northwestern end by the Euphrates and Tigris. Clastic
siliceous sediments are also being added from the
north, from the mountains in Iran with abundant
limestones and dolomites. Much of the sediment therefore consists of impure carbonates (marl). Only on the
south (Trucial Coast) side are there purer carbonate
sediments, because there is very little runoff from the
deserts on the Arabian shield. Here too, though, there
is some supply of clastic material, particularly through
aeolian transport from the desert. Sedimentation has
been proceeding in a marine basin/embayment in this
area since Mesozoic times.
The sea temperature in the Persian Gulf varies from
20
C in winter to 34
C in summer, and in the shallow
areas it can be even hotter. The salinity in the open
water is around 39–42‰, and may be higher due to
evaporation in the lagoons. The tidal range varies from
0.5 to 2 m. The innermost supratidal area consists of a
10–15 km broad marginal belt covered by
cyanobacteria and salt deposits. This belt is called
sabkha in Arabic, and the word has become a geological term.
Because of the high temperature and salinity few
organisms can live on the sabkha, which is dominated
by algal mats (stromatolites) that form a crust of
precipitated carbonate and gypsum. The stromatolites
may grow in layers parallel to bedding or develop
dome-shaped, columnar or irregular structures. The
stromatolite structures are often broken into polygons
which are separated from each other by sedimentfilled cracks. The cracks are the posthumous reflection
of shrinkage cracks as a result of drying.
The slope of the sediment surface here is only 0.4
m/km and during powerful storms water is driven in
over the sabkha and later evaporates. Anhydrite, dolomite, magnesite and halite may be precipitated, though
the halite is easily dissolved again. The precipitation
of gypsum and anhydrite leads to the water developing
a higher Mg
2þ
=Ca
2þ ratio, which favours the formation of dolomite and magnesite.
In the Persian Gulf green calcareous algae such as
Halimeda and Penicillus, which are important producers
of lime mud in the Bahamas, are for the most part absent.
Chemical precipitation is therefore probably the most
important process, particularly during periods when
diatoms proliferate and raise the pH by consuming
CO 2 and thereby reducing the solubility of calcite. On
the continental shelf, biogenic carbonate accumulates in
the form of shells of foraminifera, molluscs, ostracods,
bryozoans and echinoderms. Especially in shallow water
environments the skeletal material can be heavily
affected by post-mortem micritisation if exposed on the
sea bottom for some time. Along the outer edge of the
shelf are reefs, built of coral species that are particularly
well adapted to the high salinity.
Tidal channels connect the lagoons with the gulf.
At the mouths of the channels there are ebb-tidal deltas
with oolite banks in the shallowest parts (<2 m). In
addition to oolites there are grains of bioclastic material. Tidal deltas of this sort would form good oil
reservoirs because of the high primary porosity, and
in the Mesozoic series we find similar reservoir rocks.
Shoreface deposits are not as well sorted as we would
normally expect as they consist of fine sand and lime
mud. This is possibly because the shoreline is
stabilised by plants which bind the sediments.
Gastropods are particularly important in the
lagoons and the intertidal environment. On the
beach, shrimps and crabs burrow and disturb the
lamination in the sediments. The lagoons are
surrounded by swamps with bushes and mangroves,
or algal flats. Landward of the algal mat facies,
which is most typically developed at the high water
mark, the sabkha facies continues. The degree of
evaporation is higher here in the supratidal zone
and dilution with seawater rarer. The sulphates,
especially gypsum or anhydrite, are precipitated
within the sediment near the groundwater table.
The sabkha flats pass landwards into drier areas
dominated by aeolian sand.
The facies distribution we have just described, from
the open marine lagoon to the supratidal environment,
will form a characteristic vertical sequence in the
event of relative sea level fall and progradation.
Periods with transgressions and subsequent regressive
outbuilding will result in a series of sequences
(cycles), starting with shallow marine (subtidal)
sediments and culminating with evaporites (anhydrite)
at the top. These cycles are also found in the Mesozoic
and the impervious anhydrite beds forms an ideal cap
rock above oil reservoir rocks. Carbonate sand facies,
pellet facies, and particularly oolites, make good
reservoir rocks, while the marine muds are source
rocks.
192
N.-M. Hanken et al.
siliceous sediments are also being added from the
north, from the mountains in Iran with abundant
limestones and dolomites. Much of the sediment therefore consists of impure carbonates (marl). Only on the
south (Trucial Coast) side are there purer carbonate
sediments, because there is very little runoff from the
deserts on the Arabian shield. Here too, though, there
is some supply of clastic material, particularly through
aeolian transport from the desert. Sedimentation has
been proceeding in a marine basin/embayment in this
area since Mesozoic times.
The sea temperature in the Persian Gulf varies from
20
C in winter to 34
C in summer, and in the shallow
areas it can be even hotter. The salinity in the open
water is around 39–42‰, and may be higher due to
evaporation in the lagoons. The tidal range varies from
0.5 to 2 m. The innermost supratidal area consists of a
10–15 km broad marginal belt covered by
cyanobacteria and salt deposits. This belt is called
sabkha in Arabic, and the word has become a geological term.
Because of the high temperature and salinity few
organisms can live on the sabkha, which is dominated
by algal mats (stromatolites) that form a crust of
precipitated carbonate and gypsum. The stromatolites
may grow in layers parallel to bedding or develop
dome-shaped, columnar or irregular structures. The
stromatolite structures are often broken into polygons
which are separated from each other by sedimentfilled cracks. The cracks are the posthumous reflection
of shrinkage cracks as a result of drying.
The slope of the sediment surface here is only 0.4
m/km and during powerful storms water is driven in
over the sabkha and later evaporates. Anhydrite, dolomite, magnesite and halite may be precipitated, though
the halite is easily dissolved again. The precipitation
of gypsum and anhydrite leads to the water developing
a higher Mg
2þ
=Ca
2þ ratio, which favours the formation of dolomite and magnesite.
In the Persian Gulf green calcareous algae such as
Halimeda and Penicillus, which are important producers
of lime mud in the Bahamas, are for the most part absent.
Chemical precipitation is therefore probably the most
important process, particularly during periods when
diatoms proliferate and raise the pH by consuming
CO 2 and thereby reducing the solubility of calcite. On
the continental shelf, biogenic carbonate accumulates in
the form of shells of foraminifera, molluscs, ostracods,
bryozoans and echinoderms. Especially in shallow water
environments the skeletal material can be heavily
affected by post-mortem micritisation if exposed on the
sea bottom for some time. Along the outer edge of the
shelf are reefs, built of coral species that are particularly
well adapted to the high salinity.
Tidal channels connect the lagoons with the gulf.
At the mouths of the channels there are ebb-tidal deltas
with oolite banks in the shallowest parts (<2 m). In
addition to oolites there are grains of bioclastic material. Tidal deltas of this sort would form good oil
reservoirs because of the high primary porosity, and
in the Mesozoic series we find similar reservoir rocks.
Shoreface deposits are not as well sorted as we would
normally expect as they consist of fine sand and lime
mud. This is possibly because the shoreline is
stabilised by plants which bind the sediments.
Gastropods are particularly important in the
lagoons and the intertidal environment. On the
beach, shrimps and crabs burrow and disturb the
lamination in the sediments. The lagoons are
surrounded by swamps with bushes and mangroves,
or algal flats. Landward of the algal mat facies,
which is most typically developed at the high water
mark, the sabkha facies continues. The degree of
evaporation is higher here in the supratidal zone
and dilution with seawater rarer. The sulphates,
especially gypsum or anhydrite, are precipitated
within the sediment near the groundwater table.
The sabkha flats pass landwards into drier areas
dominated by aeolian sand.
The facies distribution we have just described, from
the open marine lagoon to the supratidal environment,
will form a characteristic vertical sequence in the
event of relative sea level fall and progradation.
Periods with transgressions and subsequent regressive
outbuilding will result in a series of sequences
(cycles), starting with shallow marine (subtidal)
sediments and culminating with evaporites (anhydrite)
at the top. These cycles are also found in the Mesozoic
and the impervious anhydrite beds forms an ideal cap
rock above oil reservoir rocks. Carbonate sand facies,
pellet facies, and particularly oolites, make good
reservoir rocks, while the marine muds are source
rocks.
192
N.-M. Hanken et al.