Arid sabkha flats
Arid shorelines are found today in places such as
the Arabian Gulf, where they are sites of evaporite
formation within the coastal sediments. These arid
coasts are called sabkhas (Kendall 1992); they typically
have a very low relief and there is not always a
well-defined beach (Fig. 15.6). The coastal plain of a
sabkha is occasionally wetted by seawater during
very high tides or during onshore storm winds, but
more important is also a supply of water through
groundwater seepage from the sea (Yechieli & Wood
2002). The surface of the coastal plain is an area of
evaporation and water is drawn up through the sediment
to the surface. As the water rises it becomes
more concentrated in salts that precipitate within
the coastal plain sediments, and a dense, highly concentrated
brine is formed. Gypsum and anhydrite
grow within the sediment while a crust of halite
forms at the surface.
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 cycle may be repeated many times if there is continued
subsidence along the coastal plain (Kendall & Harwood
1996). The displacive growth of the gypsum within
the sediment is a distinctive feature of sabkhas, which
allows the deposits of these arid coasts to be distinguished
from other marine evaporite deposits (15.5).
Similar evaporite growth occurs within continental
sediments in arid regions (10.3).