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					                                  Lagoon Environment

1. Definition:

Lagoons are coastal bodies of water that have very limited connection to the open ocean.
Seawater reaches a lagoon directly through a channel to the sea or via seepage through a barrier;
fresh water is supplied by rainfall or by surface run-off from the adjacent coastal plain. If a
lagoon is fed by a river it would be considered to be part of an estuary system. They are typically
very shallow, reaching only a few meters in depth. Lagoons generally develop along coasts
where there is a wave-formed barrier and are largely protected from the power of open ocean
waves (Reading & Collinson 1996).

Waves are generated by wind blowing across the surface of the water, but the fetch of the waves
will be limited by the dimensions of the lagoon. Ripples formed by waves therefore affect the
sediments only in very shallow water. The wind may also drive weak currents across the lagoon.
Tidal effects are generally small because the barrier– lagoon morphology is only well developed
along coasts with a small tidal range. Fine-grained clastic sediment is supplied to lagoons as
suspended material in seawater entering past the barrier and in overland flow from the adjacent
coastal plain. Organic material may be abundant from vegetation which grows on the shores of
the lagoon. In tropical climates, trees with aerial root systems (mangroves) colonise the shallow
fringes of the lagoon. Mangroves cause the shoreline to prograde into the lagoon as they act as
sites for accumulation of sediment and organic matter along the water’s edge.

In more temperate climates, saline-tolerant grasses, shrubs and trees may play a similar role in
trapping sediment. Coarser sediment may enter the lagoon when storms wash sediment over the
barrier as washover deposits, which are thin layers of sand reworked by waves. Sand is also
blown into the water by onshore winds picking up material from the dunes along the barrier.

An important characteristic of lagoons is their water chemistry. Due to the limited connection to
open ocean, it is common for lagoon water to have either higher or lower salinity than seawater.
Low salinity, brackish water will be a feature of lagoons in areas of high rainfall, local run-off of
fresh water from the coastal plain or small streams. Mixing of the lagoon water with the seawater
is insufficient to maintain full salinity in these brackish lagoons. In more arid settings the
evaporation from the surface of the lagoon may exceed the rate at which seawater exchanges
with the lagoon water and the conditions become hypersaline, that is, with salinities higher than
that of seawater. If salinities become very elevated, precipitation of evaporite minerals will
occur. A lagoonal succession is typically mudstone, often organic-rich, with thin, wave-rippled
sand beds (Boggs 2006).

The deposits of lagoons can be difficult to distinguish from those of lakes with similar
dimensions and in similar climatic settings. The processes are almost identical in the two settings
because they are both standing bodies of water. Two lines of evidence can be used to identify
lagoonal facies.

      First, the fossil assemblage may indicate a marine influence, and specifically a restricted
       fauna may provide evidence of brackish or hypersaline water.
      Second, the association with other facies is also important: lagoonal deposits occur above
       or below beach/barrier island sediments and fully marine shoreface deposits.

2. Type of lagoon
2.1.Atoll lagoons

Atoll lagoons form as coral reefs grow upwards while the islands the reefs surround subside,
until eventually only the reefs remain above sea level. Reef-building corals will thrive only in
warm tropical and subtropical waters of oceans and seas, and therefore atolls are only found in
the tropics and subtropics.

Atolls are the product of the growth of tropical marine organisms, and so these islands are only
found in warm tropical waters. Volcanic islands located beyond the warm water temperature
requirements of reef building, organisms become seamounts as they subside and are eroded away
at the surface. An island that is located where the ocean water temperatures are just sufficiently
warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin
Point.

2.2.Coastal lagoons

Coastal lagoons form along gently sloping coasts where barrier islands or reefs can develop off-
shore, and the sea-level is rising relative to the land along the shore (either because of an intrinsic
rise in sea-level, or subsidence of the land along the coast). Coastal lagoons do not form along
steep or rocky coasts, or if the range of tides is more than 4 meters (13 ft). Due to the gentle
slope of the coast, coastal lagoons are shallow. They are sensitive to changes in sea level. A
relative drop in sea level may leave a lagoon largely dry, while a rise in sea level may let the sea
breach or destroy barrier islands, and leave reefs too deep under water to protect the lagoon..
Coastal lagoons are young and dynamic, and may be short-lived in geological terms. Coastal
lagoons are common, occurring along nearly 15 percent of the world's shorelines. In the United
States, lagoons are found along more than 75 percent of the eastern and Gulf coasts.

Coastal lagoons are usually connected to the open ocean by inlets between barrier islands. The
number and size of the inlets, precipitation, evaporation, and inflow of fresh water all affect the
nature of the lagoon. Lagoons with little or no interchange with the open ocean, little or no
inflow of fresh water, and high evaporation rates
2.2.1. Sand Barriers

Elongate sandy islands that parallel the shoreline and are separated from it by lagoons or
marshes.

In contrast to river deltas, which result from interaction of fluvial and marine processes, barriers
and strand plains are controlled entirely by marine processes. They are formed on coastlines
where the wave processes are more important than tidal currents.

The environments of sand deposition include:

(1) Beach and shoreface environments on the seaward side of barriers and strand plains.

(2) Inlet channels and tidal deltas, separating barriers laterally.

(3) Washover fans on the landward or lagoon ward side of barriers. Seaward or longshore
migration of these environments results in facies sequences constituting much of the volume of
many coastal sand bodies.

Sand Barrier Facies

Distribution of facies, external geometry of sand bodies, and nature of associated facies are
variable and depend on sediment supply and relative sea-level changes

- Cross- Laminated Sandstone

- Laminated Sandstone

- Bioturbated Sandstone

- Clean sandstone

Hydrocarbon Potentiality

• Barrier Islands generate long thin sand bodies of excellent porosity & permeability within
impermeable shale sequences. It is excellent reservoir rocks for petroleum.

2.2.2. Coastal Lagoons characteristics
     The key characteristics of saline lagoons are that they are shallow, quiet water bodies,
       adjacent to the sea but sheltered from its direct effects.
     They exhibit great diversity of form (Barnes 1988; 1989a,b), ranging from fully natural
       water bodies enclosed by gravel or sandy barriers, or rock outcrops, through systems
    exhibiting varying degrees of human alteration, to wholly artificial water bodies
    impounded by human structures (Conlan et al. 1992).
   Coastal Lagoons exhibit great diversity in substrate (bedrock, sand, gravel, and mud),
    salinity, depth and stratification, and marginal habitats. They range in size from over 800
    hectares (ha), such as the Loch of Stenness in Orkney, to less than 1 ha.
   They exchange water with the sea via seepage through barriers, overtopping, and direct
    discharge through permanent or temporary surface water connections that may be
    artificial or natural.
   Depending on salinity, the dominant fringing vegetation type ranges from reeds (e.g.
    Phragmites species) to saltmarsh species (e.g. Puccinellia maritima).
   There is often a diverse submerged aquatic plant community as well, ranging from water
    lilies to seagrasses (e.g. Ruppia maritima); macrophytes can root on lagoon floors
    because of low current speeds. The fauna tends to reflect the species pool in neighbouring
    waters (Bamber et al. 2001), but lagoon specialists can occur (Ivell 1979; Barnes 1989b).

3. Lagoonal Deposits Characteristics

1. Lagoonal successions commonly contain interbedded sandstone, shale, siltstone, and coal
   facies characteristic of a number of overlapping depositional environments. Sand facies
   include washover sheet deposits and sheet and channel-fill deposits of flood-tidal-delta
   origin. Fine-grained sediments include those of the lagoon and tidal flats, which are
   situated adjacent to the barrier or on the landward side of the lagoon abutting the
   hinterland marsh and swamp flatlands.
2. Generally the lagoon is fed with marine waters that run through the numerous channels in
   the barrier-island system. However, where lagoons are developed adjacent to rivers and
   estuaries, lagoonal waters may often be brackish to nearly fresh.
3. Because of the inherent low energy of most lagoons (little current activity of any kind),
   fine-grained sediments are common. Often lagoons are the site of prolific production of
   plants and burrowing organisms that feed on the decaying organic matter. As a result,
   lagoonal sediments are rich in organics, are highly Bioturbated, and may form coal seams
   in the geologic record.
4. Other than the sands that are swept into the lagoon adjacent to ebb tidal deltas, the only
   dominant source of sand is from the growth of washover fans. These form during storms,
   when water is piled up against the beach face on the oceanic side of the barrier island.
   Commonly, the barrier island is breached at low points between the dune fields that form
   the top of the island; the water pours through and entrains abundant sand en route to
   flushing it through to the lagoon. Over the course of several storms, washover fans may
   actually prograde out into the lagoon.
5. Usually among them are burrowers, such as segmented worms, which disturb the muddy
   lagoonal sediments, leaving them mottled and largely devoid of bedding structures.
6. When a barrier island-lagoon complex receives sediment at a sufficiently high rate, it
   prograde-that is, it migrates seaward-like the active lobe of a delta.
7. Unlike the migration of a delta, however, this progradation takes place along a broad belt
   of shoreline. As the shoreline of a sea migrate seaward, marsh and tidal-flat deposits
   prograde over sediments of the lagoon and its associated tidal channels. All of these
   sediments in turn build out over deposits of the barrier islands and over the tidal deltas
   and marshes behind them. Thus the horizontal sequence of depositional environments
   (Barrier Island, marsh or tidal delta, lagoon, tidal flat, and marsh) comes to be
   represented by a corresponding vertical sequence of sedimentary deposits, in accordance
   with Walther's law.

				
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posted:8/3/2012
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