Page 50 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 3: GEOMETRY OF CARBONATE ACCUMULATIONS 41

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the mound and sometime dip at over 40 (Lees and Miller,
A) B) 1995). In contrast to reefs, the top of the mound normally
flank facies reduced or absent flank facies important does not prograde; only the base of slope progrades to the
degree required by the increase in height of the mound.
The convex top of the mounds probably reflects their for-
mation in deeper water below the “shaving” effect of wave
action. Where mounds build into the zone of intensive wave
action, their top flattens and their facies changes. Con-
versely, the geometry of slowly drowning reefs may grad-
ually change from flat platform to mound as they subside
below wave action (see also p. 122f; Neuhaus et al., 2004;
Zampetti et al., 2004).

Carbonate platform. Carbonate platform is a widely used and
aggregates rather loose term. The dictionary definition precisely cap-
C) D)
tures the essence of the usage of this term in carbonate sed-
imentology: “a horizontal flat surface usually higher than
the adjoining area” (Webster’s Dictionary). Shoal-water car-
bonates form flatter tops than other depositional systems
because (1) the high-production zone is abruptly limited by
E) sea level, (2) waves and currents efficiently redistribute sedi-
ment from high-production areas to fill up depressions, and
(3) the platform margin tends to develop a wave-resistant
rim that protects the sediment of the platform interior. The
term carbonate platform is broadly synonymous to “carbon-
mainly in-situ
off-bank flank ate shelf” (Wilson, 1975). I think platform is more appropri-
construction
ate because “shelf” has the connotation of being attached to
land. Platform is neutral in this regard.
Fig. 3.4.— The M factory typically produces upward-convex ac- Shoalwater carbonate accumulations that lack a rim will
cumulations, the mounds, that interfinger laterally with the sur- strive towards a profile of equilibrium between the sedi-
rounding facies. Flank facies are present when the factory sheds ment and the wave energy in the overlying water column.
much debris. However, the km-wide debris aprons of tropical In these instances, the carbonate-specific morphology is lost
platforms are rare. Based on Lees and Miller (1995), Keim and and the geometric discrimination of carbonates and silici-
Schlager (2001) and Schlager (2003).
clastics may become impossible.
Size is not a rigid criterion for defining carbonate plat-
forms. However, the term is usually applied to features
where energy flux in the water column frequently changes that are kilometers to hundreds of kilometers across and rise
direction dependent on weather conditions. Patch reefs in many tens of meters to thousands of meters above the adja-
lagoons or epeiric seas commonly display this geometry. cent basin floor.
Carbonate platforms may be attached to a land mass or
Mound. James and Bourque (1992) divide organic carbon- detached, isolated features that are surrounded by deeper
ate buildups into reefs and mounds. In this classification, water on all sides.
“mound” denotes a rounded, hill-like, submarine structure Platforms may prograde, aggrade vertically, or retro-
composed of skeletal material or automicrite of predomi- grade. A geometry characteristic of carbonate platforms is
nantly microbial origin. Mounded accumulations of skeletal “backstepping” (Fig. 3.5). It implies that the platform retro-
material can be interpreted as hydrodynamic structures just grades in discrete steps such that discrete margin positions
like any other detrital accumulations. Siliciclastic and car- are recognizable and separated by flat seafloor. Backstep-
bonate sediment drifts in the deep sea indicate that muddy ping is common with rimmed platforms. This seems para-
sediments may still exhibit bedding structures related to doxical at first because rims normally grow faster than plat-
bedload transport. However, the vast majority of the car- form interiors (p. 25). Thus, if the rim goes under, the entire
bonate mud mounds of the sedimentologic literature do not platform should be lost. The main advantage of backstep-
fall in this category and merit special attention. ping is the reduced power of destructive waves in a back-
Carbonate mud mounds are isometric or elongate sub- stepped position. Waves that reach a backstepped margin
marine hills. Their top is normally convex and lacks the have lost part of their energy to bottom friction as they trav-
horizontal surface of carbonate platforms or of reefs built elled over shallow ground. Other reasons for backstepping
to sea level. Like reefs, mud mounds commontly show a are elevated topography or, on large passive margins, shift
core flanked by debris aprons that steepen with the height of to areas of slower subsidence.