Page 82 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 5

Rhythms and events in carbonate stratigraphy

The standard facies model depicts depositional systems AUTOCYCLES
where sedimentation is in approximate equilibrium with in-
trinsic feedbacks and extrinsic controls. The assumption of
equilibrium conditions is often unjustified and this chapter Most depositional systems are complex and endowed
examines important causes of change in carbonate sedimen- with many internal feedbacks. Consequently, stable steady-
tation with time. state operation is rarely ever reached. In place of that,
Chapter 2 presented evidence that sedimentation is inher- the systems oscillate within certain limits, comparable to a
ently episodic or pulsating and that the record is riddled heater coupled to a thermostat. The heater kicks on when
with hiatuses in a wide range of scales. The scaling of sedi- the room temperature reaches the lower limit and turns off
mentation rates and the intense lamination of unbioturbated at the upper limit whence the room slowly cools to the lower
sediments are two major arguments in favor of a non-steady limit and the cycle is repeated. Among carbonates, the T fac-
model of sedimentation. This “Cantor model” of sedimen- tory is particularly prone to act as a limit-driven oscillator.
tation (Plotnick, 1986) does not invalidate facies models but The depth window of production is narrow, production po-
limits their use. Facies models should be viewed as ideal- tential is very high and the production curve is sigmoidal
ized equilibrium states that depositional systems strive to with a lag phase at the beginning (see chapter 2).
but do not always reach before being disturbed by extrin- Autocycles are easily produced in computer models by as-
sic factors. The episodic nature of sedimentation implies suming that carbonate production starts very slowly when
that stratigraphic documentations solely in terms of stan- a supratidal area is re-flooded and that tidal flats rapidly
dard facies belts, is inadequate and cannot do justice to the prograde (Drummond and Wilkinson, 1993; Demicco, 1998).
complexity of vertical successions. For carbonates, in par- Fig. 5.1 illustrates shoaling cycles generated by the computer
ticular, we need to consider changes through time imposed program STRATA (appendix B). The cycles form by the in-
by system-internal feed back (“autocycles”), orbital cycles terplay of linear subsidence and a T factory growth function
of the ocean-atmosphere system, organic evolution and long with a time lag.
oscillations induced by plate tectonics and cosmic processes.

Fig. 5.1.— Shoaling autocycles
on a carbonate platform generated
with program STRATA (appendix B).
water depth of deposition Thickness scale on the left. Col-
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 ors refer to depth of deposition.
Depositional environment becomes
10 km shallower by sediment accumulation.
0
When the sediment pile reaches sea
10 level, production ceases and is re-
sumed with a time lag of 7 ky. Thus,
20
depth scale 40 blue represent a 7 ky hiatus. Au-
the sharp boundaries from yellow to
30
tocycles arise from the interplay of
sediment overproduction, production
shut down at sea level, and resumed
50
production after a time lag during
which continued subsidence creates
accommodation for the next cycle.
Lag phase corresponds to the slow
initial growth phase in the logistic
equation (Fig. 1.12).

73