Geology Reference
In-Depth Information
RESULTS
similar to those of subaerial erosion. Increasing
shallow-marine erosion again reduces the size of
the FST and eventually creates the anatomy of the
standard model (Fig. 5).
The overall effect of marine erosion on FST
development in C3D models is similar to subaerial
erosion but there are important differences with
regard to the rates required to suppress the FST
(expressed as surface lowering in
Numerical modelling
The program C3D was used to explore under which
conditions a falling-stage systems tract would
develop and when the confi guration of the stan-
dard model would be realized. 'Standard-model
confi guration' implies that sea-level fall created a
continuous surface of erosion or non-deposition
and left no sediment accumulations in the area
swept by the falling sea level. The depositional
record of the time of sea-level fall consists only
of deeper-water sediments on the lower slope and
basin fl oor, i.e. the area below the sea-level min-
imum. This section presents the most important
parameters controlling FST development and
summarizes their effects as observed in about
300 modelling runs.
m yr
1
). The
two erosive processes also create somewhat differ-
ent morphology on the erosion surface. Subaerial
carbonate erosion is assumed to be uniform over
the exposed area and to proceed by dissolution,
thus all eroded material leaves the model space.
Marine erosion varies with water depth and much
eroded material is redeposited within the model.
Marine erosion is less effective in destroying the
FST because its maximum rates are only realized
in a narrow depth zone and most eroded material
is redeposited further downslope and thus needs
to be reworked repeatedly during sea-level falls.
In this study, the 'FST destruction effi ciency' of
marine and subaerial erosion was found to be
linearly correlated in a wide range of conditions
(Fig. 6). This allows the substitution of marine for
subaerial erosion and vice versa in modelling runs
and plot them on one axis in parameter space.
In summary, the modelling runs clearly indic-
ate that changes in rates of sea-level fall and rates
of erosion have similar effects on depositional
anatomy. In both instances, increasing rates shift
the depositional system in the direction of the
STM, decreasing rates shift it in the direction of
the FST.
Rate of sea-level fall
Arguing from fundamental principles of erosion
and deposition, one can already predict that the
development of a FST is more likely if sea level
falls slowly whereas rapid falls will favour the
standard model. The numerical models clearly
confi rm this view and provide quantitative estim-
ates on the rates of fall required to create FST or
STM anatomy respectively (Fig. 3). The runs were
made for a tropical carbonate platform able to
build a wave-resistant margin and maintain steep
slopes. Both subaerial and shallow-marine erosion
are set to zero in Fig. 3. Under these conditions,
a sinusoidal sea-level cycle produces a large
falling-stage systems tract. Increasing asymmetry
of the sea-level cycle and increasing rates of fall
reduce the size of the FST and fi nally lead to the
standard model. It should be noted that the rate
required to completely suppress the FST is also
a function of the spatial and temporal resolution
used as input for the modelling runs.
Carbonate production
Carbonate production is a third important control
on FST development. Production supplies sedi-
ment for the construction of the FST and therefore
directly counteracts erosion. Figure 7 illustrates
the effect of increasing production while the rates
of erosion and sea-level fall remain unchanged.
Both highstand and lowstand tract increase in
volume as production increases. The FST appears
when production rate is so high that erosion
is unable to remove all the sediment produced
during sea-level fall.
Rate of erosion
The C3D program allows one to separately
model the effects of subaerial and marine ero-
sion. The effect of increasing subaerial erosion is
shown in Fig. 4. Much like changing rates of sea-
level fall, varying rates of erosion shift the system
from the FST mode to the mode of the standard
model and vice versa. In the present context, the
effects of shallow-marine erosion (by currents and
waves, or by boring or grinding organisms) are
Slope angle
The FST sediments are deposited on slopes and
this may cause resolution problems if slopes
