Beach barrier lagoons
Lagoons form along carbonate coastlines where a
beach barrier wholly or partly encloses an area of
shallow water (Fig. 15.4). The character of the lagoon
deposits depends on the salinity of the water and this
in turn is determined by two factors: the degree of
connection with the open ocean and the aridity of the
climate.
Carbonate lagoons
Carbonate lagoons are sites of fine-grained sedimentation
forming layers of carbonate mudstone and
wackestone with some grainstone and packstone
beds deposited as washovers near the beach barrier.
Where a barrier island ridge is cut by tidal channels in
a mesotidal regime, the tidal currents passing through
form flood- and ebb-tidal deltas in much the same way
as in clastic barrier island systems (13.3). The shape
and internal sedimentary structures of these deposits
are also similar on both clastic and carbonate coastlines,
with lenses of cross-bedded oolitic and bioclastic
packstone and grainstone formed by subaqueous
dunes on flood-tidal deltas. The nature of the carbonate
material deposited on ebb- and flood-tidal deltas
depends on the type of material being generated in
the shallow marine waters: it may be bioclastic debris
or oolitic sediment forming beds of grainstone and
packstone (Fig. 15.4). The source of the fine-grained carbonate sediment
in lagoons is largely calcareous algae living in the
lagoon, with coarser bioclastic detritus from molluscs.
Pellets formed by molluscs and crustaceans are abundant
in lagoon sediments. The nature and diversity of
the plant and animal communities in a carbonate
lagoon is determined by the salinity. Lagoons in mesotidal
coastlines tend to have better exchange of seawater
through tidal channels than more isolated
lagoons in microtidal regimes. Where the climate is
relatively humid evaporation is lower, and as the
lagoon has near-normal salinities a diverse marine
fauna is present. In more arid regions the lagoon
may become hypersaline and there will be a restricted
fauna, with organisms such as stromatolites and marine
grasses (Thalassia) abundant.