Salt Layers
Sea water is about 3% salts, mostly NaCl, with a little CaCO3 and CaSO4.
As sea water evaporates, the salts precipitate when different fractions have evaporated:
CaSO4 when 50% evaporated
CaCO3 when 80% evaporated
NaCl when 90% evaporated
If sea water has evaporated to:
Below 10%, get a triplet layer of salts
10-20%, get a doublet layer
20-50%, get only a singlet layer