erosional surface that separate essentially
  parallel sedimentary strata. These are probably
  the most common types of unconformities and are
  often the most difficult to recognize. The is
  especially so when the rock types are similar
  above and below the erosion surface.

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Disconformity
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Disconformity
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Disconformity
Along a roadcut in Gladeville, TN
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III. Correlation
Determining the age relationships between rock
units or geologic events in separate areas

• A. SIMILARITY OF ROCK TYPES
• 1. __________________________________
  
  
• Being able to trace physically the course
  of a rock unit.
• 2. Walking the Outcrop

Lithostratigraphic Correlation
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Lithostratigraphic Correlation

• Correlation of lithostratigraphic units such as
  formations
• traces rocks laterally across gaps

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Correlation Using Lines Drawn to Points of
Equivalence
Disconformities
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Fossils
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1. ____________________
Index (Guide) Fossils

1. Short lived species
2. Easily identifiable
3. Wide geographic distribution

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Index (Guide) Fossils

• The brachiopod Lingula
• is not useful because,
• although it is easily identified
• and has a wide geographic extent,
• it has too large a geologic range
• The brachiopod Atrypa
• and trilobite Paradoxides
• are well suited
• for time-stratigraphic correlation,
• because of their short ranges
• They are guide fossils

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2. ______________________
Fossil Assemblage

• Several different fossil species in a rock layer
• All species existed when the sediment was
  deposited.
• Use a fossil range chart.

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Fossil Ranges
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Fossil Assemblages