In general, anhydrite forms in the hotter, drier
sabkhas and gypsum where the temperatures are
lower or where there is a supply of fresh, continental
water to the sabkha. Both gypsum and anhydrite are
formed in some sabkhas: close to the shore in the
intertidal and near-supratidal zone gypsum crystals
grow in the relatively high flux of seawater through
the sediment, whereas further up in the supratidal
area conditions are drier and nodules of anhydrite
form in the sediment. The gypsum and anhydrite
grow by displacement within the sediment, with the
gypsum in clusters and the anhydrite forming amorphous
coalesced nodules with little original sediment
in between. These layers of anhydrite with remnants
of other sediment have a characteristic chicken-wire
structure. Halite crusts are rarely preserved because
they are removed by any surface water flows. The
terrigenous sediment of the sabkha is often strongly
reddened by the oxidising conditions.
The succession formed by sedimentation along an
arid coast starts with beds deposited in a wavereworked
shallow subtidal setting and overlain by
intertidal microbial limestone beds. Gypsum formed
in the upper intertidal and lower supratidal area
occurs next in the succession, overlain by anhydrite
with a chicken-wire structure. Coalesced beds of
anhydrite formed in the uppermost part of the sabkha
form layers, contorted as the minerals have grown,
known as an enterolithic bedding structure. This