Some coastal environments, don't have very strong tidal currents.
Without the influence of these strong currents dominating the sedimentary
environment, waves can play more of a role in depositing and reshaping sediments.
On coast lines where wave energy is high, barrier islands
and spits can form parallel to the coast line and behind them are lagoons:
areas of quiet, low energy water.
Sheltered by the barrier island, lagoons are not greatly effected by currents,
tides, or waves and therefore, fine sediments can be deposited here.
If lagoons are very shallow, enough water may able to evaporate
so that the water left behind concentrates minerals and
can become much saltier than ocean water.
If the water in a lagoon continues to evaporate, these minerals will precipitate
out of solution forming a sedimentary rock called an evaporite.
Common evaporite minerals are calcite, gypsum, anhydrite, and halite.
Now, calcite is a calcium carbonate.
Anhydrite is a calcium sulfate.
And halite is sodium chloride, which is what we usually mean, when we say salt.
These can be interbedded with muds, as the water level in the lagoon rises and
falls as new sea water flows in periodically.
If we find layers of these minerals in the geological record,
there's a good chance we're looking at what used to be a shallow lagoon.