Geology Reference
In-Depth Information
waves and currents such that Pleistocene relict
material forms the sea fl oor (James
et al
., 2001). It
is unlikely that a healthy tropical carbonate fac-
tory could evolve to this extreme state because
reef builders would occupy this space. However,
the Australian shaved shelves are important as
extreme members of the erosion spectrum.
the FST question. In all experiments, the slope
interval that needed to be examined for presence
of FST fell in a rather narrow range of 15-25
.
These slopes are far below the angle of repose of
reef talus in tropical carbonates. Even the occa-
sionally considered slopes of 40
fall within the
observed range of detrital carbonate slopes (Kenter,
1990; Grammer
et al
., 1993). Moreover, frame-
building and marine cementation allow reefs to
maintain slopes far above the angle of repose of
loose material (Hopley, 1982; Kenter, 1990; Ebren,
1996). Thus, the runs were realistic in the sense
that they did not make unjustifi ed assumptions
on preserving FST on excessively steep slopes.
The narrow range of slopes in the presence/
absence tests also means that the resolution var-
ied little among runs, thus avoiding the second
problem. It must be noted, however, that the
effect of deposition-related changes in slope angles
on FST development is not zero.
Effect of carbonate production
In the results section it was mentioned that changes
of carbonate production and erosion have opposite
effects on FST development. As a consequence,
rates of production are as important for FST devel-
opment as rates of erosion or rates of sea-level fall.
The ability of tropical carbonate systems
to keep pace with relative sea-level rise, their
growth potential, is fairly well known from
observations on modern reefs as well as growth
rates gleaned from the geological record. It is
also well established that sedimentation rates
and, by analogy also the growth potential of the
T factory, decrease as the time window of obser-
vation increases (Sadler, 1981; Plotnick, 1986;
Schlager, 1999). In order to defi ne the geologically
probable parameter space for the FST, one needs
to estimate the maximum growth rates that are
reasonable with respect to the time required
for the sea-level fall. The shortest sea-level fall
in the numerical experiments took 0.7 kyr, the
longest 111 kyr; the vast majority of falls lasted
between 1 and 50 kyr. Based on the compilations
of Schlager (1999), this translates into accept-
able maximum growth rates of 40,000
Composite FST by superposition of
sea-level cycles
Two styles of FST have been described in the lit-
erature, a gradual downward shift or downward
shift in discrete steps. Both geometries indicate
sedimentation during sea-level fall. The stepped
record additionally indicates that stillstands or
short oscillations of sea level were superimposed
on a longer falling trend. In tropical carbonates,
downstepping can be expected to be common
because the factory usually builds fl at topsets
at sea level, wave-resistant rims and very steep
upper slopes. If sea level falls from the platform
top, it is likely to leave little sediment accumula-
tion until it reaches a zone of lower slope angle.
Consequently, carbonate FSTs with smoothly dip-
ping surfaces usually are truncated clinoforms
(e.g. Nijar Ridge, Fig. 11; Pomar, 1991; Franseen
et al
., 1993, Fig. 3). Pomar (1991) elegantly applied
the depth zonation of reef corals to demonstrate
that erosion surfaces on downstepping clinoforms
indeed record a downward shift of relative sea
level. In seismic data, the distinction between
stepped and gradually shifting FSTs frequently is
obscured by limited resolution.
m yr
1
and
m yr
1
for the extremes and approximately
15,000-1000
700
m yr
1
for the commonly used
time window. This result implies that the growth
rates used in the simulations were all within the
geologically probable range.
Effect of slope angle
The angle of depositional slope may infl uence
the numerical FST analysis in two ways. First,
in loose sediment accumulations the preserva-
tion potential of the FST decreases as the slope
approaches the angle of repose; it becomes zero
at the angle of repose. Second, in modelling runs
it may be diffi cult to resolve FST accumulations
defi ned by time lines on steep slopes because the
lateral separation of the lines decreases with the
cosine of the slope (see Results section).
In this study, settings were chosen to min-
imize the effect of slope angle and resolution on
'Geologically probable' part of parameter space
The boundaries between FST and STM domains in
Fig. 8 are based on numerical modelling. The 'geo-
logically probable space' in Fig. 8a and c indicates
conditions supported by geological observation.
