thickness (and density ρ), shortly after deposition
(Stickler and Watts 1978b, Bally et al. 1981). If we
know the variations in sea level and the depth of the
water from environmental interpretations, for example, these can be inserted into the equation so that the
primary tectonic movement can be worked out.
The backstripping technique which is used to reconstruct the subsidence history of different parts of a
sedimentary basin lends itself very well to computer
modelling. The data obtained on the depth and temperature history of the source rock in particular has allowed
much better assessments of kerogen maturity and the
times of oil expulsion and migration. One parameter
which is crucial but difficult to estimate is the variation
of the geothermal gradient as a function of geological
time. It is also often difficult to estimate the palaeodepth
during the deposition of different sedimentary
formations. Whether a formation was deposited at a
depth of 200 or 1,000 m will make a very significant
difference, and it is often difficult to make accurate
palaeoenvironmental estimates of the palaeobathymetry.
To form a sedimentary basin with a thick infill of
sediments requires a crustal depression large enough
to provide the accommodation space for the deposits.
This may be a result of large-scale crustal movements
like seafloor spreading and crustal thinning (extension). The supply of sediments is also critical and
clastic sediments have to be supplied from adjacent
land areas which are being uplifted and eroded.
Chemical and biogenic sediments are formed
locally from seawater and thus are not reliant on sediment supply from land areas. This typically applies to
limestones, which accumulate where there is little
clastic sedimentation. Sedimentary basins that are
more or less isolated from the sea in arid regions can
be filled up with evaporites at a rather high sedimentation rate.
8.9
Continental Rifting
Stretching and thinning of the continental crust takes
place by tensional tectonics in connection with rifting.
Crustal stretching may be a result of tensional forces
and uplift due to the high geothermal gradients
associated with rifting. This leads to a thinning of the
continental crust, causing isostatic subsidence.
Fracture zones in the continental crust (rift valleys)
produce subsidence which often is located in a
sedimentary basin because the continental crust is
thin and because heavy rocks from the mantle push
their way up, thus increasing the average density of the
rocks. Those parts of the rift valley system which have
much volcanism will have less space for the
sediments. Along the margins of a rift system, where
the continental crust is not stretched, uplift occurs due
to the higher geothermal gradient. This causes the
basement rock at the surface to slope away from the
rift valley as the geothermal gradients are reduced.
This will to some extent be compensated for by the
formation of erosional valleys which cut backwards
into the raised shoulders on the sides of the rift valley.
Because of the high relief around these basins and
the short transport distance for the sediment eroded
from the bedrock, these basins will be characterised by
mineralogically very immature sediments, largely
arkoses and conglomerates deposited in fan deltas
along the active faults. In the central and deeper
parts of the basins we find deposition of finer-grained
sandstones and clayey sediments. Rift valley basins
may be continental lacustrine basins as in East Africa,
or marine as those offshore East Africa and the Jurassic basins of the North Sea. Horsts, which are
unstretched (thick) pieces of continent crust, may
become topgraphically very high due to high heat
flow. The Ruwenzori Mountains of East Africa,
reaching more than 5,000 m, are one example. Both
marine and lacustrine rift basins will tend to have
reducing conditions in the deeper part due to limited
circulation of oxygenated water.
Rift basins formed in areas with wet climates will
be occupied by large lakes. Lacustrine basins often
have an even better potential for producing source
rocks than marine rift basins, because water stratification (density stratification) is usually more marked in
lakes. We will therefore often find black, organic-rich
shales in these basins.
Rift basins formed in arid zones are characterised
by evaporite deposits. Block faulting will readily lead
to isolated basins or horsts which cut off contact with
the open sea. Evaporites are typical of rift deposits
today, e.g. in East Africa, and were widespread in
Europe and North America during the Permo-Triassic,
before the ocean-floor spreading which created the
Atlantic Ocean started in the Mid-Jurassic. The
Zechstein salt deposits in Germany and the North
Sea are typical examples. Jurassic and early Cretaceous rifting during the early phase of the opening of
266
K. Bjørlykke
(Stickler and Watts 1978b, Bally et al. 1981). If we
know the variations in sea level and the depth of the
water from environmental interpretations, for example, these can be inserted into the equation so that the
primary tectonic movement can be worked out.
The backstripping technique which is used to reconstruct the subsidence history of different parts of a
sedimentary basin lends itself very well to computer
modelling. The data obtained on the depth and temperature history of the source rock in particular has allowed
much better assessments of kerogen maturity and the
times of oil expulsion and migration. One parameter
which is crucial but difficult to estimate is the variation
of the geothermal gradient as a function of geological
time. It is also often difficult to estimate the palaeodepth
during the deposition of different sedimentary
formations. Whether a formation was deposited at a
depth of 200 or 1,000 m will make a very significant
difference, and it is often difficult to make accurate
palaeoenvironmental estimates of the palaeobathymetry.
To form a sedimentary basin with a thick infill of
sediments requires a crustal depression large enough
to provide the accommodation space for the deposits.
This may be a result of large-scale crustal movements
like seafloor spreading and crustal thinning (extension). The supply of sediments is also critical and
clastic sediments have to be supplied from adjacent
land areas which are being uplifted and eroded.
Chemical and biogenic sediments are formed
locally from seawater and thus are not reliant on sediment supply from land areas. This typically applies to
limestones, which accumulate where there is little
clastic sedimentation. Sedimentary basins that are
more or less isolated from the sea in arid regions can
be filled up with evaporites at a rather high sedimentation rate.
8.9
Continental Rifting
Stretching and thinning of the continental crust takes
place by tensional tectonics in connection with rifting.
Crustal stretching may be a result of tensional forces
and uplift due to the high geothermal gradients
associated with rifting. This leads to a thinning of the
continental crust, causing isostatic subsidence.
Fracture zones in the continental crust (rift valleys)
produce subsidence which often is located in a
sedimentary basin because the continental crust is
thin and because heavy rocks from the mantle push
their way up, thus increasing the average density of the
rocks. Those parts of the rift valley system which have
much volcanism will have less space for the
sediments. Along the margins of a rift system, where
the continental crust is not stretched, uplift occurs due
to the higher geothermal gradient. This causes the
basement rock at the surface to slope away from the
rift valley as the geothermal gradients are reduced.
This will to some extent be compensated for by the
formation of erosional valleys which cut backwards
into the raised shoulders on the sides of the rift valley.
Because of the high relief around these basins and
the short transport distance for the sediment eroded
from the bedrock, these basins will be characterised by
mineralogically very immature sediments, largely
arkoses and conglomerates deposited in fan deltas
along the active faults. In the central and deeper
parts of the basins we find deposition of finer-grained
sandstones and clayey sediments. Rift valley basins
may be continental lacustrine basins as in East Africa,
or marine as those offshore East Africa and the Jurassic basins of the North Sea. Horsts, which are
unstretched (thick) pieces of continent crust, may
become topgraphically very high due to high heat
flow. The Ruwenzori Mountains of East Africa,
reaching more than 5,000 m, are one example. Both
marine and lacustrine rift basins will tend to have
reducing conditions in the deeper part due to limited
circulation of oxygenated water.
Rift basins formed in areas with wet climates will
be occupied by large lakes. Lacustrine basins often
have an even better potential for producing source
rocks than marine rift basins, because water stratification (density stratification) is usually more marked in
lakes. We will therefore often find black, organic-rich
shales in these basins.
Rift basins formed in arid zones are characterised
by evaporite deposits. Block faulting will readily lead
to isolated basins or horsts which cut off contact with
the open sea. Evaporites are typical of rift deposits
today, e.g. in East Africa, and were widespread in
Europe and North America during the Permo-Triassic,
before the ocean-floor spreading which created the
Atlantic Ocean started in the Mid-Jurassic. The
Zechstein salt deposits in Germany and the North
Sea are typical examples. Jurassic and early Cretaceous rifting during the early phase of the opening of
266
K. Bjørlykke