140
organisms attached to the reef. Furthermore, skeletal
debris provided by physical breakup or bioerosion as
well as biomicrite mentioned above can fill cavities
within the reef and contribute considerably to the
buildup of the reef core (cf. Fig. 3.26b). Encrusting
organisms often grow over the dead parts of the reef
surface and aid in stabilizing the structure.
The upward growth of reefs terminates within the
tidal zone, where they form a flat surface eovered by
water during high tide. A certain amount of reef destruction seems to be necessary for continued reef
growth, because in this way new solid substrate is
provided for the colonization of sessil reef organisms. Reef talus and fine-grained debris delivered by
reef destruction are exported into both the backreef
zone and to the slope and deepcr parts of the forereef
zone where they form part of the reef complex.
Planation and partial destruetion of reefs is aecomplished
by several processes: (l) storm waves and tidal CUITents
eroding instable parts of the reef structure; (2) grazing and
rasping organisms such as fish, some molluscs, and sea
urgins (bioerosion; e.g. Stearly and Ekdale 1989) which
can cause severe damage to modem eoral reefs; (3) boring
endolithic micro- and macro-organisms (algae, fungi,
sponges, bivalves, ete.; Vogel 1993; Vogel et al. 1996). All
these types of reef destruetion can operate relatively fast
(downward erosion of 0.1 to 1 mika) and even reach the
same order of magnitude as reef growth. Reef destruction,
especially by mechanical processes, is most active when
the reefhas grown up to sea level and consumed the available vertical sediment accommodation space (cf. Sect. 7.4).
ReefTypes
Despite the great variability of the reef-building communities in space and time and the complex internal
struetures of reefs, most of the reef bodies can be
simply c1assified according to their external geometry. They range from flat and small to usually thiek
and large structures:
Flat muddy banks, one to several km lang, rise
some decimeters or meters above the surrounding sea
floor. They are partially built by sedentary organisms
such as algae, sca grass, branching corals, molluscs
and many other groups. Tropical banks, for example
on the Florida shelf, are characterized by thick and
rapidly growing shells, but skeletal-rich banks also
occur in non-tropical waters (foramol association).
The latter typically occupy the deeper shelf and the
tops of isolated plateaus at depths of hundred to several hundreds of meters. Ahermatypic corals may be
present and form patches rising somewhat over the
neighboring sea floor. Cool-water banks often contain glauconite and are generally more sandy and
gravelly (inc1uding coarse shell debris and fragments
of calcareous algae) than tropical counterparts. CarChapter 3 Coastal and Shallow Sea Sediments
bonate banks in both shallow and deeper water can
be stabilized by early cementation generating
erosion-resistant tops or hardgrounds, especially under warm-water conditions. Continuous upbuilding
of carbonate banks up to a certain distance from the
coast can generate an aggraded platform in contrast
to reef-rimmed platforrns (Hunt and Tucker 1993, cf.
Fig.3.27).
Tbe bank-type reefs of the present-day oceans are considered to be counterparts of the frequently described
biostromes in ancient sediments (Fig. 3.25a). By definition,
these are bedded structures, such as shell beds or coral-rich
beds exhibiting part of their fauna in situ, but they did not
grow fast enough in their central part to form lens-like or
mound-like bodies as was the case for bioherms.
Superposition of biostromes or small bioherms can result
in thick reef structures ("stratigraphie" or stratified reefs),
but these never developed a substantial primary relief. In
contrast, "ecologic" reefs form prominent topographie,
wave-resistant features such as baITier reefs, patch reefs,
pinnacle reefs (Fig. 3.25a and d).
Reef mounds and mud mounds are flat, long lenses
or steeper conical piles of incomplete reef structures
(James 1983). They vary in size, but often have
lengths on the order of 100 m. They occur on the
gently dipping slopes of carbonate platforrns, in tranquil reef lagoons, shelf seas, and occasionally in
deeper water (Figs. 3.25a and 3.26a). Mud mounds
consist of poorly sorted bioc1astic lime mud with minor amounts of delicate to dendroid skeletons of
sessil organisms growing on top of the muddy matrix
(e.g., sponges, algae, bryozoans, small branching
corals, bivalves). Larger skeletons of typical framebuilders are generally not found. For that reason, reef
mounds cannot grow up to the zone of strong and
constant wave turbulence.
Mud mounds have been recently studied in great detail
(e.g. James and Bourque 1992; Monty et al 1995; Reitner
and Neuweiler 1995). Tbey mainly represent slowly growing microbe-sponge associations which are known since
the early Cambrian. The presence of biofilms allows metazoans to colonize and contribute to carbonate buildup. The
cap of mud mounds may be formed by a thin layer of encrusting organisms or an early lithified limestone crust
(Fig. 3.25a). Very large mud mounds and mud ridges (up
to 40 m and 100 m high, respective1y) have been described
from the Middle Devonian of the Algerian Sahara (Wendt
et al. 1997). They formed on a carbonate shelf at water
depths probably ranging from 100 to 200 m. Frame builders are scarce in these structures.
Knoll reefs and patch reefs are frame-built, isolated,
roughly circular bioherrns (Fig. 3.25a). They grow on
the upper foreslope of medium-energy shelf or platform margins, where they often form a reef barrier or
a gently seaward dipping ramp in combination with
forereef or interreef debris. Patch reefs usually grow
in the shallow outer part of a protected reef lagoon
organisms attached to the reef. Furthermore, skeletal
debris provided by physical breakup or bioerosion as
well as biomicrite mentioned above can fill cavities
within the reef and contribute considerably to the
buildup of the reef core (cf. Fig. 3.26b). Encrusting
organisms often grow over the dead parts of the reef
surface and aid in stabilizing the structure.
The upward growth of reefs terminates within the
tidal zone, where they form a flat surface eovered by
water during high tide. A certain amount of reef destruction seems to be necessary for continued reef
growth, because in this way new solid substrate is
provided for the colonization of sessil reef organisms. Reef talus and fine-grained debris delivered by
reef destruction are exported into both the backreef
zone and to the slope and deepcr parts of the forereef
zone where they form part of the reef complex.
Planation and partial destruetion of reefs is aecomplished
by several processes: (l) storm waves and tidal CUITents
eroding instable parts of the reef structure; (2) grazing and
rasping organisms such as fish, some molluscs, and sea
urgins (bioerosion; e.g. Stearly and Ekdale 1989) which
can cause severe damage to modem eoral reefs; (3) boring
endolithic micro- and macro-organisms (algae, fungi,
sponges, bivalves, ete.; Vogel 1993; Vogel et al. 1996). All
these types of reef destruetion can operate relatively fast
(downward erosion of 0.1 to 1 mika) and even reach the
same order of magnitude as reef growth. Reef destruction,
especially by mechanical processes, is most active when
the reefhas grown up to sea level and consumed the available vertical sediment accommodation space (cf. Sect. 7.4).
ReefTypes
Despite the great variability of the reef-building communities in space and time and the complex internal
struetures of reefs, most of the reef bodies can be
simply c1assified according to their external geometry. They range from flat and small to usually thiek
and large structures:
Flat muddy banks, one to several km lang, rise
some decimeters or meters above the surrounding sea
floor. They are partially built by sedentary organisms
such as algae, sca grass, branching corals, molluscs
and many other groups. Tropical banks, for example
on the Florida shelf, are characterized by thick and
rapidly growing shells, but skeletal-rich banks also
occur in non-tropical waters (foramol association).
The latter typically occupy the deeper shelf and the
tops of isolated plateaus at depths of hundred to several hundreds of meters. Ahermatypic corals may be
present and form patches rising somewhat over the
neighboring sea floor. Cool-water banks often contain glauconite and are generally more sandy and
gravelly (inc1uding coarse shell debris and fragments
of calcareous algae) than tropical counterparts. CarChapter 3 Coastal and Shallow Sea Sediments
bonate banks in both shallow and deeper water can
be stabilized by early cementation generating
erosion-resistant tops or hardgrounds, especially under warm-water conditions. Continuous upbuilding
of carbonate banks up to a certain distance from the
coast can generate an aggraded platform in contrast
to reef-rimmed platforrns (Hunt and Tucker 1993, cf.
Fig.3.27).
Tbe bank-type reefs of the present-day oceans are considered to be counterparts of the frequently described
biostromes in ancient sediments (Fig. 3.25a). By definition,
these are bedded structures, such as shell beds or coral-rich
beds exhibiting part of their fauna in situ, but they did not
grow fast enough in their central part to form lens-like or
mound-like bodies as was the case for bioherms.
Superposition of biostromes or small bioherms can result
in thick reef structures ("stratigraphie" or stratified reefs),
but these never developed a substantial primary relief. In
contrast, "ecologic" reefs form prominent topographie,
wave-resistant features such as baITier reefs, patch reefs,
pinnacle reefs (Fig. 3.25a and d).
Reef mounds and mud mounds are flat, long lenses
or steeper conical piles of incomplete reef structures
(James 1983). They vary in size, but often have
lengths on the order of 100 m. They occur on the
gently dipping slopes of carbonate platforrns, in tranquil reef lagoons, shelf seas, and occasionally in
deeper water (Figs. 3.25a and 3.26a). Mud mounds
consist of poorly sorted bioc1astic lime mud with minor amounts of delicate to dendroid skeletons of
sessil organisms growing on top of the muddy matrix
(e.g., sponges, algae, bryozoans, small branching
corals, bivalves). Larger skeletons of typical framebuilders are generally not found. For that reason, reef
mounds cannot grow up to the zone of strong and
constant wave turbulence.
Mud mounds have been recently studied in great detail
(e.g. James and Bourque 1992; Monty et al 1995; Reitner
and Neuweiler 1995). Tbey mainly represent slowly growing microbe-sponge associations which are known since
the early Cambrian. The presence of biofilms allows metazoans to colonize and contribute to carbonate buildup. The
cap of mud mounds may be formed by a thin layer of encrusting organisms or an early lithified limestone crust
(Fig. 3.25a). Very large mud mounds and mud ridges (up
to 40 m and 100 m high, respective1y) have been described
from the Middle Devonian of the Algerian Sahara (Wendt
et al. 1997). They formed on a carbonate shelf at water
depths probably ranging from 100 to 200 m. Frame builders are scarce in these structures.
Knoll reefs and patch reefs are frame-built, isolated,
roughly circular bioherrns (Fig. 3.25a). They grow on
the upper foreslope of medium-energy shelf or platform margins, where they often form a reef barrier or
a gently seaward dipping ramp in combination with
forereef or interreef debris. Patch reefs usually grow
in the shallow outer part of a protected reef lagoon