the South Atlantic Ocean, evaporite basins were
located in the arid regions of the time. They can
form almost perfect cap rocks that are not likely to
leak. If they are thick enough they may form salt
domes which produce structural traps in overlying
rocks, as at Ekofisk and in the Gulf Coast basin. Rising
salt domes also strongly influence the clastic sediment
distribution in a basin.
Salt has high conductivity causing the temperatures
above the salt to be higher than normal and the
sediments below the salt to be cooler than normal.
This must be taken into account when modelling the
maturation of source rocks associated with the salt.
These temperature anomalies may also be important
when modelling diagenesis and reservoir properties.
Large subsalt discoveries have been made in recent
years both in the Gulf Coast and offshore Brazil.
The subsidence due to rifting renders the adjacent
rocks unstable and promotes gravitational sliding of
blocks in the crust, in towards the rift structure. There
is a tendency for listric faults to form, i.e. parallel,
curved fault planes which start as normal faults and
curve round with depth until they are almost horizontal. The blocks then become rotated so that they tilt
over and slope away from the rift. During the initial
part of the spreading phase, basins with limited circulation will be formed so that evaporites and carbonates
are often deposited. Upper Jurassic and Lower Cretaceous deposits of this sort are found extensively along
the margins of the Atlantic Ocean.
8.10 Subsidence Along Passive Margins
Passive margins develop from a rift phase to a spreading phase. The transition between continental crust
and oceanic crust therefore consists of a thinned continental crust with listric faults and horsts formed
during the rifting phase (Fig. 8.9). As ocean-floor
spreading progresses, the geothermal gradients in this
part of the continental shelf will decline, resulting in
cooling and thermal subsidence of the continental
margins. The oceanic crust will also experience thermal subsidence, as a function of the age of the seafloor.
Subsidence flexure will develop where the subsidence
is most rapid, on the outer parts of the continental shelf
and slope nearest the oceanic crust. Further in from the
passive margin the subsidence rate will be slower.
Eventually sedimentation fills the prism between the
oceanic and continental crust. With cooling, the rigidity of the crust increases, so that the bending of the
continental crust near the continental margin broadens
and there is overall subsidence of the continental shelf,
and in consequence transgression and onlap. It will
subside isostatically and make accommodation space.
During high sea level the clastic supply is pushed back
onto the continent and during low stand a
progradational sequence is formed (Fig. 8.10).
In other places the drainage discharge from land
focuses the sediment in large delta areas. This is the
situation with the Mississippi delta in the Mexican
Gulf, which has been receiving sediment from large
expanses of the North American continent since Mesozoic times.
The sediment supply is controlled by the drainage
system on the continents, which may follow old rift
systems because most of the sediment is produced on
the continents and has plenty of space to be deposited
along the margin of the deep ocean.
The Niger delta represents a similar focusing of
sedimentation on the African side of the Atlantic.
Sediment basins along passive margins are deposited
above old rift basins which have cooled. If basin
subsidence is rapid, this will also contribute to a low
geothermal gradient because the sediments have to be
heated during burial (20–30
C/km). A relatively great
thickness of sediment (4–5 km) must therefore be
deposited for the underlying source rocks to achieve
maturation. That is to say, attain 100–150
C,
depending on the subsidence rate and hence the time
available for the heating. This kind of sedimentary
basin situated along a passive margin is called a
“cold basin”.
Foreland basins form in front of mountain chains
when they are uplifted and eroded. The advancing
nappes help to depress the continental crust and provide accommodation space for the sediments shed
from the mountains. Foreland basins tend to be broad
and gently folded into giant structures in the distal
parts. The Middle East is a large foreland basin in
front of the alpine mountain chain running along Iran
and Turkey. The Persian Gulf is a continuation of this
basin collecting sediments coming from the mountains
of Iran. The Jurassic and Cretaceous sequences in Iraq,
Saudi Arabia, Kuwait and The Emirates contain some
of the largest oil fields in the world.
On the east side of the Rocky Mountains we have
similar foreland basins from the Denver Basin all the
8 Seismic Stratigraphy, Sequence Stratigraphy and Basin Analysis
267
located in the arid regions of the time. They can
form almost perfect cap rocks that are not likely to
leak. If they are thick enough they may form salt
domes which produce structural traps in overlying
rocks, as at Ekofisk and in the Gulf Coast basin. Rising
salt domes also strongly influence the clastic sediment
distribution in a basin.
Salt has high conductivity causing the temperatures
above the salt to be higher than normal and the
sediments below the salt to be cooler than normal.
This must be taken into account when modelling the
maturation of source rocks associated with the salt.
These temperature anomalies may also be important
when modelling diagenesis and reservoir properties.
Large subsalt discoveries have been made in recent
years both in the Gulf Coast and offshore Brazil.
The subsidence due to rifting renders the adjacent
rocks unstable and promotes gravitational sliding of
blocks in the crust, in towards the rift structure. There
is a tendency for listric faults to form, i.e. parallel,
curved fault planes which start as normal faults and
curve round with depth until they are almost horizontal. The blocks then become rotated so that they tilt
over and slope away from the rift. During the initial
part of the spreading phase, basins with limited circulation will be formed so that evaporites and carbonates
are often deposited. Upper Jurassic and Lower Cretaceous deposits of this sort are found extensively along
the margins of the Atlantic Ocean.
8.10 Subsidence Along Passive Margins
Passive margins develop from a rift phase to a spreading phase. The transition between continental crust
and oceanic crust therefore consists of a thinned continental crust with listric faults and horsts formed
during the rifting phase (Fig. 8.9). As ocean-floor
spreading progresses, the geothermal gradients in this
part of the continental shelf will decline, resulting in
cooling and thermal subsidence of the continental
margins. The oceanic crust will also experience thermal subsidence, as a function of the age of the seafloor.
Subsidence flexure will develop where the subsidence
is most rapid, on the outer parts of the continental shelf
and slope nearest the oceanic crust. Further in from the
passive margin the subsidence rate will be slower.
Eventually sedimentation fills the prism between the
oceanic and continental crust. With cooling, the rigidity of the crust increases, so that the bending of the
continental crust near the continental margin broadens
and there is overall subsidence of the continental shelf,
and in consequence transgression and onlap. It will
subside isostatically and make accommodation space.
During high sea level the clastic supply is pushed back
onto the continent and during low stand a
progradational sequence is formed (Fig. 8.10).
In other places the drainage discharge from land
focuses the sediment in large delta areas. This is the
situation with the Mississippi delta in the Mexican
Gulf, which has been receiving sediment from large
expanses of the North American continent since Mesozoic times.
The sediment supply is controlled by the drainage
system on the continents, which may follow old rift
systems because most of the sediment is produced on
the continents and has plenty of space to be deposited
along the margin of the deep ocean.
The Niger delta represents a similar focusing of
sedimentation on the African side of the Atlantic.
Sediment basins along passive margins are deposited
above old rift basins which have cooled. If basin
subsidence is rapid, this will also contribute to a low
geothermal gradient because the sediments have to be
heated during burial (20–30
C/km). A relatively great
thickness of sediment (4–5 km) must therefore be
deposited for the underlying source rocks to achieve
maturation. That is to say, attain 100–150
C,
depending on the subsidence rate and hence the time
available for the heating. This kind of sedimentary
basin situated along a passive margin is called a
“cold basin”.
Foreland basins form in front of mountain chains
when they are uplifted and eroded. The advancing
nappes help to depress the continental crust and provide accommodation space for the sediments shed
from the mountains. Foreland basins tend to be broad
and gently folded into giant structures in the distal
parts. The Middle East is a large foreland basin in
front of the alpine mountain chain running along Iran
and Turkey. The Persian Gulf is a continuation of this
basin collecting sediments coming from the mountains
of Iran. The Jurassic and Cretaceous sequences in Iraq,
Saudi Arabia, Kuwait and The Emirates contain some
of the largest oil fields in the world.
On the east side of the Rocky Mountains we have
similar foreland basins from the Denver Basin all the
8 Seismic Stratigraphy, Sequence Stratigraphy and Basin Analysis
267