Title: Barrier Island Formation and Stratigraphy of Overwash Deposits
1Barrier Island Formation and Stratigraphy of
Overwash Deposits
nasa.gov
www.geology.iupui.edu
www.livingocean.org
2What 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).
3Cross Section of a Barrier Island and
Backbarrier (from Beaches and Coasts By R. A.
Davis, 2004)
4How do barrier islands form?
- Three Theories
-
- Offshore bar theory
- -Beaumont (1845)
- Spit accretion theory
- -Gilbert (1885)
- Submergence theory
- -McGee (1890)
Davis and Fitzgerald (2004)
5www.nasa.gov
Examples of Barrier Island Theories
Davis and Fitzgerald (2004)
Penland and Boyd (1985)
6What promotes island development?
- High wave vs. tidal energy
- Sediment Supply
- Wide flat continental Shelf
www.nasa.gov
7Wave Versus Tidal Energy Hayes Model
Davis and Hayes (1984)
8Mixed Energy
Wave Dominated
pubs.usgs.gov
georgiamagazine.com
9Prograde, 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)
10science.howstuffworks.com
11Retrograding/Overwash
Davis and Fitzgerald (2004)
12Chandeleur Islands usgs.gov
13Importance 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!!!
-
14Washover Deposits
- Washover that deposits into the backbarrier
lagoons and marshes - Washover deposited in horizontal landward dipping
strata.
15Washover 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
16Tidal Range on Washover Fans
Sedgwick and Davis (2003)
17Conclusion
- 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.