Page 59 - Carbonate Sedimentology and Sequence Stratigraphy

50 WOLFGANG SCHLAGER

Because of the higher growth potential of the rim, “empty cumulations of muddy sediment is the abundant evidence
buckets”, i.e. raised rims and empty lagoons are commonly for syndepositional lithification of the structure. This evi-
found in the geologic record. Retrogradation of rimmed dence consists primarily of microborings, cracks and large
platforms usually proceeds in discrete steps because once cavities filled by coeval sediment and clasts of automi-
the rim is drowned much of the adjacent lagoon is also lost. crite. Additional arguments for pervasive lithification of the
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Fig. 3.5 illustrates possibilities for backstepping platforms to mound edifice are the steep dip (commonly 40-50 )of the
establish a new “line of defense”. micritic flank deposits, meter-size, angular boulders of au-
Platform morphologies are common with the T-factory tomicrite facies at the toe-of-slope (Lees and Miller, 1995)
but they are not diagnostic. If the T factory is weak, the M and fractures filled by coeval sediment in automicrite de-
factory may occupy the niche and build platforms with flat posits. The debris aprons of mud mounds are small. In
tops (and rims) at sea level (Fig. 3.22). most instances, the calcareous flank deposits of automicrite
mounds extend only meters to few hundreds of meters into
Cfactory. Modern cool-water carbonates essentially lack the
the flat-lying basin deposits surrounding the mound (e.g.
ability to build rims. Consequently, their characteristic de-
Lees and Miller, 1995).
positional geometry is that of a ramp (Fig. 3.1). Many cool-
In settings with high external sediment input, the mud-
water accumulations lie in zones of strong winds and high
wave energy (such as the “hailing fourties” of the south- mound factory may be unable to create mounds. One such
ern hemisphere, Fig. 3.20). Consequently, accumulation in situation are steep platform slopes where automicrite layers
the shallow part of the ramp may be low and hardgrounds have been found to alternate with layers of platform debris.
abundant (see chapter 4). The mud-mound factory betrays its presence by large boul-
ders of automicrite boundstone that accumulate at the toe-
Mfactory. The favorite setting for accumulations of the mud- of-slope. (e.g. Russo et al., 1997).
mound factory are deeper-water environments in the ther- The M factory has no monopoly on mound geometries
mocline with low lateral influx of sediment. When left to even though they are very common in this factory. Algal
its own dynamics in this setting, the M factory will produce mounds of the T factory (e.g. Roberts et al., 1988) and bry-
mounds, i.e. circular or elliptical, upward-convex accumu- ozoan mounds of the C factory (e.g. James et al., 2000) illus-
lations that rise above the adjacent sea floor (Figs 3.4, 3.21). trate that the mound geometry is not diagnostic of a factory.
What sets these mounds apart from purely mechanical ac-

Fig. 3.22.— Sella Mountain in the Southern Alps, Italy, a carbonate platform created by the M factory. Top was built to sea level
as documented by abundant supratidal deposits. Slope consists of a 1:1 mix of automicrite and cement in situ, and reworked
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sand and rubble. Note planar slope clinoforms built to over 35 , the approximate angle of repose of this material. Interpretation
based on Keim and Schlager (2001), photo by J.A.M. Kenter.