Geology Reference
In-Depth Information
Case studies in the northern Atlantic underline the
great importance of subtidal kelp forests, which harbor
a great number of carbonate-secreting and sediment-
producing organisms. These forests occur along the
wave-exposed flanks of rocky coastal platforms. In con-
trast, pure coralline red algal gravel deposits ('maerl')
develop in more protected parts of the platforms.
'Maerl'
is a Breton term, and refers to unattached
crust-forming branched or nodular coralline algae and
algal gravel, including their widely distributed depos-
its (Pl. 4/2). Maerl is one of the most abundant shal-
low-water carbonates of cool-temperate environments.
Modern maerl beds are found throughout the Mediter-
ranean Sea, and are common on terrigenous-free At-
lantic coasts in depths < 20-30 m, from Norway and
Denmark in the north to Portugal in the south, extend-
ing to Morocco and Mauretania on the African coast.
Maerl deposits have been studied in detail in Brittany,
northwestern France (Henrich and Freiwald 1995,
Bader 2001), British Islands (Farnham and Jephson
1977), western Ireland (Bosence 1985) and in the Medi-
terranean Sea.
The French term
'coralligène'
refers to recent cor-
alline algal buildups of the Mediterranean (Bosence
1985). These carbonates occur in intertidal and sublit-
toral rocky coasts in water depths between about 20
and 160 m; they are represented by reefs formed by
corallinacean frameworks. The coralligène can be con-
sidered to be a good recent analog for some Miocene
coralline limestones of the Tethyan region that exhibit
a very similar generic composition and similar growth
forms of red algae.
Skeletal grains:
Common bioclasts are bryozoans (Pl. 4/5), foramin-
ifera, molluscs, echinoids and calcareous red algae.
Other organisms include barnacles, serpulids, brachio-
pods, ahermatypic corals and sponges. The main groups
occur in various proportions, depending on the water
temperature, salinity, nutrients and grain size of the sedi-
ment. Compositional differences allow 'skeletal grain
assemblages' characterized by the dominance of par-
ticular grain types to be defined (e.g. bryozoans and
mollusks = bryomol association occurring on deep shelf
banks of the northern Atlantic; Pl. 4/4), which are in-
dicative of special environmental conditions and par-
ticular parts of the shelf (Sect. 12.2.2.1). Non-skeletal
grains (peloids, intraclasts) are rare, ooids are absent.
Cool-water reefs: Diverse in composition and distri-
bution:
Lower
slope deep-water reefs ('lithoherms'):
Neu-
mann et al. (1977) described elongate coral bioherms
in water depths of several hundred meters, that are axi-
ally aligned with the flow field of the Florida current.
The upcurrent end of the mounds consists of dense
thickets of branching ahermatypic corals; the middle
and downcurrent sections are rocky, and strongly bio-
eroded with attached sponges, alcyonarians, gorgon-
ians, and crinoids. Sediments from this zone are a mix-
ture of planktonic foraminifers and pteropods, with skel-
etal grains from lithoherm organisms. Large amounts
of aragonite mud are trapped in the coral thicket at the
upcurrent ends of the mounds or in sponge-boring cavi-
ties within the lithoherms. The description of these
Table 2.3.
Generalized comparison of modern warm-water shallow-marine reefs and cold/cool-water deeper-marine coral
reefs.
Warm and shallow water reefs
Cold and deeper water reefs
Sea water temperature range
20 - 28 °C
4 - 12 °C
Bathymetrical range
-50 m
250 m to >1 000 m
Light
Photic zone
Aphotic to dysphotic zones
Zonation patterns
Windward-leeward
Current zonation
Light zonation
Taphonomic zonation
Nutrional regime
Oligotrophic
Eutrophic
Trophic modes
Photo- and heterotrophy
Heterotrophy
Diversity of coral framework builders
High
Low
Diversity of associated fauna
High
High
Diversity of associated flora
High
Non existent
Biological framework stabilization
Coralline Algae
Encrusting sponges
Bioerosional impact
High
High
Diagenetic impact
High
Low
Interskeletal cementation
Very common
Rare
Intraskeletal cementation
Very common
Low to common
Internal sediment
Very common
Very common
Automicrite
Common
Very common
