Page 156 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 8

Sequence stratigraphy of C and M factories

INTRODUCTION ➤ The depth window of production is much wider than
that of the T factory. Like the T and M factories, the C
The discussion of sequence stratigraphy started in chap- factory cannot produce above sea level; only clastic ac-
ter 6 by examinig the standard model and its observational cumulations of marine material may occur in the form
support based largely on siliciclastics. Chaper 7 introduced of eolian dunes. The lower limit of C-factory produc-
carbonate sequence stratigraphy with a detailed look at the tion usually is set by influx of terrigenous or plank-
T factory - the best known carbonate system and the most tic fine material. Below a certain depth, skeletal pro-
productive one. The present chapter deals with the sequence ductionsimply drownsinmud. However, oncurrent-
stratigraphy of the C and M factories. It does so by high- swept sea floors the C factory may produce even in the
lighting differences to the T factory. This is not to say, how- deep bathyal and abyssal environment.
ever, that the principles of marine carbonate production out- ➤ Lithification within the depositional environment usu-
lined in chapters 2 and 7 are no longer valid simply because ally is scarce or absent. The reason is the scarcity of
they are not discussed at length here. In the C and M facto- soluble aragonite in the sediment and the low carbon-
ries, just as in the T factory, sediment is largely produced ate saturation of the sea water. Consequently, sea-floor
by organic activity within the depositional setting. Envi- lithification distinctly increases in warm temperate en-
ronmental factors therefore strongly influence the sediment vironments where the C factory gradually passes into
type and the rate of production. Consequently, environmen- the T factory.
tal change is a major competitor of sea-level change in shap- ➤ The ability to build protective rims is weak. Reef-
ing the sequence record. building communities exist but they are either entirely
independent of light, or include only a small fraction of
phototrophic organisms, mostly red algae. As a conse-
CFACTORY quence, the reef comunities of the C factory rarely build
to sea level. A structure that rises slightly above the
Overview adjacent sediment on the shelf or upper slope will do.
As they remain below the zone of permanent wave ac-
The hallmark of C-factory sequences are coasts formed tion, the tops of the reefs are not planed off. The typical
by cliffs or sandy beaches, consistently seaward-dipping geometry is that of a convex mound. Internal lithifi-
shelves, and sigmoidal shelf breaks that bend down to rela- cation is scarce and the structures are easily reworked.
tively gentle slopes. Reefs, if present, have low-lying crests, It seems that the cool-water reefs are less long-lived
and are widely scattered over the outer parts of the shelf and than tropical reefs. Reef patches start and die more
the upper slope. The shelf breaks of the C factory gener- frequently. The result are swarms of reefs scattered in
ally lack wave-breaking rims. Therefore it is not surprising loose sediment.
that there are no detailed reports of empty buckets either.
With regard to sequence stratigraphy, one can say that the Sequence facies and bounding surface
deposits of the C-factory behave similarly to siliciclastics in
the marine environment and their response to exposure is The facies succession from coast to basin of the C factory
somewhere between that of siliciclastics and tropical carbon- has been presented in chapter 4. The facies belts typically
ates. develop on a ramp - a smooth surface that dips seaward
◦
These characteristics and their causes will be discussed in with anglesof lessthan1.5 . The ramp facies model ap-
more detail below. The following attributes of the C factory plies to shelves of continents and islands as well as epeiric
are crucial in a sequence-stratigraphic context. seas. This is advantageous because the C factory, too, may
➤ The output of the C factory consists almost exclusively develop in all these settings.
of skeletal particles ranging in size from coarse silt to On the shelves and upper slopes, standard sequence-
pebble. Mud, if present, stems from abrasion and bio- stratigraphic techniques can be applied but the differences
degradation of the coarse particles and from occasional to T carbonates described in chapter 7 remain significant.
input by the planktic factory of the open ocean. The scarcity of syndepositional cementation, together with

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