11.3 Dynamic Denudation Accumulation Systems
Modeling the situation of the early Mesozoie Newark
basin (shown on map in Fig. 12.7), Schlische and Olsen
(1990) found a fairly good agreement between their model
(Fig. 11.12c,d) and the gross stratigraphie and depositional
development of this basin (Fig. 11.12e, Stage 4). In order
to apply the more simple full-graben model to the basin,
the half-graben cross section of the basin was transformed
into a full-graben. The subsidence rate was inferred from
the sedimentation rate of time intervals during which both
rates were equal. Over long periods, the cumulative sediment thickness and the sedimentation rate of the model approximates the true values. Major discrepancies between
model and nature were caused by vo1canic activity and climatic change.
A somewhat different, more quantitative approach to
the longer term evolution of a rift basin is demonstrated in the model in Fig. 11.13. Both the
terrigenous sediment supply (global mean value of
mechanical denudation DR.nc=50 nun/1m) and the rate
at which the rift or ocean basin is widening are still
assumed to be constant.
The rate of extension is 0.25 cmla for a narrow basin (beginning with 50 km width) in its early stage of evolution,
or it is 2.5 cmla for a wider ocean basin (500 km). In both
cases the time period needed for a doubling of the basin
width is 20 Ma. The initial tectonic subsidence history is
linear, but later, due to crustal cooling, it shows a concaveup curve (cf.Sect. 8.3 and Fig. 11.10b). Both the early and
late portions ofthis subsidence curve are stro!\gly modified
by the increasing sediment load which, in this example,
completely fills up the basin for a long time.
The model starts with a drainage area two times larger
than the basin area (Fig. 11.13a, left-hand side). The sedimentation rate SR",e for solid matter (p=2.5 g/cm 3 ) is therefore twice as high (100 mmlka) than DRme (Fig. 11.l3b).
The actual sedimentation rate for porous sediment with
lower bulk density is of course higher as shown in the diagram. Due to basin widening, SRmc decreases with time and
the average sediment density simultaneously grows with
the thickening sedimentary fill. Surplus sediment which
cannot be accommodated by the basin is bypassed to other
regions. After a time period of about 40 Ma the incoming
sediment cannot fill up the basin any more, i.e. it is transformed into a lake or shallow sea. Then subsidence proceeds more slowly than it would do for a completely
sediment-filled basin (Fig. 11.13b).
Fig. 11.12. Model of extensional rift basin filling
with the assumptions of constant sediment input and
uniform subsidence. a Full-graben filled with fluvial
and lake deposits under hydrologically open (1 to 3)
and closed conditions (4); E extension (%). b Twodimensional model of progressive, complete fill of
half-graben growing in size; cumulative sediment
thickness in total basin (average) and basin center,
and declining sedimentation rates through time.
501
Rift Basins with Varying Sediment Supply
So far, achanging input of terrigenous material has
not been considered. As discussed in Section 9.3, the
specific sediment yield of drainage areas can greatly
vary and, in addition, their areal extent can change
significantly with time. These effects are indicated in
Fig. 11.11 b,c where an elongate basin (e.g. a graben
structure) receives river sediment from several sides.
During the initial phase of tectonic basin formation,
both the relief and the river gradients are commonly
high causing enhanced specific sediment yields. In
the course of time, the sediment yields may decrease
due to the lowering of the landscape. On the other
hand, the drainage areas of some of the feeder SYSc
tems grow with time as a result of headward river
erosion preventing a significant decrease in sediment
influx. In addition, climate change may have some
influence on the sediment supply. Taking into account all these factors (changes in relief, drainage
area, climate), the sediment supply may vary by at
least one order of magnitude during the history of an
individual basin.
A rift basin model where the terrigenous sediment
influx and thus also the mean mechanical sedimentation rate varies on two occasions by a factor of 2 is
shown in Fig. Il.l4a.
In contrast to the previous model (Fig. 11.13b and c), the
rates of subsidence and sedimentation (instead of sediment
thickness or depth) are now plotted over the time scale.
The sedimentation rates decrease along the solid curves in
the figure.
The rate of total subsidence reaches a maximum at
the transition from continental to marine deposits and
thereafter decreases because then the mean sedimentation rate in the widening basin lags behind subsidence. Both the response time (since the starting
point of the model) and the time interval after which
the subsidence rate has reached a maximum, increase
with the sedimentation rate operating at the onset of
basin evolution.
If the period of subsidence of a graben or
halfgraben is limited and the rate of total subsidence
starts to decrease early (Fig. l1.l4a, lowermost
curve) the basin filling sediments often exhibit the
tripartite sequence as mentioned above. This is
c Transformation of half-graben into full-graben to
apply full-graben filling model (d) based on data
from Triassic-Jurassic Newark basin (see Fig. 12.7).
e Adjustment of deposition to asymmetric, uniform
basin subsidence (1), accelerated tilting (2), additional faultin~ (3), and (4) situation after further tilting and erOSiOn. (After Schlische and Olsen 1990;
e4 from Manspeizer 1988b)
Modeling the situation of the early Mesozoie Newark
basin (shown on map in Fig. 12.7), Schlische and Olsen
(1990) found a fairly good agreement between their model
(Fig. 11.12c,d) and the gross stratigraphie and depositional
development of this basin (Fig. 11.12e, Stage 4). In order
to apply the more simple full-graben model to the basin,
the half-graben cross section of the basin was transformed
into a full-graben. The subsidence rate was inferred from
the sedimentation rate of time intervals during which both
rates were equal. Over long periods, the cumulative sediment thickness and the sedimentation rate of the model approximates the true values. Major discrepancies between
model and nature were caused by vo1canic activity and climatic change.
A somewhat different, more quantitative approach to
the longer term evolution of a rift basin is demonstrated in the model in Fig. 11.13. Both the
terrigenous sediment supply (global mean value of
mechanical denudation DR.nc=50 nun/1m) and the rate
at which the rift or ocean basin is widening are still
assumed to be constant.
The rate of extension is 0.25 cmla for a narrow basin (beginning with 50 km width) in its early stage of evolution,
or it is 2.5 cmla for a wider ocean basin (500 km). In both
cases the time period needed for a doubling of the basin
width is 20 Ma. The initial tectonic subsidence history is
linear, but later, due to crustal cooling, it shows a concaveup curve (cf.Sect. 8.3 and Fig. 11.10b). Both the early and
late portions ofthis subsidence curve are stro!\gly modified
by the increasing sediment load which, in this example,
completely fills up the basin for a long time.
The model starts with a drainage area two times larger
than the basin area (Fig. 11.13a, left-hand side). The sedimentation rate SR",e for solid matter (p=2.5 g/cm 3 ) is therefore twice as high (100 mmlka) than DRme (Fig. 11.l3b).
The actual sedimentation rate for porous sediment with
lower bulk density is of course higher as shown in the diagram. Due to basin widening, SRmc decreases with time and
the average sediment density simultaneously grows with
the thickening sedimentary fill. Surplus sediment which
cannot be accommodated by the basin is bypassed to other
regions. After a time period of about 40 Ma the incoming
sediment cannot fill up the basin any more, i.e. it is transformed into a lake or shallow sea. Then subsidence proceeds more slowly than it would do for a completely
sediment-filled basin (Fig. 11.13b).
Fig. 11.12. Model of extensional rift basin filling
with the assumptions of constant sediment input and
uniform subsidence. a Full-graben filled with fluvial
and lake deposits under hydrologically open (1 to 3)
and closed conditions (4); E extension (%). b Twodimensional model of progressive, complete fill of
half-graben growing in size; cumulative sediment
thickness in total basin (average) and basin center,
and declining sedimentation rates through time.
501
Rift Basins with Varying Sediment Supply
So far, achanging input of terrigenous material has
not been considered. As discussed in Section 9.3, the
specific sediment yield of drainage areas can greatly
vary and, in addition, their areal extent can change
significantly with time. These effects are indicated in
Fig. 11.11 b,c where an elongate basin (e.g. a graben
structure) receives river sediment from several sides.
During the initial phase of tectonic basin formation,
both the relief and the river gradients are commonly
high causing enhanced specific sediment yields. In
the course of time, the sediment yields may decrease
due to the lowering of the landscape. On the other
hand, the drainage areas of some of the feeder SYSc
tems grow with time as a result of headward river
erosion preventing a significant decrease in sediment
influx. In addition, climate change may have some
influence on the sediment supply. Taking into account all these factors (changes in relief, drainage
area, climate), the sediment supply may vary by at
least one order of magnitude during the history of an
individual basin.
A rift basin model where the terrigenous sediment
influx and thus also the mean mechanical sedimentation rate varies on two occasions by a factor of 2 is
shown in Fig. Il.l4a.
In contrast to the previous model (Fig. 11.13b and c), the
rates of subsidence and sedimentation (instead of sediment
thickness or depth) are now plotted over the time scale.
The sedimentation rates decrease along the solid curves in
the figure.
The rate of total subsidence reaches a maximum at
the transition from continental to marine deposits and
thereafter decreases because then the mean sedimentation rate in the widening basin lags behind subsidence. Both the response time (since the starting
point of the model) and the time interval after which
the subsidence rate has reached a maximum, increase
with the sedimentation rate operating at the onset of
basin evolution.
If the period of subsidence of a graben or
halfgraben is limited and the rate of total subsidence
starts to decrease early (Fig. l1.l4a, lowermost
curve) the basin filling sediments often exhibit the
tripartite sequence as mentioned above. This is
c Transformation of half-graben into full-graben to
apply full-graben filling model (d) based on data
from Triassic-Jurassic Newark basin (see Fig. 12.7).
e Adjustment of deposition to asymmetric, uniform
basin subsidence (1), accelerated tilting (2), additional faultin~ (3), and (4) situation after further tilting and erOSiOn. (After Schlische and Olsen 1990;
e4 from Manspeizer 1988b)