Relative Dating Techniques Sierra Nevada, California R.M. Burke and Peter W. Birkeland

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Relative Dating TechniquesSierra Nevada,
CaliforniaR.M. Burke and Peter W. Birkeland

• A review by
• Karen K. Shell
• ESS 433

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Overview

• Defining Glaciations using RD Techniques
• Field Case Studies
• Questions

• Environmental Factors
• Weathering Criteria
• Moraine Morphology

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Environmental Factors

• Setting of Deposits
• Vegetation
• Lithology

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Environmental FactorsSettings of Deposits

• Valleys
• Elevation of valleys
• Constraining bedrock
• Location with respect to mountain range

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Environmental FactorsVegetation

• Forests
• Brushes
• Smaller Trees
• Grasses
• Soil Development

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Environmental FactorsLithology

• Importance of minimizing variables
• Limit to particular grain sizes and weathering
  resistant minerals
• Granodiorite or Quartz monzonite sampled
• Correlation between sample sites

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Weathering CriteriaMoraine Surface Boulders

1. Fresh-to-weathered ratio
2. Pitted-to-non pitted ratio
3. Pit Depth
4. Maximum height of resistant mafic inclusions
5. Rind-to-no rind ratio

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Weathering CriteriaMoraine Surface Boulders

• Average rind thickness
• Hammer-blow weathering ratio
• Surface boulder frequency
• Granitic boulder-to-non granitic boulder ratio
• Split-to-non split ratio
• Oxidation ratio

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Weathering CriteriaMoraine Surface Boulders
(continued)

• Fresh-to-weathered ratio
• Requires 50 of boulder to exhibit weathering
• Rough texture
• Pitted-to-non pitted ratio
• Has one or more concave depressions (pits)
• Pits are not required to be circular in shape
• Counts are made separate from boulder counts
• Caused by break down of grains

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Weathering CriteriaMoraine Surface Boulders
(continued)

• Pit depth
• Depth is measured from lowest point of pit to
  current boulder surface
• Maximum height of resistant mafic inclusions
• Measured from top of inclusion to average
  position of the adjacent rock surface
• Rind-to-no-rind ratio
• Discolored parallel to outer surface

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Weathering CriteriaMoraine Surface Boulders
(continued)

• Average rind thickness
• Measured after rind-to-no rind ratio
• Measured to nearest millimeter
• Hammer-blow weathering ratio
• Based on sound heard when striking boulder with
  hammer. Determines whether boulder is fresh,
  weathered or grusified
• Surface boulder frequency
• Measurement of boulder frequency along crest of
  moraine within an area abundant with boulders

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Weathering CriteriaMoraine Surface Features
(continued)

• Granitic boulder-to-non granitic boulder ratio
• 50 samples taken on boulders greater than 50 cm
  in diameter
• Split-to-non split ration
• Breaks appear along planar cracks
• Not the result of spalling
• Oxidation Ratio
• Relative visual discoloration within and between
  50 samples measured

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Weathering CriteriaSubsurface Features

1. Grusified granitic boulders beneath surface
2. Soil properties

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Weathering CriteriaSubsurface Features
(continued)

• Grusified granitic boulders
• Diameters of 30 cm or greater are measured
• Grusification is the intense disintegration of
  grains throughout the clast
• Classification
• Fresh
• Grusified
• Grusified and un-oxidized
• Grusified and oxidized

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Weathering CriteriaSubsurface Features
(continued)

• Soil Properties
• Measured in field and analyzed in laboratory
• Extracted from pits dug along moraine crests.
• Field Properties
• Horizonation
• Color
• Texture
• Consistence
• Structure
• pH
• Other diagnostic features (clays, carbonates,
  etc.)

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Weathering CriteriaSubsurface Features
(continued)

• Soil Properties (continued)
• Laboratory Properties
• Particle size distribution
• Percent loss during ignition
• Organic matter
• pH
• Dry color

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Weathering CriteriaMoraine Morphology

• Width of crest of moraine
• Angle of slopes of moraine
• Inner and outer slopes
• Possible Downfalls
• Precision and accuracy require a single person to
  take measurements

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Defining a Glaciation Using RD Techniques

• How much variation occurs between deposits?
• How much variation occurs between a stade?
• Splitters vs. Lumpers
• Burke and Birkelands methodology

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Defining a Glaciation Using RD Techniques
(continued)

• Splitters
• Researchers that make the subdivision at the
  maximum interval of the sequence, namely at the
  stade level.
• Subtle variations are of greater importance
• Good for determining moraine positions and
  cross-cutting moraine relationships

• Lumpers
• Researchers that are more inclined to make
  distinctions between glaciations at larger scale
• Consistent recognition of weather features
  between research locations (valley to valley)

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Defining a Glaciation Using RD Techniques
(continued)

• Burke and Birkelands Methodology
• Methodology largely influenced by the focus of
  the research on moraines that are in close
  proximity (adjacent, local)
• Determined that no single value could be set for
  determining glaciations in all locales

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Defining a Glaciation Using RD Techniques
(continued)

• Burke and Birkelands Methodology
• Burke and Birkeland settled for a magnitude of 2
  times or greater in variation to separate
  glaciations
• Allows for distinctions to be made between
  first-order glaciations rather than secondary
  fluctuations of the first-order glaciations

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Field Case Studies
Bloody Canyon
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Field Case StudiesBloody Canyon

• Previous Studies
• Some studies divided morphological features into
  four glaciations Mono Basin, Tahoe, Tioga and
  Tenaya. (McGee, 1885 Russell, 1887 others)
• Other proposed two specific glaciations prior to
  Burke and Birkeland Tioga and Tahoe (Putnam,
  1949 Kistler, 1966)

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Field Case StudiesBloody Canyon (continued)

• Burke and Birkelands Assessment
• Tioga
• Lie above Tahoe deposits and through RD
  Techniques is similar in morphology, lithology,
  etc to the Tenaya deposits.
• Determined that Tioga and Tenaya are part of the
  same glacial event.
• Younger than Tahoe deposits as determined by
  degree of weathering and superposition

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Field Case StudiesBloody Canyon (continued)

• Burke and Birkelands Assessment
• Tioga
• Evidence?
• Presence of ash deposits above Till
• Absence of clay build-up except near the top of
  the profile
• Granitic boulders are fresh throughout the
  soils

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Field Case StudiesBloody Canyon (continued)

• Burke and Birkelands Assessment
• Tahoe
• Due to extent of moraines, several sites were
  studied
• Vegetation at each of the study sites is crucial
  to correlation of RD data
• Ideal scenario would be the absence of vegetation
  or if not available, two sites of similar
  vegetation

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Field Case StudiesBloody Canyon (continued)

• Burke and Birkelands Assessment
• Tahoe
• Evidence?
• Little difference in subsurface age between Mono
  Basin and Tahoe deposits
• Minimal soil development
• Both have clay development in the B-horizon of
  the soils. Tahoe deposits exhibits clay in
  horizons above and below the B-horizon
• Clay development is the result of eolian influx
  and in situ weathering

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Field Case StudiesBloody Canyon (continued)

• Burke and Birkelands Assessment
• Tahoe
• Evidence? (continued)
• Surface boulders are limited and when found a
  moderately to completely grusified.
• Since differentiation between the sub-surfaces
  cannot be distinguished from one another, the
  Mono Basin and Tahoe deposits cannot be proven to
  be the result of separate glaciations.
• 7. Only distinction is the moraine morphology
  (Mono Basin exhibits less relief than Tahoe
  Moraine).

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Discussion Questions

• What are the positive/negative aspects of the RD
  Techniques?
• Does the RD technique have differing results when
  applied to different types of glaciers (i.e.
  warm-based vs. cold-based, etc)?
• Is the elimination of the Mono Basin and Tenaya
  glaciations valid?
• Is the justification of order of magnitude for
  determining/separating glaciations a valid way to
  go about distinguishing glaciations?

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