Geology Reference
In-Depth Information
more than 100 kyr of karst erosion in a humid
tropical climate.
Based on the data above, the upper limit of
subaerial denudation rates supported by geolo-
gical observations is estimated to be 200 μm yr
1
.
(Marine erosion, plotted on the same axis, is dis-
cussed below.)
The mix of mechanical and biotic processes
as well as the feedback between erosion and
growth make it exceptionally diffi cult to deter-
mine what are reasonable long-term rates of
marine erosion in tropical carbonates. Arguably
the best-quantifi ed process is the notch-cutting
bio-erosion at and just below the intertidal zone.
Surface retreat of 500-2500
m yr
1
has been
observed in this setting (Spencer, 1992; Neumann &
Hearty, 1996, p. 776). Downward shift of this
zone of rapid bio-erosion during sea-level fall
may be highly effective in eroding the FST in trop-
ical carbonates, particularly in combination with
intensive wave action on the platform margins.
However, little is known about the effects of this
process on geological timescales of 10
4
-10
6
yr. In
Fig. 8 it was assumed that surface lowering by
marine erosion in tropical carbonates can achieve
rates of 100
Marine erosion of tropical carbonates
It is a fundamental principle that the wave-agitated
part of the sea is a zone of intensive erosion and
reworking of loose sediment. Relict bedforms
rarely survive in this zone. The situation frequently
differs in tropical carbonate settings because
organic frame-building and submarine cementa-
tion counteract erosion by waves and currents.
The effect of frame-building and cementation
is strengthened by an important feedback loop.
Both processes tend to intensify where wave
agitation, and thus erosion, are strong. This implies
that where waves intensively erode and lower the
sea bed, similarly intensive frame-building and
cementation repair the damage.
A peculiarity of carbonate sediments is that
they can be eroded by boring and rasping organ-
isms that attack loose grains, organic framework
and hardgrounds. This bio-erosion is signifi cant
throughout the shallow-water carbonate domain
(James & Macintyre, 1985); a narrow maximum in
the uppermost water column produces the char-
acteristic 'intertidal' notch on carbonate sea cliffs
(Spencer, 1992; Neumann & Hearty, 1996). The
process is particularly effi cient because it under-
cuts sea cliffs and creates boulders that come
to rest on the abrasion platform at the foot of
the cliffs thus providing extra surface area for the
notch-cutting bio-eroders.
While the principles of mechanical and biotic
erosion in carbonates are fairly well understood,
their long-term effects, such as rates of lowering of
the sea fl oor, are diffi cult to quantify. Unfortunately
such rates are required for the present discussion.
The emphasis on long-term, geologically relevant
rates is particularly important in view of the above-
mentioned feedback between erosion and frame-
building. If space created by erosion is completely
fi lled by accelerated frame-building, the long-term
effect of erosion for the present analysis would be
zero. Thousands of kilometres of modern barrier
reefs that have built to sea level clearly indicate
that reef communities can maintain a surface at
the upper limit of the reef habitat and thus locally
override the destructive effect of marine erosion.
m yr
1
(expressed in subaerial units
after conversion according to the equation in
Fig. 6). However, this value is one of the least
constrained elements in the present analysis.
Erosion of cool-water carbonates
Frame-building as well as marine and terrestrial
cementation are signifi cantly less than in tropical
settings owing to lower temperatures and lower
contents of metastable aragonite and magnesian
calcite in the sediment (overviews in James, 1997;
Schlager, 2005). As a consequence, the conditions
for erosion shift in the direction of siliciclastic
settings. The seaward dipping shelves and
rounded shelf breaks of most cool-water carbonate
provinces (James, 1997) indicate the dominance
of marine erosion over frame-building and
cementation. It is important to note that sea-fl oor
morphology is determined by the balance of ero-
sional and constructional processes, not their
presence or absence. There is considerable ero-
sion and removal of detrital material from tropi-
cal platform rims and there is much production
and even frame-building on cool-water carbon-
ate shelves. The contrasting morphologies of
the two settings result from different balances
of destructional and constructional processes -
widespread dominance of construction in the
T factory and equally wide-spread dominance
of erosional destruction in the C factory. Certain
shelves of southern Australia occasionally may
represent an end-member in the changing bal-
ance between production and marine erosion:
on these 'shaved shelves' cool-water carbonate
is produced but almost immediately removed by
