Page 243 - Sedimentology and Stratigraphy

230 Shallow Marine Carbonate and Evaporite Environments

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Fig. 15.5 A carbonate-dominated coast with a barrier island in an arid climatic setting: evaporation in the protected lagoon
results in increased salinity and the precipitation of evaporite minerals in the lagoon.

The source of the fine-grained carbonate sediment (3.2.1) to form by growing upwards from the lagoon
in lagoons is largely calcareous algae living in the bed (Warren & Kendall 1985). Connection with the
lagoon, with coarser bioclastic detritus from molluscs. ocean may be via gaps in the barrier or by seepage
Pellets formed by molluscs and crustaceans are abun- through it. Variations in the salinity within the
dant in lagoon sediments. The nature and diversity of lagoon may be because of climatically related changes
the plant and animal communities in a carbonate in the freshwater influx from the land or increased
lagoon is determined by the salinity. Lagoons in meso- exchange with open seawater during periods of
tidal coastlines tend to have better exchange of sea- higher sea level. The extent of the lagoon and the
water through tidal channels than more isolated minerals precipitated in it are therefore likely to be
lagoons in microtidal regimes. Where the climate is variable, resulting in cycles of sedimentation, includ-
relatively humid evaporation is lower, and as the ing layers of carbonate deposited during periods when
lagoon has near-normal salinities a diverse marine the salinity was closer to normal marine values. An
fauna is present. In more arid regions the lagoon alternation between laminated gypsum deposited sub-
may become hypersaline and there will be a restricted aqueously in a lagoon and nodular gypsum formed in
fauna, with organisms such as stromatolites and mar- a supratidal sabkha (see below) around the edges of
ine grasses (Thalassia) abundant. the water body may represent fluctuations in the area
of the water body.

Arid lagoons
15.2.3 Supratidal carbonates
In hot, dry climates the loss of water by evaporation
from the surface of a lagoon is high. If it is not and evaporites
balanced by influx of fresh water from the land or
Supratidal carbonate flats
exchange of water with the ocean the salinity of the
lagoon will rise and it will become hypersaline (10.3), The supratidal zone lies above the mean high water
more concentrated in salts than normal seawater mark and is only inundated by seawater under excep-
(Fig. 15.5). An area of hypersaline shallow water tional circumstances, such as very high tides and
that precipitates evaporite minerals is known as a storm conditions. Where the gradient to the shoreline
saltern. Deposits are typically layered gypsum and/ is very low the supratidal zone is a marshy area where
or halite occurring in units metres to tens of metres microbial (algal and bacterial) mats form (Fig. 15.6).
thick. In the restricted circulation of a lagoon condi- Aeolian action may also bring in carbonate sand
tions are right for large crystals of selenitic gypsum and dust that is bound by the microbes and, as