Page 66 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 4: CARBONATE FACIES MODELS 57

rived from the siliciclastic beach-shelf model. All one needs FACIES BELTS OF THE T FACTORY
to do is insert a wave-resistant rim (composed of reefs or
partly lithified sand shoals) and adjust the facies for the ef-
The succession of facies on rimmed tropical platforms has
fects of this rim.
The morphologic transition from ramp to rimmed plat- been cast into a standard facies model by Wilson (1975). The
model, based on two decades of case studies by numerous
form is characterized by a gradual increase in slope and
researchers, has passed the test of time. It has become a
an accentuation of the platform margin. It has been ob-
widely accepted framework for presentation of carbonate fa-
served in seismic data (e.g. Sheriff, 1988, Fig. 1; Harris and
cies (e.g. Tucker and Wrigth, 1990; Wright and Burchette,
Saller, 1999,p. 40), in correlated boreholes (e.g. Burchette
1996; Flügel, 2004) and is reproduced in Fig.4.3. Despite
and Wright, 1992, Fig. 10) and in outcrops (e.g. Stanton and
Flügel, 1989; Kerans and Tinker, 1999). The observational the remarkable success of the model, some modifications are
data do not constrain the transition in all details but some called for. They are shown in Fig. 4.4 and discussed below.
general trends emerge. The standard model is “overcomplete”. It contains more
It seems that the formation of a rim normally starts at the facies belts than one normally finds on any one platform.
inner ramp in one of two ways: (1) growth of patch reefs that Platforms with a reduced number of facies belts may be per-
form in the inner ramp seaward of the nearshore sand and fectly normal and healthy. In the clinoform and fondoform
subsequently coalesce to a shore-parallel belt; (2) the high- settings, facies 2 (deep shelf) will only be present in addi-
energy shoal itself becomes an quasi-stationary barrier by tion to facies 1 if the platform has recently backstepped or
high sediment production and syndepositional lithification. has been structurally deformed. If a deep shelf exists, the
Both reefs and sand shoals are likely to produce more sedi- structure of the lithospere virtually dictates that it be con-
ment than they can store, they therefore fill the lagoon and nected with the deep basin floor of facies 1 by a slope. In
then prograde seaward. As the crest remains a sea level but epeiric seas, the facies succession may terminate with facies
the ramp surface dips basinward, a slope gradually forms 2. In the undaform setting, the rim need not consist of facies
and its declivity increases with increasing height (Fig. 3.8). 5 plus 6. Many healthy platforms have either facies 5 (reefs)
When should we call this prograding system a rimmed plat- or facies 6 (sand shoals) as a rim.
form? If the geometry is well preserved and slope angles In one instance, the Wilson model lacks a facies belt. Fa-
well measurable, such as in many seismic data, I recom- cies 9 of Wilson (1975) was defined for arid settings only.
◦
mend drawing the boundary at approximately 1.5 .Sys- However, the equivalent deposits in humid climates have
tems with a barrier belt and a foreslope distinctly greater been described and are added here as facies "9-humid". It
◦
than 1.5 should be called rimmed platforms (see chapter should be noted that facies 9-arid and 9-humid are alterna-
3). If slope angles are not available, facies may serve as a tives that will not occur side by side in one shore-to-basin
proxy for morphology. On rimmed platforms, the rim belt succession.
should be reasonably continuous (rim index of ≥ 0.25) and Another characteristic of the Wilson model is that it uses a
its seaward slope should show evidence of slumping or of discrete horizontal scale with sharp boundaries between fa-
bypassing by sediment gravity flows, as indicated by gullies cies. In nature, these facies boundaries may be gradational
or canyons, increase in thickness and abundance of debrites and irregular. For instance, the subdivision of slopes in fa-
and turbidites at the toe of slope etc. cies 3and 4is often impossible and acombinedbelt 3/ 4 may
In the climax stage of platform evolution, the wave- be more appropriate. The boundary between facies 7 and 8
resistant rim sits at the platform edge, directly atop the is oftenverygradual. Inthese instancesitispreferable to
slope. This position is rather stable because of the dynam- designate a combined facies belt 7/ 8 and express increasing
ics of platform growth: a rim that originally forms in a more restriction by biotic indices or a larger number of subfacies.
bankward position will rapidly prograde if it produces more The standard model says nothing about windward-lee-
sediment than is needed to match relative sea-level rise; ward differentiation. Most platforms develop asymmetries
such excess production is likely considering the high pro- in response to dominant wind directions (Fig. 4.9 and chap-
ductivity of rims. At the platform edge, the rate of progra- ter 7). Seismic surveys reveal these asymmetries better than
dation slows down because the high slope requires much most other techniques.
larger sediment volumes for the same amount of prograda-
tion (Fig. 3.9). Thus, further progradation will be slower but
Discription of the standard facies belts
the slope will tend to steepen to the angle of repose because
of the high sediment supply from the productive rim. On 1A) Deep Sea. Setting: Below wave base and below
the platform side, the rim sheds excess sediment ranging in euphotic zone; part of deep sea, i.e. reaching through the
size from clay to boulder. Beyond the reach of rim debris, thermocline into the realm of oceanic deep water. Sedi-
an open lagoon may develop with a bottom profile in equi- ments: Entire suite of deep-sea sediments such as pelagic
librium with wave action. However, at the climax stage of clay, siliceous and carbonate ooze, hemipelagic muds
platform evolution, this lagoon is largely filled and replaced including turbidites; adjacent to platforms we find mixtures
by tidal flats that expand seaward almost to the platform of pelagic and platform-derived materials in the form
rim. of peri-platform oozes and muds. Biota: Predominantly