180
The first strata on top of the evaporites are dark grey,
dolomitic silty claystones and early diagenetie dolomites.
Intervals with high organic earbon and pyrite contents
point to periods of restricted water eirculation in the basin
(Fig. 4.6d). Then hemipelagie nanno oozes, marls and
marly claystones follow, which again exhibit dark, organicrieh interbeds. Locally, vo1canic ashes are interealated. The
youngest sediments consist of foraminiferal nanno ooze
and ehalk, which are more or less diluted by clay and silt
ftom terrestrial sourees.
Qnaternary Sediments in tbe Red Sea Controlled
by Sea-Level Cbanges
A characteristic feature of the young, unconsolidated
(or only slightly consolidated) Red Sea sediments is
the occurrence of so-called "aplanktic" horizons and
many lithified layers, containing high proportions of
chemically precipitated aragonite and Mg-calcite.
These layers obviously forrned during times of glacial sea-levellowstand and increased salinity.
Further evidence for this assumption (Reiss and Hottinger
1984) is the faunal and isotopie composition of Holoeene
and late Pleistocene sediments in the central and southem
Red Sea (Reiss et al. 1980; Locke and Thunell 1988). Glacial surface and bottom water salinities were significantly
higher than at present, up to 53-55%0 (HernIeben et al.
1996). The planktonic foraminifera vanished completely
during these intervals. Water exchange between the Gulf of
Aden and the Red Sea was markedly reduced and the bottom waters were low in oxygen. At the same time the climate was extremely arid.
Ouring such periods, fertility in the photic zone was
relatively high in the northem Red Sea (Gulf of Aqaba),
whereas planktonic carbonate production was limited in the
southem re/fions. The resulting "aplanktic" layers also exhibit high Ö 80 values (Fig. 4.6d). Ouring deglaeiation, i.e.,
about 11 000 to 8000 years B.P., the sea level began to
rise, but the bottom water in the Red Sea was still
hypersaline and rather stagnant beneath a surface water
zone of lower salinity and increasing exchange with the
open sea. The sediments of this transitional stage are relatively rich in organic matter.
While the Strait of Bab al Mandab was kept practically
ftee of sediment by the outtlowing bottom eurrents, sediment aecumulation in the Gulf of Aden south of the strait
was high during this period.
4.3.4 Tbe Persian (Arabian) Gnlf
General Setting
The shallow Persian (Arabian) Gulf has been a
shrunken foreland basin in front of the Zagros fold
belt since the Upper Cretaceous (Fig. 4.7; cf. Sect.
14.3 and Fig. 14.12). Its deepest portion (80 to 100
m) is a structural feature running parallel to this
mountain range in the northeast.
References include Purser (1973), Kassler (1973), Stotfers
and Ross (1979), Murris (1985), Uchupi et al. (1996). The
Chapter 4 Adjacent Seas
Mesozoic and Paleogene development of this basin is
brietly outlined in Sec!. 14.3. Oligocene and Mioeene
sandstones and limestones, which are sealed by evaporites,
are the principal reservoirs of several giant oil fields. They
store hydrocarbons derived from older source rocks. The
present configuration and hydrographie situation of the
gulf is unfavorable for the accurnulation of significant
amounts of organic matter.
Qnaternary and Holocene Sediments
The young sediments in front of the fold belt are
mainly siliciclastic sands tones, silts, and shales, but
farther to the southwest an open-marine basin has
been maintained. This is bordered by a shelf region
characterized by carbonate and evaporite deposition.
In the central basin, marls and calcareous shales have
accumulated, because carbonate production has been
diluted by suspended river load carried into the basin
from the east and northwest. In the northwest, the
Tigris-Euphrates River system has built a fluviodeltaic complex out into the gulf.
The laIe Pliocene and Qualernary his tory of the
gulf was strongly affected by the glacial regressions
and transgressions. During low sea-level stands (up
to 110-120 m below the present level), the basin became almost completely dry. Emerged shallow-marine skeletal carbonates, tidal carbonates, and oolites
were cemented and formed lithified layers, or they
were reworked. Fluvial sediments and eolian sand
dunes migrated into and across the basin (Fig. 4.7b).
Thus, the Holocene marine sediments rest on terrestrial deposits in large areas of the gulf.
During the early Holocene transgression (18 to 12
ka B.P.), the climate was more humid than at present.
Large areas in the northem and northwestem gulf
were covered with terrigenous material. Since ~8 ka
the climate became dryer, allowing the deposition of
evaporites and eolian sediments in shallow gulf regions. In areas covered by the transgressing sea, carbonate sands and light muds rich in aragonite needles
and pellets (carbonate content ;: 7 0%) accumulated.
Older aragonitic muds were partly reworked and redistributed throughout the gulf. Under the present
conditions, silty marls (carbonate content ab out 55%)
form in wide areas of the gulf. They are rich in
biogenic constituents and contain abundant high
magnesian calcite.
Thicknesses and sedimentation rates of the Holocene carbonate sands and ca1careous muds vary according to the
irregular pre-Holocene topography. Average sedimentation
rates are in the order of 10 to 20 cmlka; in depressions,
sediments accumulated at a rate up to about 300 cmlka,
whereas topographie highs may be almost free ofHolocene
sediments.
A specific feature of the present-day sea floor of the gulf
is a network of pockmarks produced by seepages of
thermogenie gas (Uchupi et al. (1996).
The first strata on top of the evaporites are dark grey,
dolomitic silty claystones and early diagenetie dolomites.
Intervals with high organic earbon and pyrite contents
point to periods of restricted water eirculation in the basin
(Fig. 4.6d). Then hemipelagie nanno oozes, marls and
marly claystones follow, which again exhibit dark, organicrieh interbeds. Locally, vo1canic ashes are interealated. The
youngest sediments consist of foraminiferal nanno ooze
and ehalk, which are more or less diluted by clay and silt
ftom terrestrial sourees.
Qnaternary Sediments in tbe Red Sea Controlled
by Sea-Level Cbanges
A characteristic feature of the young, unconsolidated
(or only slightly consolidated) Red Sea sediments is
the occurrence of so-called "aplanktic" horizons and
many lithified layers, containing high proportions of
chemically precipitated aragonite and Mg-calcite.
These layers obviously forrned during times of glacial sea-levellowstand and increased salinity.
Further evidence for this assumption (Reiss and Hottinger
1984) is the faunal and isotopie composition of Holoeene
and late Pleistocene sediments in the central and southem
Red Sea (Reiss et al. 1980; Locke and Thunell 1988). Glacial surface and bottom water salinities were significantly
higher than at present, up to 53-55%0 (HernIeben et al.
1996). The planktonic foraminifera vanished completely
during these intervals. Water exchange between the Gulf of
Aden and the Red Sea was markedly reduced and the bottom waters were low in oxygen. At the same time the climate was extremely arid.
Ouring such periods, fertility in the photic zone was
relatively high in the northem Red Sea (Gulf of Aqaba),
whereas planktonic carbonate production was limited in the
southem re/fions. The resulting "aplanktic" layers also exhibit high Ö 80 values (Fig. 4.6d). Ouring deglaeiation, i.e.,
about 11 000 to 8000 years B.P., the sea level began to
rise, but the bottom water in the Red Sea was still
hypersaline and rather stagnant beneath a surface water
zone of lower salinity and increasing exchange with the
open sea. The sediments of this transitional stage are relatively rich in organic matter.
While the Strait of Bab al Mandab was kept practically
ftee of sediment by the outtlowing bottom eurrents, sediment aecumulation in the Gulf of Aden south of the strait
was high during this period.
4.3.4 Tbe Persian (Arabian) Gnlf
General Setting
The shallow Persian (Arabian) Gulf has been a
shrunken foreland basin in front of the Zagros fold
belt since the Upper Cretaceous (Fig. 4.7; cf. Sect.
14.3 and Fig. 14.12). Its deepest portion (80 to 100
m) is a structural feature running parallel to this
mountain range in the northeast.
References include Purser (1973), Kassler (1973), Stotfers
and Ross (1979), Murris (1985), Uchupi et al. (1996). The
Chapter 4 Adjacent Seas
Mesozoic and Paleogene development of this basin is
brietly outlined in Sec!. 14.3. Oligocene and Mioeene
sandstones and limestones, which are sealed by evaporites,
are the principal reservoirs of several giant oil fields. They
store hydrocarbons derived from older source rocks. The
present configuration and hydrographie situation of the
gulf is unfavorable for the accurnulation of significant
amounts of organic matter.
Qnaternary and Holocene Sediments
The young sediments in front of the fold belt are
mainly siliciclastic sands tones, silts, and shales, but
farther to the southwest an open-marine basin has
been maintained. This is bordered by a shelf region
characterized by carbonate and evaporite deposition.
In the central basin, marls and calcareous shales have
accumulated, because carbonate production has been
diluted by suspended river load carried into the basin
from the east and northwest. In the northwest, the
Tigris-Euphrates River system has built a fluviodeltaic complex out into the gulf.
The laIe Pliocene and Qualernary his tory of the
gulf was strongly affected by the glacial regressions
and transgressions. During low sea-level stands (up
to 110-120 m below the present level), the basin became almost completely dry. Emerged shallow-marine skeletal carbonates, tidal carbonates, and oolites
were cemented and formed lithified layers, or they
were reworked. Fluvial sediments and eolian sand
dunes migrated into and across the basin (Fig. 4.7b).
Thus, the Holocene marine sediments rest on terrestrial deposits in large areas of the gulf.
During the early Holocene transgression (18 to 12
ka B.P.), the climate was more humid than at present.
Large areas in the northem and northwestem gulf
were covered with terrigenous material. Since ~8 ka
the climate became dryer, allowing the deposition of
evaporites and eolian sediments in shallow gulf regions. In areas covered by the transgressing sea, carbonate sands and light muds rich in aragonite needles
and pellets (carbonate content ;: 7 0%) accumulated.
Older aragonitic muds were partly reworked and redistributed throughout the gulf. Under the present
conditions, silty marls (carbonate content ab out 55%)
form in wide areas of the gulf. They are rich in
biogenic constituents and contain abundant high
magnesian calcite.
Thicknesses and sedimentation rates of the Holocene carbonate sands and ca1careous muds vary according to the
irregular pre-Holocene topography. Average sedimentation
rates are in the order of 10 to 20 cmlka; in depressions,
sediments accumulated at a rate up to about 300 cmlka,
whereas topographie highs may be almost free ofHolocene
sediments.
A specific feature of the present-day sea floor of the gulf
is a network of pockmarks produced by seepages of
thermogenie gas (Uchupi et al. (1996).