The Vanishing Islands: A Look at the Louisiana
Coasts Barrier Island System
Western Governors University
One of the youngest geologic structures already in danger of vanishing. Formed in
the Holocene Epoch apart of microtidal and mesotidal regimes( 4,000 to 6,000
years ago) upon the decline of rapid sea level rise and sea level stabilization.
Clastic sedimentary marine deposits
The geologic mechanisms that formed barrier islands are still in question, though
most recent findings coincide with spit-building processes.
1919 Theory (Johnson) -
Influx of sediments
Spits are breached by tidal
action and separated from the
mainland forming a barrier
Sea level stabilizes and
tidal actions rework
sediments into strips of
Erosional drift of sediment
accretes down current of
mainland forming spits
A Decline in Rapid
Sea Level Rise
Dunes - Mounds of sand crucial for stabilizing the barrier island and preventing erosion.
Wash-over - Inundation of the dunes by seawater resulting in run-off
basin-ward through the sand flats. Exhibited morphology is of narrow-bands
that expand into funnel-like areas for vegetation in thin laminate sheets (<2m). Isolated
lenses of basal sediments from spit-formations are in units. Wash-overs gradually
supersede existing flood deltas.
Lagoon - Muddy waters , truncated areas from washover that become anoxic
organic environments. Preserved units indicate impermeable mud, organic
material bound by sand (see stratigraphic controls) that result in petroleum
and coal deposits.
Flood Tidal Delta -Basinward deposit of sediments that reactivate in a
transgressive environment causing higher rates of erosion.
Spits - Rare features known for their sediment accreting properties in rapid
island building processes and proposed to have the primary role in barrier
island creation. The littoral drift caused by an oblique angle of waves along
with bioturbation of sediments forms these structures. In landward
deposition, preserved currents can be found in units near the back-barrier.
Steep to gentle stratification indicate both a landward and seaward dip with
deposits consisting of shell fragments. ( elaborate on spatial trend, highest
subsidence rates always at these locations)
“Outcrop of Mississippean barrier island tidal channels on Route 23
Kentucky at Pound Gap.” ( Woodworth, Mendenhall, Knight,
Shoreface - Where the ocean and barrier island come together. Shallow sandbar
below the water line. Waves have lost their intensity and fossils are found as a
result of bioturbation. Silt, mud, and fine sand are the sediments here
Middle - No bioturbation as a result of varying wave intensity. Size of sand grains
Foreshore - An area of high and low tide where coarser sand allows for cross-
Backshore - Above water where berms are located where wind dominates
“ Major Depositional Settings of Barrier Island Systems ” SEPM Sequence
As Mother Natures first line of defense, barrier islands protect the
mainland and the wetlands against storms. The USGS Geological Scale affect
of storms to barrier islands can be assessed on an impact scale from 1-4. As a
result of an Impact 4, duly noted as Hurricane Katrina, the barrier islands
suffered severe net loss to land. They serve as system controls to wetland
environments along with shielding pipelines and industry terminals from
Back-barrier- An area that the barrier island separates from the ocean where many
deposited units can be determined. There is a progradation trend in upper 6m of strata,
where oldest to youngest deposits can further be separated. In their units a switch in
stacking patterns can be identified, where the continental shelf space opens up and
deposition is less, accommodating the shelf mudstone in the change of a microtidal
regime. Barrier islands serve as a gateway in the exchange of sediments that pass through
to flood tidal deltas and those that nourish the dunes of the islands.
Timbalier Island - Known for its historical processes and the most morphological
changes. Exhibits most land migration. Accretion on the west end as a result of
island spit-building process causes lateral migration and shows to be the best
means for rapid growth at the cost of erosion.
Chandeleur Islands - Suffered devastating effects from Hurricane Katrina and the
recent Deepwater Horizon oil spill.
Raccoon Island/Pass - Flood tidal inlet with diverse stratagraphic depositions and a
transgressive trend. Its crossing with Timbalier Bay has led concurrent projects including
beach nourishment. Underlying are buried remnants of Mississippi distribution channels
that underwent a regressive event and are possibly the cause of high erosion rates.
Currently, the shoreline of this barrier island is retreating.
Lafourche Spit - Cores taken from deposits here are unique in that they blend in with
the flood-tidal sand-rich facies, though have a subtle identifiable feature to allow for
further separation. It has been found that a bed of species, the Mulina, and fragments of
them are living at the basal contact zone. A “hash horizon” about .25m thick capped with
finer grain sediments is found in cores at this location.
A Google Earth Closer Look:
Specific Features of Barrier Islands
Historical Distributaries of Mississippi
River By Jakob Rosenweig, Tulane
With closing of the Mississippi River Gulf Outlet Canal historical channels can be
restored to nourish wetlands
Increase plant cultivation on Hammock (forested) Islands
Review and support those entities actively engaged in restoring the barrier islands.
hasexcellentrootworkproperties. The UnitedStatesDepartmentofAgriculture(USDA)haslisted
plants toproliferate intheareas (aslistedbelowright.)
The state of Louisiana is not highlighted in
the USDA map of native status, though several
states have similar wetland conditions that
satisfy their growth in buffer zones. Rhode
Island is such a state that has also had beach
nourishment activities due to erosion.
“Stratigraphic Architecture of a Transgressive Tidal Inlet-Flood
Tidal Delta System: Raccoon Pass, Louisiana” ( Kulp, M.A.,
Fitzgerald, D., Penland, S., Motti, J. Brown, J., Flocks, J.,
Miner, M., McCarty, P., Mobley, C.)