Page 72 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 4: CARBONATE FACIES MODELS 63

FACIES BELTS OF THE C FACTORY Rimmed M platforms (Fig. 4.11B) develop where the T
factory is hampered by environmental or evolutionary fac-
The facies patterns of the C factory essentially follow the tors; examples are the Famennian of the Canning Basin and
ramp model as shown in Fig. 4.10. However, ramps of cool- some Triassic platforms in the Alps (Playford, 1980; Play-
water carbonates differ from their tropical counterparts in ford et al., 2001; Russo et al., 1997; Keim and Schlager,
several ways: 2001). Where studied in detail, the rim consists of au-
➤ Ramps of tropical carbonates have a strong tendency tomicrite (thrombolite, stromatolitic crusts) and fibrous ma-
to evolve to rimmed platforms; in cool-water carbon- rine cements. Skeletal framebuilders, such as stromato-
ates, the ramp is a stable configuration also during cli- poroids, sponges or serpulid worms occur in highly vari-
max conditions. Differences in rim building and in the able amounts and indicate the gradual boundary to the T
depth window of production are the reason for the dif- factory. The platform interior of the M factory shows fa-
ference. In the T factory, maximum production is in the cies belts 7, 8 and 9 based on sedimentary structures and
uppermost water column and the factory always has textures. Determining restriction is difficult because of the
some rim-building capability. Together, these attributes low content of skeletal grains. However, extreme salinities
quickly drive the system to form a rimmed platform can be recognized by the presence of evaporites. Slopes of
whose slope steepens as it progrades. In the C factory, the M platforms still are part of the factory such that layers
high production is maintained over a wide depth range of automicrite alternate with layers of mud and deposits of
and the ability to build wave-resistant rims is weak or sediment-gravity flows (Blendinger et al., 1997; Keim and
absent. Thus, the system’s equilibrium profile resem- Schlager, 1999). Megabreccias are common. Their clasts, of-
bles that of a siliciclastic shelf. tenover 10min diameter, stemfromthe platform rimor the
➤ Mud content on cool-water ramps is significantly lower automicrite layers of the slope. In contrast to the the spec-
than in tropical ramps. There is less primary production tacular breccias, turbidite aprons in the basin usually are in-
of mud in the C factory and mud produced by abrasion conspicuous. It seems that the M platforms export little fine
is easily moved to greater depth because of the high material because most micrite is already hard upon forma-
turbulence. The coastal zone of cool-water ramps often tion and skeletal detritus is scarce.
consists of sandy beaches, flanked by calcareous eolian
dunes. CARBONATE FACIES OF EPEIRIC SEAS
➤ Wave abrasion on Neogene cool-water shelves of the
southern hemisphere may become so intensive that Epeiric seas are shallow seas on continents. I consider
parts of the middle part of the shelf produce sediment the term epeiric sea synonymous to epicontinental sea.
but cannnot accumulate any of it (Fig. 4.10b; James et These epicontinental seas differ clearly from the pericon-
al., 2001). It is not clear if this pattern was common in tinental shelf seas overlying continental margins (Leeder,
the past. 1999). Process-oriented carbonate sedimentology emerged
mainly from the study of two recent systems: pericontinen-
tal shelves and detached carbonate platforms in the mid-
FACIES BELTS OF THE M FACTORY dle of the ocean, usually on volcanic basement. Carbonate
deposits of epeiric seas are common in the geologic record
In chapter 3 we have seen that the most common mor- but scarce at present. Moreover, recent epeiric seas are
phology of the M factory is a ramp studded with mounds. small compared to some of their ancient counterparts. Irwin
However, rimmed platforms may develop if competition by (1965) summarized the situation as follows: “The present is
the T factory is subdued. In both instances, one needs to ad- the key to the past’ may be a misleading statement when
just the respective facies models developed for the T factory. considering epeiric sedimentation. We simply have no ex-
On ramps (Fig. 4.11A), sediment production in the form isting models of epeiric seas to guide our investigations ...
of peloids, micritic oncoids and layers of automicrite is ”. Carbonate researchers have echoed these concerns (e.g.
widespread. Locally favorable conditions lead to growth of Aigner, 1985; Wright and Burchette, 1996). The question is
mud-mounds. The mounds may occur isolated or in clus- both legitimate and important. Epeiric sediments are im-
ters and belts parallel to the strike of the slope. Clusters portant because they are more likely to be preserved than
may coalesce to a network of ridges (see chapter 8). The deposits of continental marginsor ocean interiors. Thus, in
sunlit and wave-agitated part of the ramp usually is dom- most instances epeiric deposits are our principal resource for
inated by skeletal carbonate. The mounded belt typically reconstructing the conditions of the distant past. The strong
lies in the thermocline below the oceanic mixed layer, in the regional differentiation of epeiric seas (e.g. Simo et al., 2003)
thermocline where strong density gradients impede verti- puts serious limitations on attempts to reconstruct the con-
cal mixing, oxygen is scarce and nutrient concentration high ditions in the open oceanic domains.
(Fig. 1.1 and Fig. 2.18; Stanton et al., 2000; Neuweiler et al., Personally, I believe that actualism should be used with
2003). There is circumstantial evidence that upward growth discretion but remains a powerful concept, also for inter-
of mounds has been governed by the limits of the oxygen- preting the record of ancient epeiric seas. To start with,
minimum zone (Stanton et al., 2000). carbonate epeiric seas do exist at present and can be used