138
(e.g. Rao 1996). F oraminifera, diatoms, sponge debris, and bryozoans tend to be more abundant in the
sand fraction than in coarser material.
3.4.4 Carbonate Ramps
A carbonate ramp is a gently sloping sea floor (generally less than 1 to 2 degrees) without a marked
break in slope. It therefore deviates from most
present-day shelves, which are characterized by a
distinct shelf edge.
See, e.g., Wilson and Jordan (1983), Read (1985), Wright
(1986). Modem examples of ramp settings are the shallow
region of the southern ArabianIPersian Gulf (Purser 1973;
cf. Sect. 4.4.3), the deeper shelf and upper slope of West
Florida (Mullins et al. 1988), the Libyan coastal zone of
the Mediterranean, and the shelves of Yucatan and southern Australia.
On a carbonate ramp, the high-energy facies of the
wave-dominated nearshore zone gradually pass
downslope into deeper water and finally into basinal
sediments deposited under low-energy conditions
(Fig. 3.24). Landward, ramps are characterized by a
high-energy grainstone belt, whereas rimmed carbonate shelves exhibit such a zone on their basinward
edge. Continuous, large reef structures and sediment
gravity flow deposits containing clasts of cemented,
shallow-water facies at the foot of the gentle ramp
slope are generally absent.
The nearshore zone of shallow, high-energy gravel
and sands or skeletal shoal complexes separates a
protected landward area with lagoons, coastal and
peritidal clastics, and possibly tidal flats from an
open marine, high-energy foreshore and shallow-water belt (Fig. 3.24a). The back-barrier area is dominated by low-diversity euryhaline faunal assemblages
which tolerate a wide range of salinities. The inner
ramp, seaward of the beach and baITier facies, is an
efficient "carbonate factory", providing carbonate for
both the back-baITier region and the outer, deeper
ramp.
In addition to biogenic carbonate production, the fonnation
of oolitic and peloidal sands may contribute significantly to
the overall carbonate production in wann waters.
Hardgrounds appear to be a common feature on inner
ramps. Nonnal wave action and frequent storms transport
biogenic sands and larger skeletal particJes (caJcarenites
and caJcrudites) along shore and as traction carpets into
somewhat deeper water. Rare large stonns produce sandand silt-sized biocJastic tempestites (Sect. 3.1.2) which
extend into the deeper ramp and basin, where they alternate
with lime muds or marls. Here, pelagic sediments, such as
planktonic foraminiferal and nannofossil oozes (cf. Sect.
5.3.1) dominate, but some isolated carbonate buildups may
also occur on the outer ramp. The ichnofacies shows a distinct trend from the inner to the outer ramp.
Chapter 3 Coastal and Shallow Sea Sediments
It appears that carbonate ramps preferentially fonned in
geologic periods or environments in which reef builders
secreting large, rigid skeletons were rare. The organic assemblages of ramps function mainly as grain and mud producers, as weil as trappers and binders to form carbonate
banks. Ramps arr. possibly more common in temperate climatic zones (e.g. the modem shelf of southem Australia;
e.g. Boreen and James 1993) than in wanner regions.
Carbonate ramps are common on passive (extensional) continental margins and in epicon-tinental
seas. On continental margins, carbonates frequently
overlie volcanics, evaporites, and clastic sediments,
representing an early phase of deposition in a rift basin or a young ocean basin (cf. Sect. 12.1). Ramps
tend to evolve into rimmed carbonate shelves (Fig.
3 .24b) when carbonate production on the developing
shelf edge is sufficiently high, and the production
and deposition of carbonate in deeper water remains
limited. The buildup of a rim with a steepening slope
can be promoted by mud mounds which contain only
minor proportions of biogenic framework (see below). Such structures may be confused with normal
reefs where frame-building organisms play a great
part (Flügel 1989). The transformation of a rimmed
shelfinto a ramp is less common (Fig. 3.24c); it may
occur, for example, where the shelf is drowned and
buried by prograding siliciclastics, or when the basin
is filled rapidly from another side.
3.4.5 Reef Types
and Composite Carbonate Buildups
General Aspects
At first glance, organic reefs appear to be relatively
simple structures which rise above sea floor as a result of high skeletal calcium carbonate production,
low siliciclastic input, and stabilization against the
attack of waves and currents. In reality, however,
reefs and their associated sediments represent very
complex structures (e.g. Hüssner 1994). In contrast
to other sedimentary bodies, the growth and geometry of reefs are controlled mainly by organisms, the
reef community, and early cementation. The physical
processes of their environment play an indirect or
secondary role. It is only the upper part of a reef
which is organically active, because reef-builders
take their food and nutrients from the surface water.
Calcareous algae or hermatypic corals, which live in
symbiosis with microscopic algae (zooxanthellae)
need sunlight for photosynthesis. Hence, these reefs
can only grow in shallow water up to a depth of
about 50 to 80 m, but they grow optimally just a few
meters below sea level.
(e.g. Rao 1996). F oraminifera, diatoms, sponge debris, and bryozoans tend to be more abundant in the
sand fraction than in coarser material.
3.4.4 Carbonate Ramps
A carbonate ramp is a gently sloping sea floor (generally less than 1 to 2 degrees) without a marked
break in slope. It therefore deviates from most
present-day shelves, which are characterized by a
distinct shelf edge.
See, e.g., Wilson and Jordan (1983), Read (1985), Wright
(1986). Modem examples of ramp settings are the shallow
region of the southern ArabianIPersian Gulf (Purser 1973;
cf. Sect. 4.4.3), the deeper shelf and upper slope of West
Florida (Mullins et al. 1988), the Libyan coastal zone of
the Mediterranean, and the shelves of Yucatan and southern Australia.
On a carbonate ramp, the high-energy facies of the
wave-dominated nearshore zone gradually pass
downslope into deeper water and finally into basinal
sediments deposited under low-energy conditions
(Fig. 3.24). Landward, ramps are characterized by a
high-energy grainstone belt, whereas rimmed carbonate shelves exhibit such a zone on their basinward
edge. Continuous, large reef structures and sediment
gravity flow deposits containing clasts of cemented,
shallow-water facies at the foot of the gentle ramp
slope are generally absent.
The nearshore zone of shallow, high-energy gravel
and sands or skeletal shoal complexes separates a
protected landward area with lagoons, coastal and
peritidal clastics, and possibly tidal flats from an
open marine, high-energy foreshore and shallow-water belt (Fig. 3.24a). The back-barrier area is dominated by low-diversity euryhaline faunal assemblages
which tolerate a wide range of salinities. The inner
ramp, seaward of the beach and baITier facies, is an
efficient "carbonate factory", providing carbonate for
both the back-baITier region and the outer, deeper
ramp.
In addition to biogenic carbonate production, the fonnation
of oolitic and peloidal sands may contribute significantly to
the overall carbonate production in wann waters.
Hardgrounds appear to be a common feature on inner
ramps. Nonnal wave action and frequent storms transport
biogenic sands and larger skeletal particJes (caJcarenites
and caJcrudites) along shore and as traction carpets into
somewhat deeper water. Rare large stonns produce sandand silt-sized biocJastic tempestites (Sect. 3.1.2) which
extend into the deeper ramp and basin, where they alternate
with lime muds or marls. Here, pelagic sediments, such as
planktonic foraminiferal and nannofossil oozes (cf. Sect.
5.3.1) dominate, but some isolated carbonate buildups may
also occur on the outer ramp. The ichnofacies shows a distinct trend from the inner to the outer ramp.
Chapter 3 Coastal and Shallow Sea Sediments
It appears that carbonate ramps preferentially fonned in
geologic periods or environments in which reef builders
secreting large, rigid skeletons were rare. The organic assemblages of ramps function mainly as grain and mud producers, as weil as trappers and binders to form carbonate
banks. Ramps arr. possibly more common in temperate climatic zones (e.g. the modem shelf of southem Australia;
e.g. Boreen and James 1993) than in wanner regions.
Carbonate ramps are common on passive (extensional) continental margins and in epicon-tinental
seas. On continental margins, carbonates frequently
overlie volcanics, evaporites, and clastic sediments,
representing an early phase of deposition in a rift basin or a young ocean basin (cf. Sect. 12.1). Ramps
tend to evolve into rimmed carbonate shelves (Fig.
3 .24b) when carbonate production on the developing
shelf edge is sufficiently high, and the production
and deposition of carbonate in deeper water remains
limited. The buildup of a rim with a steepening slope
can be promoted by mud mounds which contain only
minor proportions of biogenic framework (see below). Such structures may be confused with normal
reefs where frame-building organisms play a great
part (Flügel 1989). The transformation of a rimmed
shelfinto a ramp is less common (Fig. 3.24c); it may
occur, for example, where the shelf is drowned and
buried by prograding siliciclastics, or when the basin
is filled rapidly from another side.
3.4.5 Reef Types
and Composite Carbonate Buildups
General Aspects
At first glance, organic reefs appear to be relatively
simple structures which rise above sea floor as a result of high skeletal calcium carbonate production,
low siliciclastic input, and stabilization against the
attack of waves and currents. In reality, however,
reefs and their associated sediments represent very
complex structures (e.g. Hüssner 1994). In contrast
to other sedimentary bodies, the growth and geometry of reefs are controlled mainly by organisms, the
reef community, and early cementation. The physical
processes of their environment play an indirect or
secondary role. It is only the upper part of a reef
which is organically active, because reef-builders
take their food and nutrients from the surface water.
Calcareous algae or hermatypic corals, which live in
symbiosis with microscopic algae (zooxanthellae)
need sunlight for photosynthesis. Hence, these reefs
can only grow in shallow water up to a depth of
about 50 to 80 m, but they grow optimally just a few
meters below sea level.