Page 218 - Sedimentology and Stratigraphy

Barrier and Lagoon Systems 205

13.3.2 Lagoons
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
(13.6). They are typically very shallow, reaching only
a few metres in depth.
Lagoons generally develop along coasts where
there is a wave-formed barrier and are largely pro-
tected from the power of open ocean waves (Reading
Fig. 13.9 Beach-barrier bars along a wave-dominated
coastline. & Collinson 1996). Waves are generated by wind
blowing across the surface of the water, but the
fetch of the waves (4.4) will be limited by the dimen-
restricted inlet would not be able to let the water pass sions of the lagoon. Ripples formed by waves therefore
through at a high enough rate. Barrier systems are affect the sediments only in very shallow water. The
therefore best developed in microtidal (Fig. 13.8) and, wind may also drive weak currents across the lagoon.
to some extent, mesotidal settings (Fig. 13.10). Third, Tidal effects are generally small because the barrier–
barrier islands generally form under conditions of lagoon morphology is only well developed along
slow relative sea-level rise (Hoyt 1967; FitzGerald & coasts with a small tidal range.
Buynevich 2003). If there is a well-developed beach Fine-grained clastic sediment is supplied to lagoons
ridge, the coastal plain behind it may be lower than as suspended material in seawater entering past the
the top of the ridge. With a small sea-level rise, the barrier and in overland flow from the adjacent coastal
coastal plain can become partially flooded to form a plain. Organic material may be abundant from vege-
lagoon, and the beach ridge will remain subaerial, tation which grows on the shores of the lagoon.
forming a barrier. For the barrier to remain subaerial In tropical climates, trees with aerial root systems
as sea level rises further, sediment must be added to (mangroves) colonise the shallow fringes of the
the beach to build it up, that is, the first condition of lagoon. Mangroves cause the shoreline to prograde
high sediment supply must be satisfied. into the lagoon as they act as sites for accumulation of

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Fig. 13.10 Morphological features of a coastline influenced by wave processes and tidal currents.