deposited under the control of greater subsidence
rates.
In the evolving graben which should be
characterised by increasing fault activity and eventual
magmatism, the continuity of those sand bodies would
be later broken.
Units related to transverse sediment transport
encompass both pure stratigraphic and mixed traps
(Fig. 12.11, trap type 5). Alluvial fan systems, which
offer pure stratigraphic traps with potentially great
lateral extension and considerable thickness, might
represent the most important type. The general
model for this stage in the graben development
suggests that axial transport systems dominate, but
that most of the axial transport comes ultimately
from transverse systems upstream.
Clastic fans related to primary synthetic faultgrowth represent a mixed trap type, which depends
on proximal sealing faults and a distal pinch-out
(Fig. 12.11, trap types 1 and 2). Taking the likely
moderate relief, low surface gradient and simple
fault geometry into consideration, it is probable that
the reservoirs in this type of trap are characterised by
good lateral continuity in the dip direction. The thickness of the source rock may be considerable in stable
basins.
Clastic fans related to accommodation structures
along master faults (Fig. 12.11, trap type 1b) classify
as mixed palaeomorphic/structural traps. Because of
the steep geometries of the master faults in the initial
rifting stage, the accommodation structures will be
laterally restricted transverse strike, and to constitute
a trap of some size accordingly demands good continuity along strike. This is especially the case in late
stages of development where channel amalgamation
and progradation lead to a sheet-like geometry.
12.3.1.2 The Active Stretching Stage
During this stage the area of subsidence narrows, and
the structural elements typical for mature grabens start
to appear. The subsidence, and hence the sedimentation pattern, which to a large extent will be influenced
by the faulting within the graben area, is characterised
by numerous fault-bounded depocentres that are only
partly interconnected. The geometry of faults will
change from steep planar to low-angle, either by
fault-plane rotation or by development of listric faults.
Both low-angle rotational and listric faulting will
enhance the internal graben relief by rotation and
upheaval of fault-block crests and by isostatic and
flexural adjustments of the edges of the fault blocks.
The fault pattern, which is initially relatively simple and stable and consists of isolated fault strands,
develops into linked structures, and the relief is
enhanced. This gives the possibility of accumulation
of considerable thicknesses of sediments, and with
potentially good strike-continuity in the hangingwall
fault blocks of the master faults. Such catchments may
be fed with sediments which are transported across the
fault scarp from the foot-wall hinterland, and also
from the eroded crests of the neighbouring
hangingwall dip slope. With continued subsidence,
the initiation of accommodation structures
(hangingwall anticlines, antithetic faults and forced
folds) will restrict both the area of deposition and the
continuity of sands. Simultaneously, fault complexity
will increase with the development of different types
of fault-related transfer zones and steps, which in turn
may act as foci for drainage systems.
Finer-grained pelagic and other mature sediments
derived from the hinterlands would dominate over the
coarser sandy or conglomeratic deposits eroded from
local structural highs, although the latter may dominate in isolated basins. Initially, the sediment transport
is still essentially axial, but due to enhanced relief in
the central parts of the basin will be branched to some
extent. A growing local influence from relief caused
by rotated fault blocks is expected. This may cause
sediment transport parallel to the graben axis also in
the more distal parts of the graben system. Locally this
pattern will be broken by transverse transport systems
cutting across highs related to rotated fault blocks, or
in connection with relay ramps and bridges between
fault blocks. In broader tilted fault-blocks or in
grabens immediately adjacent to the hinterlands, the
volume of transversely-transported coarse sediment
may be great.
Compared to the initial stage, the final burial depth
attained by traps generated in the active stretching stage
will of course be shallower. Due to the likely marine
influx and sub-basinal restricted conditions, the chances
for generation of source rocks will also be higher, and
semi-regional or local areas of enhanced subsidence
may promote the presence of local pockets of source
rocks in addition to those of regional significance.
Fans related primarily to synthetic growth along
faults give potentially a mixed trap type, but may also
334
R.H. Gabrielsen
rates.
In the evolving graben which should be
characterised by increasing fault activity and eventual
magmatism, the continuity of those sand bodies would
be later broken.
Units related to transverse sediment transport
encompass both pure stratigraphic and mixed traps
(Fig. 12.11, trap type 5). Alluvial fan systems, which
offer pure stratigraphic traps with potentially great
lateral extension and considerable thickness, might
represent the most important type. The general
model for this stage in the graben development
suggests that axial transport systems dominate, but
that most of the axial transport comes ultimately
from transverse systems upstream.
Clastic fans related to primary synthetic faultgrowth represent a mixed trap type, which depends
on proximal sealing faults and a distal pinch-out
(Fig. 12.11, trap types 1 and 2). Taking the likely
moderate relief, low surface gradient and simple
fault geometry into consideration, it is probable that
the reservoirs in this type of trap are characterised by
good lateral continuity in the dip direction. The thickness of the source rock may be considerable in stable
basins.
Clastic fans related to accommodation structures
along master faults (Fig. 12.11, trap type 1b) classify
as mixed palaeomorphic/structural traps. Because of
the steep geometries of the master faults in the initial
rifting stage, the accommodation structures will be
laterally restricted transverse strike, and to constitute
a trap of some size accordingly demands good continuity along strike. This is especially the case in late
stages of development where channel amalgamation
and progradation lead to a sheet-like geometry.
12.3.1.2 The Active Stretching Stage
During this stage the area of subsidence narrows, and
the structural elements typical for mature grabens start
to appear. The subsidence, and hence the sedimentation pattern, which to a large extent will be influenced
by the faulting within the graben area, is characterised
by numerous fault-bounded depocentres that are only
partly interconnected. The geometry of faults will
change from steep planar to low-angle, either by
fault-plane rotation or by development of listric faults.
Both low-angle rotational and listric faulting will
enhance the internal graben relief by rotation and
upheaval of fault-block crests and by isostatic and
flexural adjustments of the edges of the fault blocks.
The fault pattern, which is initially relatively simple and stable and consists of isolated fault strands,
develops into linked structures, and the relief is
enhanced. This gives the possibility of accumulation
of considerable thicknesses of sediments, and with
potentially good strike-continuity in the hangingwall
fault blocks of the master faults. Such catchments may
be fed with sediments which are transported across the
fault scarp from the foot-wall hinterland, and also
from the eroded crests of the neighbouring
hangingwall dip slope. With continued subsidence,
the initiation of accommodation structures
(hangingwall anticlines, antithetic faults and forced
folds) will restrict both the area of deposition and the
continuity of sands. Simultaneously, fault complexity
will increase with the development of different types
of fault-related transfer zones and steps, which in turn
may act as foci for drainage systems.
Finer-grained pelagic and other mature sediments
derived from the hinterlands would dominate over the
coarser sandy or conglomeratic deposits eroded from
local structural highs, although the latter may dominate in isolated basins. Initially, the sediment transport
is still essentially axial, but due to enhanced relief in
the central parts of the basin will be branched to some
extent. A growing local influence from relief caused
by rotated fault blocks is expected. This may cause
sediment transport parallel to the graben axis also in
the more distal parts of the graben system. Locally this
pattern will be broken by transverse transport systems
cutting across highs related to rotated fault blocks, or
in connection with relay ramps and bridges between
fault blocks. In broader tilted fault-blocks or in
grabens immediately adjacent to the hinterlands, the
volume of transversely-transported coarse sediment
may be great.
Compared to the initial stage, the final burial depth
attained by traps generated in the active stretching stage
will of course be shallower. Due to the likely marine
influx and sub-basinal restricted conditions, the chances
for generation of source rocks will also be higher, and
semi-regional or local areas of enhanced subsidence
may promote the presence of local pockets of source
rocks in addition to those of regional significance.
Fans related primarily to synthetic growth along
faults give potentially a mixed trap type, but may also
334
R.H. Gabrielsen