Page 162 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 8: SEQUENCE STRATIGRAPHY OF C AND M FACTORIES 153

rubble (Fig. 3.13), indicate that the flanks are part of the Calvet and Tucker (1995). The Triassic mounds turned into
stiff framework of the mounds rather than a talus cone that mini-platforms as they grew higher; simultaneously, the fa-
covers the framework. The toe-of-slope progrades as the cies changed from a framework of automicrite, sessile mi-
mound increases in height. The vast Devonian exposures crobes and cement to skeletal sands with coral patches. The
in the Sahara show that chains of mounds may coalesce to change to a wave-swept environment goes hand in hand
kilometer-long ridges (Fig. 8.7, Wendt et al. 1993; Wendt with increased lateral progradation, probably because of in-
and Kaufmann, 1998). Spectacular networks of mud-mound creased sediment supply to the slope.
chains were revealed by 3D seismic data of the Permo- Bounding surfaces of depositional units develop in simi-
Carboniferous in the Barents Sea (Fig. 8.8, 8.9; Elvebakk et lar fashion as in the T factory. Marine cementation is rapid
al. 2002). The buildups grew during 35 My, extend over and pervasive. Consequently, flooding events frequently
2
hundreds of km and reach thicknesses of 350 – 1,200 m. Ge- are accompanied by hardgrounds. This also applies to M
ometrically similar, albeit smaller, patterns develop in coral platforms where sedimentation fills all accommodation and
reefs of the T factory where isolated patch reefs have been ends in supratidal flats. Hard crusts were common on these
observed to expand and coalesce to networks with polygo- flats and are likely to turn into hardgrounds or layers of
nal depressions in between. lithoclasts during the subsequent transgression.
Convex mounds that turn flat during growth are of spe- Patterns and rates of karst formation on M factory carbon-
cial interest to the sequence stratigrapher (with the proviso ates have not been studied in detail. The fact that the frac-
that the flat surface is not the result of coincidental coa- tion of metastable carbonates is similar to the T factory sug-
lescence of several mounds). Flattend mounds are likely gests that the rate of karst formation is similar, too. Terres-
to have grown into the zone of wave action - waves, af- trial cementation rates may be lower because the aragonite
ter all, are the most effective planing tool in marine sedi- content of M carbonates seems to be somewhat lower (Fig.
mentation. For some mounds, growth into shoal-water can 2.15). From the Devonian M platform of the Canning Basin,
be demonstrated. Fig. 8.10 is an example, contributed by Playford (2002) reports karst with karren and caves that

1°15'E

26°06'N

N

2 km

N

1000 m
mud ridges
with mound nuclei
nodular wackestones (Givetian)

mud ridges
Early Devonian
siliciclastica +
Middle Devonian
carbonates
Fig. 8.7.— Two examples of mounds that coalesced to kilometer-long ridges in the Devonian of southern Algeria. Compiled from
Wendt et al. (1993) and Wendt and Kaufmann (1998).