Glacial Terraces

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Glacial Terraces

• Tom Isaacs
• April 3, 2009

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Glacial Terrace Formation

• Terrace Formation
• The variability in vertical incision rates to
  lateral planation rates ( Gilbert)
• Terrace formation has been ascribed to changes in
  sediment supply and/or water discharge produced
  by late Quaternary climatic fluctuations.
  (Anderson 2002)

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Glacial Terrace Formation

• Process
• Sediment impute vs. stream incision
• Time
• Glacial vs. Interglacial
• Location
• Landscape directly effected by glacial sediment
  supply and melt water discharge from a glaciated
  region.

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Process

• Planation
• Deposition of sediment across broad valley
• Abandonment
• Changes course through time
• Incision
• Bedrock
• River Sediment
• Formation of terraces lags by several thousand
  years from the input changes that cause their
  formation (Anderson 2002).
• Record of Climate change?

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Location

• Where glacial erosion supplies sediment into a
  stream with changing ratios of planation verses
  incision rate.
• Preservation potential?
• Accommodation space?

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Time
When does sediment supply decrease and discharge
increase for vertical incision to occur and a
terrace to form?

• Glacial transitions
• peak glaciations
• deglaciations
• Interglaciations

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Case Study
A 900 k.y. record of terrace formation during
glacial-interglacial transitions in northwest
China. By Pan, Burbank, Wang, Wu, Li, and
Guan (2003)
Objectives - Date paired bedrock terrace
sequences -Thermoluminescence, -Radiocarbon
-Palemagnetic - Determine when terraces form in
relation to glaciations, deglaciations, or
interglaciations
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Case Study

• Shaque River
• 50 km
• 3500 meters
• Secession of five preserved glacial terraces
• Terraces grade down stream to alluvial fan
• Capped with thin veneer of gravel
• 200m of loess (Aeolian)

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Map/Cross Sectional View
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Case Study

• Summary of Pan, and Burbank Results
• Only after abandonment of terrace do nonfluvial
  processes such as loess accumulation or paleosols
  form
• Paleosols were date and correlated to well
  documented loess-paleosol stratigraphy on the
  loess plateau (Liu, 1987) this suggested that
  there were not any paleosols missing from the
  terrace sequence.
• They were then able to assign ages to the
  nonfluvial deposits.
• Key observation The first paleosol to develop
  above the bedrock terrace was on 1-2 meters of
  loess. This thin loess cover suggests that
  terrace did not form during pure interglacial
  time, because loess deposits persisted prior to
  soil formation.
• The bedrock terrace were not formed during a
  fully glacial time because loess deposits would
  be thicker before paleosol development.
• Pan, and Burbank concluded that each terrace
  was abandoned during the transition from glacial
  to interglacial climates, demonstrating a strong
  climatic control on terrace formation.

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References

• Anderson, R.S., and Hancock G.S., 2002, Numerical
  modeling of fluvial strath-terrace formation in
  response to oscillating climate, Geological
  society of America Bulletin, v.114, p. 1131-1142.
  
• Ritter, D.F., 1967, Terrace Development along the
  Front of the Beartooth Mountains, Southern
  Montana, Geological Society of America Bulletin,
  v. 78, p. 467-484.
• Pierce, K.L., and Colman, S.M., Effect of height
  and orientation (microclimate) on geomorphic
  Degradation rates and processes, late-glacial
  terrace scarps in central Idaho, Geological
  Society of America Bulletin, v. 97 p. 869-885.
• Born, S. M. and Ritter, D.F., 1970, Modern
  Terrace Development Near Pyramid Lake, Nevada,
  and its Geologic Implications, Geological society
  of America Bulletin, v. 81, p. 1233-1242.
• Pan, P., and Burbank, D., 2003 Record of strath
  terrace formation during glacial-interglacial
  transitions in northwest China, Geology, v. 31
  no.11, p. 957-960.