Page 76 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 4: CARBONATE FACIES MODELS 67

STABILITY OF FACIES PATTERNS Bioturbation of carbonate sediments may lead to “whole-
sale destruction of evidence” as Bathurst (1971, p.128) called
The standard facies belts of Wilson (1975) were conceived it. The sediments of the T factory in particular suffer inten-
as static patterns. This is a gross simplification and strati- sive bioturbation by crabs in the upper 1 -1.5 m (Shinn, 1968;
graphers certainly are aware of this. Wilson (1975) himself Bathurst, 1971). One consequence of bioturbation is the loss
dedicated most of his book to documenting the change with of hydrodynamic structures in all but the most energetic en-
time of carbonate facies and chapter 5 of this report deals vironments on carbonate platforms. Two implications of this
with the same topic. This section, by contrast, addresses a intensive turnover have already been mentioned:
very specific aspect of facies change with time: The ques- ➤ In most instances we cannot tell if the wackestones
tion of which patterns are transient and which are more sta- and packstones of platform limestones were indeed de-
ble considering the internal dynamics of facies belts and the posited by simultaneously dumping a wide range of
feed back between sedimentation, erosion and morphology. grain sizes or whether a primary alternation of mud
The rimmed platform with flat top and steep flanks is a and coarse storm layers was later mixed by burrowers.
very stable pattern of the T factory. It is maintained (or re- ➤ Hydrodynamic structures of the intertidal zone are al-
established after external disturbance) by the following pos- most entirely lost. These structures were already less
itive feedbacks: well developed in carbonates than in siliciclastics be-
➤ Production is high but constrained to a narrow part of cause the carbonate seabed is partly covered by algal
the water column. mats and seagrass; the structures that did form were
➤ High and rather uniform, wave energy leads to fairly subsequently erased by bioturbation.
even sediment distribution. Selective dissolution is another source of preservational
➤ Excess sediment can be dumped into the adjacent, bias (Flügel, 2004, p.106). Shoal-water carbonate sediments
nearly infinite accommodation of the open ocean; in are deposited as mixtures of aragonite, magnesian calcite
this way the flat top is expanded by lateral prograda- and calcite (see chapter 1). Aragonite and magnesian cal-
tion. cite normally are the most abundant minerals and both are
M factory platforms are stabilized in a similar way but their metastable. Dissolution-precipitation reactions during dia-
development in the Phanerozoic requires (temporary) shut- genesis yield stable end members in the form of dolomites
down of the T factory as a starting condition. and calcitic limestones. The vestiges of selective dissolu-
The empty bucket is an unstable pattern. It can develop tion during diagenesis can be observed in the rocks, but
only if the growth rate of the rim exceeds the rate of accom- they are only preserved if the sediment was stiff or hard
modation creation whereas the growth rate of the platform during the process and the voids did not collapse. Disso-
interior trails it. As the rim rises above the lagoon, the latter lution of metastable grains during early diagenesis, when
turns into a natural sediment trap that tends to fill up and soft sediment was being bioturbated and mechanically com-
counteract the effects of differential growth. pacted, may go unnoticed. Recently, several authors pro-
Ramps are stable or transient, depending on the factory. T- posed that this is exactly what has happened (Cherns and
factory ramps tend to be transient. High but narrowly con- Wright, 2000; Peterhänsel and Pratt, 2001; chemical consid-
strained production and formation of wave-resistant struc- erations in Morse, 2004,p.80). Work has not gone far enough
tures allow the system to quickly deviate from the hydro- to estimate how much has been lost but there can be no
dynamic equilibrium profile. The T factory tends to fill the doubt that the problem is real and needs attention.
shallow part of the ramp to sea level, then prograde seaward
and steepen the slope. The result is an expanding shallow-
TERRESTRIAL EXPOSURE
water platform. Ramps of the C factory are rather stable.
The production window is wide, and frame building and ce- In chapter 2, it was concluded that the window of marine
mentation are minor. The factory produces in a wide depth carbonate production ends in the supratidal zone. In the
range. The sediment is easily reworked and molded into a terrestrial environment, carbonate dissolution prevails and
sigmoidal profile in equilibrium with waves and currents. carbonate precipitation is limited to small fresh-water bod-
ies and other special environments. A relative sea level fall
BIAS IN THE FACIES RECORD that exposes a marine carbonate environment to terrestrial
conditions shuts down the carbonate factory immediately
It is widely accepted that sedimentary rocks chronicle and creates a break in carbonate sedimentation. As sea-level
earth history in a rather imperfect way. One distortion of the fall and exposure play a pivotal role in sequence stratigra-
record is what I termed “depositional bias” - the tendency of phy, the facies record of terrestrial exposure needs to be dis-
different sediment families to record the same events differ- cussed in some detail.
ently, just like newspapers report on human history with a First, some definitions need to be clarified. The term “sub-
certain editorial bias. Depositional bias in sequence stratig- aerial exposure“ is somewhat ambiguous. It is preferable to
raphy will be discussed in chapter 5. Besides depositional use the following terms:
bias there exists bias of preservation and this is the topic of ➤ intertidal - between normal high and low tide, alter-
this section. nately flooded and awash;