Barrier Island Formation and Stratigraphy of Overwash Deposits

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Barrier Island Formation and Stratigraphy of
Overwash Deposits
nasa.gov
www.geology.iupui.edu
www.livingocean.org

• Scott A. Maxwell

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What is a barrier island?

• Barrier islands are thin strips of land that have
  accreted vertically through the combined actions
  of wind, waves, and longshore currents. They
  consist of several parts landward margin,
  barrier interior, and beach (Davis and
  Fitzgerald, 2004).

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Cross Section of a Barrier Island and
Backbarrier (from Beaches and Coasts By R. A.
Davis, 2004)
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How do barrier islands form?

• Three Theories
• Offshore bar theory
• -Beaumont (1845)
• Spit accretion theory
• -Gilbert (1885)
• Submergence theory
• -McGee (1890)

Davis and Fitzgerald (2004)
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www.nasa.gov
Examples of Barrier Island Theories
Davis and Fitzgerald (2004)
Penland and Boyd (1985)
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What promotes island development?

• High wave vs. tidal energy
• Sediment Supply
• Wide flat continental Shelf

www.nasa.gov
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Wave Versus Tidal Energy Hayes Model
Davis and Hayes (1984)
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Mixed Energy
Wave Dominated
pubs.usgs.gov
georgiamagazine.com
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Prograde, Retrograde or Aggrade?

• Retrograding island stratigraphy (notice the
  washover deposits below the beach face)
• Prograding barrier stratigraphy
• Aggrading barrier stratigraphy (notice the
  overlying layers of washover and lagoon)

Galloway and Hobday (1980)
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science.howstuffworks.com
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Retrograding/Overwash
Davis and Fitzgerald (2004)
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Chandeleur Islands usgs.gov
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Importance of Washover Deposits

• Indicators of
• Ancient sea level rise
• Historical frequency of major storm events
• Barrier island evolution
• They can also serve as possible oil resevoirs!!!

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Washover Deposits

• Washover that deposits into the backbarrier
  lagoons and marshes
• Washover deposited in horizontal landward dipping
  strata.

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Washover Stratigraphic Units
Sedgwick and Davis (2003)

• Five Subfacies
• Stratified sand
• -Gently landward dipping beds of alternating
  laminae of shells and heavy minerals.
• -Upper plane bedforms with scour surfaces.
  (high Froude number)
• Reverse-graded sand
• -Least common subfacies. Forms from sinking
  of small heavy minerals
• Normal-graded sand
• -Most abundant in organic material (gt20
  shells)
• -Thick algal mats can overly the surface and
  separate washover events
• -Lies in the intertidal region, bioturbation is
  prevalent
• Bioturbated muddy sand
• -40-70 bioturbated due to its deposition in
  the intertidal to subtidal region
• Undifferentiated sediments
• -100 bioturbated fine grained silt and sand
• -Distal margin of the fan

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Tidal Range on Washover Fans
Sedgwick and Davis (2003)
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Conclusion

• Barrier Islands form through a variety of
  processes that cannot be pigeon-holed.
• Stabilization of barriers depends on wave and
  tide energy, sediment supply, and the topography
  of the continental shelf.
• Barrier islands retrograde primarily through the
  process of overwash and sediment transport to the
  backbarrier region.
• Washover deposits can be subdivided into five
  subfacies stratified sand, reverse-graded sand,
  normal-graded sand, bioturbated muddy sediments,
  and undifferentiated sediments.
• Preservation is most likely in intertidal and
  subtidal zones.