Key features of mixed carbonate-siliciclastic shallow-marine systems ...

Fig. 6 - Coralline algal frameworks
and associated deposits: A) detail of
the framework of Facies B1 near the
Col 3 section, showing calcareous
algae, bryozoans and minor calcarenite
of Facies B2; B) detail of the
algal laminae of Facies B1 (Col 4
section); C) sharp contact between
coralline algal frameworks (Facies
B1) and the calcarenites of Facies
B2 at the Col 4 section; D) sharp lateral
contact between stratified calcarenites
(Facies B2) and coralline
algal frameworks (Facies B1) at the
Col 5 section.
KEY FEATURES OF MIXED CARBONATE-SILICICLASTIC SHALLOW-MARINE SYSTEMS 375
coralline algal fragments. A siliciclastic component, consisting
of very coarse-grained sandstone to granule-size
micro-conglomerate, is present with variable amount.
Cavities affecting Facies B1 have commonly a vertical
development, reaching up to 2.5 m of height (the Col 4
section, fig. 3); their fill is typically structureless, and the
bioturbation is common (fig. 7). Calcarenites and calcirudites
laterally juxtaposed to the patches of Facies B1 have
a thickness similar to that of the algal frameworks themselves
(figs. 3 and 6D), whereas their lateral extent is
more difficult to evaluate, and is estimated in the order of
tens of metres. The contact with Facies B1 may be vertical,
and is very sharp and locally clearly erosional (figs. 3
and 6D). These deposits show a well developed flat to
inclined and undulate lamination and locally trough
cross-stratification (figs. 3 and 6D). Internal, irregular
erosional surfaces truncating the underlying laminae are
very common. Burrow traces are present.
Interpretation of facies association B
This facies association records optimum conditions for
the carbonate factory, that consisted in the growth of
coralline algal frameworks (Facies B1) upon the rigid substrate
provided by the skeletal accumulations of facies
association A. The optimal depth for coralline algal framework
growth in the present day Mediterranean ranges
between 30 and 60 m (LABOREL, 1961; PÉRÈS & PICARD,
1964; BOSENCE, 1983). The thinning of Facies B1 both in
seaward and landward directions, as well as its patchy di -
stribution in relatively proximal settings, are inferred to be
due to environmental conditions away from the optimum
for coralline algal growth. The mollusc shells, the echinoids
and the coarse siliciclastic fraction recognized
within Facies B1 are inferred to have deposited after major
Fig. 7 - Calcarenites of Facies B2 filling a cavity within the coralline
algal framework of Facies B1 at the Col 4 section.