homogeneous due to lack of pronounced gradients in
the basin. The marginal sediment transport system is
prone to act in concert with the axial transport system,
feeding the latter with sediments. This may consist of
braided or meandering river systems, depending on
factors like axial basin gradient and climate. Since
most rifts are generated by break-up of continents, a
terrestrial depositional environment would be most
common for the pre-rift stage, so that source rocks
and cap rocks, which are mostly of marine depositional
origin, may be scarce (Fig. 12.8a). There are, however,
numerous examples of both source rocks and cap rocks
of terrestrial origin.
In the active stretching stage extension, and hence
also subsidence, accelerate. Simultaneously, heat
input increases due to upheaval of hot layers of the
mantle lithosphere. The steep fractures generated in
the pre-rift stage will not be able to accommodate the
extension and a new set of low-angle planar or listric
faults will be activated, separating fault blocks that are
detached from the lower crust by a subhorizontal zone
of weakness. Gliding on the system of detachments,
the fault blocks and their internal beds will rotate away
from the basin axis (Figs. 12.7b and 12.9a). From the
view of the petroleum explorationist, the active
stretching stage deserves particular attention because
of the variety of structural and stratigraphic traps that
may develop. This stage is also characterised by a
complex sediment distribution system that is likely to
produce a variety of lithofacies due to the increasing
topographic relief associated with high fault activity.
The marine transgression that commonly follows the
increased subsidence of the basin floor also contributes
to this variety in sedimentary facies. Sand that is
eroded from the high-standing parts of the basin (e.g.
basin shoulders and crests of rotated fault blocks) may
be trapped in lows in various structural positions and
these units are likely later to be covered by transgressive marine sediment accumulations. The sediment
transport system in the active stretching stage is likely
to be dominated by complex transverse and locally bidirectional fluvial systems that are strongly influenced
by the elongated, rotated fault blocks, generating axisparallel transport in segments along the basin margin.
The central part of the basin may be less complex and
axial-parallel sediment transport would prevail there
(Fig. 12.8b).
In the thermal subsidence stage, thermal contraction
of the lithosphere dominates the basin subsidence pattern. Because solids typically contract during cooling,
the parts of the basin that have experienced the strongest extension (i.e. those that have been thinned the
most and hence heated the most) will contract and
subside more than other parts. In a pure-shear configuration this is most likely to be the central segment
running along the basin axis. This means that the rotation of strata upwards away from the basin axis
becomes reversed so that strata begin to rotate downwards towards the basin axis (Fig. 12.9b). This rotation
is strengthened by sediment loading and compaction
(thickest sequence in the central part of the basin).
The transverse sediment transport will persist during the thermal subsidence stage, while the basin floor
becomes gradually smoothed. An axial transport system may also still be active, but is likely to become
less pronounced through this stage of development
(Fig. 12.8c). Depending on the balance between subsidence and sediment input, the water depth will vary
from one basin to another, but the depositional environment is likely to be marine and the central part of
the basin may attain great water depth (thousands of
metres). The fault systems that dominated the basin
floor geometry during the active stretching stage are
now quiescent, and stratigraphic hydrocarbon traps
rather than structural ones are likely to be the most
common.
Syn-rift to post-rift transition. The pure-shear
model predicts that a simple geometrical change of
a
b
c
Fig. 12.7 Three major stages in the devlopment of extensional
basins. After Gabrielsen (1986)
328
R.H. Gabrielsen
the basin. The marginal sediment transport system is
prone to act in concert with the axial transport system,
feeding the latter with sediments. This may consist of
braided or meandering river systems, depending on
factors like axial basin gradient and climate. Since
most rifts are generated by break-up of continents, a
terrestrial depositional environment would be most
common for the pre-rift stage, so that source rocks
and cap rocks, which are mostly of marine depositional
origin, may be scarce (Fig. 12.8a). There are, however,
numerous examples of both source rocks and cap rocks
of terrestrial origin.
In the active stretching stage extension, and hence
also subsidence, accelerate. Simultaneously, heat
input increases due to upheaval of hot layers of the
mantle lithosphere. The steep fractures generated in
the pre-rift stage will not be able to accommodate the
extension and a new set of low-angle planar or listric
faults will be activated, separating fault blocks that are
detached from the lower crust by a subhorizontal zone
of weakness. Gliding on the system of detachments,
the fault blocks and their internal beds will rotate away
from the basin axis (Figs. 12.7b and 12.9a). From the
view of the petroleum explorationist, the active
stretching stage deserves particular attention because
of the variety of structural and stratigraphic traps that
may develop. This stage is also characterised by a
complex sediment distribution system that is likely to
produce a variety of lithofacies due to the increasing
topographic relief associated with high fault activity.
The marine transgression that commonly follows the
increased subsidence of the basin floor also contributes
to this variety in sedimentary facies. Sand that is
eroded from the high-standing parts of the basin (e.g.
basin shoulders and crests of rotated fault blocks) may
be trapped in lows in various structural positions and
these units are likely later to be covered by transgressive marine sediment accumulations. The sediment
transport system in the active stretching stage is likely
to be dominated by complex transverse and locally bidirectional fluvial systems that are strongly influenced
by the elongated, rotated fault blocks, generating axisparallel transport in segments along the basin margin.
The central part of the basin may be less complex and
axial-parallel sediment transport would prevail there
(Fig. 12.8b).
In the thermal subsidence stage, thermal contraction
of the lithosphere dominates the basin subsidence pattern. Because solids typically contract during cooling,
the parts of the basin that have experienced the strongest extension (i.e. those that have been thinned the
most and hence heated the most) will contract and
subside more than other parts. In a pure-shear configuration this is most likely to be the central segment
running along the basin axis. This means that the rotation of strata upwards away from the basin axis
becomes reversed so that strata begin to rotate downwards towards the basin axis (Fig. 12.9b). This rotation
is strengthened by sediment loading and compaction
(thickest sequence in the central part of the basin).
The transverse sediment transport will persist during the thermal subsidence stage, while the basin floor
becomes gradually smoothed. An axial transport system may also still be active, but is likely to become
less pronounced through this stage of development
(Fig. 12.8c). Depending on the balance between subsidence and sediment input, the water depth will vary
from one basin to another, but the depositional environment is likely to be marine and the central part of
the basin may attain great water depth (thousands of
metres). The fault systems that dominated the basin
floor geometry during the active stretching stage are
now quiescent, and stratigraphic hydrocarbon traps
rather than structural ones are likely to be the most
common.
Syn-rift to post-rift transition. The pure-shear
model predicts that a simple geometrical change of
a
b
c
Fig. 12.7 Three major stages in the devlopment of extensional
basins. After Gabrielsen (1986)
328
R.H. Gabrielsen