Page 34 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTION 25

such that one variable, time, appears on both axes (Gard- vertical growth determines the system’s ability to keep up
ner et al., 1987; Schlager et al., 1998). The reason for the with relative sea-level rise and remain in the photic zone.
scaling of sedimentation rates lies in the distribution of hia- Usually, the aggradation potential is significantly higher at
tuses (Sadler, 1981). Sedimentation and erosion are episodic the margins of shoal-water platforms than in the platform
or pulsating processes and the record is riddled with hia- interior (Fig. 2.22) and this may lead to a growth morphol-
tuses of highly variable duration. The fractal Cantor set is ogy of raised rims and deep lagoons (chapter 3). In the M
a good mathematical model of this situation (Fig. 2.20; Plot- factory, the vertical growth potential is critical for staying
nick, 1986). above the adjacent sea floor and avoid being buried. Where
On log-log plots, the regression lines of the rate-time plots the M factory builds platforms that rise to sea level, one oc-
show slopes of approximately -0.5. A slope of -0.5 is typi- casionally also observes raised rims and deep lagoons (e.g.
cal of random noise, for instance a trend that is generated Adams et al., 2004).
by the superposition of many unrelated effects. This is in-
deed what one may expect considering the many factors that CARBONATE-SPECIFIC ASPECTS OF DEPOSITION
can affect sedimentation rates. Sadler (1981, 1999) examined AND EROSION
very large data sets and found regression slopes of approx-
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8
imately -0.5 in the domain of 10 –10 yr, but also signif- Carbonate sedimentation obeys the same mechanical pro-
icantly higher slopes in certain time windows, such as the cesses that control sedimentation in siliciclastics or other
frequency band of the Earth’s orbital perturbations (chapter sediment accumulations. A discussion of these principles is
5). beyond the scope of this book. There are, however, some as-
The upper limit of the observed rates, the estimated pects of deposition and erosion in carbonate environments
growth potential, also scales with a factor of -0.5. In the geo- that deserve special attention.
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logically particularly relevant interval of 10 –10 years the
T-factory rates are highest, decreasing from 250 to 100 µ/y. Deposition – source and sink
C rates are about 25% of the tropical rates. Rates of the M
factory are about the same as the T rates. However, field ob- In siliciclastic systems, source and sink, i.e. erosional hin-
servations indicate that mud-mounds shed far less sediment terland and place of deposition, are normally well sepa-
into the adjacent basins. I therefore estimate the growth po- rated. In shoal-water carbonate systems, the two roughly
tential of the M factory to be only 80-90 % of that of the T coincide. However, the spatial coincidence is only approxi-
factory. mate. The action of waves and tides in shoal-water carbon-
The growth potential also varies with time. Particularly ate environments is normally so intense that redistribution
important is that during transgressions the factories need to of sediment is common and may move sediment by tens of
be started up again after exposure and this start-up phase is kilometers (Fig. 2.23). Large-scale redistribution of the out-
coupled with slow growth followed by rapid growth (Fig. put of carbonate platforms is also apparent from the record
1.12). Population dynamics as described by the logistic of the deep-water sediments surrounding the shoal-water
equation (Fig. 1.12) is the reason for the sigmoidal growth sources. Periplatform muds, turbidites, and debris flows
curve. In carbonate sedimentology, this pattern is known attest to the fact that a platform such as the Bahama Bank
as the start-up, catch-up and keep-up stages of growth exports material not just from the rims but also the plat-
and reef response to the Holocene sea-level rise is a typi- form interior (Neumann and Land, 1975; Droxler& Schlager,
cal example of this rule (Fig. 2.21; Neumann and Macintyre, 1985; Haak and Schlager, 1989; Roth and Reijmer, 2004). De-
1985). However, loose-sediment accumulations such as oo- termining extent and pattern of sediment redistribution re-
lite shoals or lagoonal muds, also display this pattern. The quires knowledge of the platform facies outlined in chapter
law of sigmoidal growth implies that the growth potential 4.
of the system is significantly lower in the start-up phase. In Another noteworthy aspect of carbonate sedimentation is
the Holocene, the effect lasts only 2,000 to 5,000 years. Lag the ability to form rigid structures upon deposition. This
effects of millions of years have been postulated for growth is done by organisms building skeletal framework, by bio-
in the wake of mass extinctions (Hottinger, 1989). induced precipitation of micrite or by abiotic precipitation of
The growth potentials derived from Fig. 2.20 should be cement. In reefs, organic framebuilding and abiotic cemen-
viewed as very crude estimates. They are based on limited tation combine to create structures that can resist the force
data and they consider only vertical aggradation which is a of all but the most turbulent marine environments.
rather imperfect substitute for sediment production by vol-
ume or mass. However, data on volumetric sediment pro- Erosion
duction of the distant geologic past are very rare and ham-
pered by the fact that carbonate factories are open systems Erosion of the surface of the solid Earth is, of course, per-
that export much sediment to the surrounding ocean where vasive and often dominant. In the context of this book, we
it dissolves or becomes highly diluted and unrecognizable. will only consider erosion that occurs within the deposi-
The ability to grow upward, the aggradation potential, is tional environment or so close to it in space or time that it
an important parameter in its own right. In the T factory, influences subsequent deposition.