3.4 Carbonates
belonging to the back reef zone behind a larger reef
barrier. Patch reefs are frequently associated with
skeletal and oolithic sand bars. In deep lagoons the
patch reefs may develop into isolated conical high
pinnacle reefs (Fig. 3.25d), surrounded largely by
pelagic carbonate.
Reef rims, fringing reefs, and barrier reefs. In
high-energy environments the frame-building organisms can produce narrow elongated structures referred to as reef rims. They grow up to or c10se to
mean sea level into the zone of greatest wave action.
This implies that their primary structure must be sufficiently solid to withstand permanent and occasionally very strong wave attack. As a result, their seaward slope tends to be very steep and their proximal
foreslope talus often consists of coarse reef debris
and fallen blocks from the active reef zone. Nevertheless, these structures are able to grow not only
upward, but also to prograde seaward toward their
food supply and on top of their foreslope talus (cf.
Fig. 3.28a).
Ifseaward outbuilding prevails and the carbonate
body is attached to land masses or islands, the resulting platforms are called fringing reefs (Fig. 3.25b).
Wide structures of this type develop when carbonate
production exceeds subsidence and, in addition, the
relative sea level remains cither constant or repeatedly reaches about the same level (cf. Sect. 7.5).
Conversely, rapidly rising sea level or strong subsidence often leads to a pronounced vertical buildup of
the reef front and thus to barrier reefs and atolls separating deepened and sometimes wide lagoons from
the open sea (Figs. 3.25c and d; see below).
Modem fringing reefs are commonly 0.5 to 1 km wide and
may include some very shallow lagoons of limited extent.
Well-known examples occur, for example, along the coasts
of the Red Sea and many tropical islands where only little
teITigenous material is swept into the sea. The most prominent modern example of a baITier reef is the 2000 km long
Great Barrier Reef along the eastern continental margin of
Australia (e.g. Davies 1989; Davies and Peerdeman 1998).
An outstanding fossil example are the Devonian baITier
reefs in the Canning basin, western Australia (e.g. Playford
et a1. 1989).
The many atolls of the present-day oceans (more than
320 examples in the Indian and Pacific oceans) are ringlike baITier reefs sUITounding a shallow lagoon in which a
central land mass is lacking (Fig. 3.25c). As already described by Darwin, most of these atolls are associated with
submerged, isolated volcanic seamounts, which previously
formed islands. These reef structures can be explained by
alternating outbuilding of fringing reefs and upbuilding of
baITier reefs related to the rapid Quaternary and slower earlier sea-Ievel fluctuations. In addition, a general tendency
of the seamounts to subside plays a role. The carbonate
buildups of some atolls reach thicknesses in excess of 1000
m and represent time spans of some tens of million years.
Their central lagoons collect reef debris and sediment produced in the lagoon itself, mainly sand- and silt-sized skeletal carbonate.
141
Composite Carbonate Buildups
Carbonate shelves and carbonate platforms generally
are composite structures, consisting of the reef front
or reef belt, the foreslope proximal and distal reef
talus (fore reet), and the back reef, inc1uding reef
flats and lagoons (Figs. 3.25a and 3.26a). In the case
of carbonate shelves, the coastal mud belt and tidal
flats form an additional part of the total carbonate
structure (cf. Sect. 3.2.2).
Carbonate buildups have received much attention by
sedimentologists in both academia and oil exploration. A
number of special volumes on this subject has been published, for example about the widespread Cretaceous platforms in the Tethys region (Simo et al. 1993), where about
16% of the world's hydrocarbon reserves are located. The
facies zones and architecture of carbonate buildups also
considerably varied through geologie time as a result ofthe
evolution of reef-building organism as mentioned earlier
(summary in James and Bourque 1992). In addition, shifts
of climate zones and long-term changes in relative sea level
contributed to this remarkable variations in the products of
reef-building organisms. The response of carbonate buildups to third order and higher-frequency sea-Ievel changes,
including fore-reef slopes, will be discussed in Sect. 7.5.
These processes largely control the formation of reservoirs
for hydrocarbons.
The lateral sediment distribution on the tops of carbonate platforms, as for example observed on the
Great Bahama Bank, commonly inc1udes (from land
to sea):
- Calcareous mud (biofacies adjusted to normal to
hypersaline conditions).
Pellet mud.
- Oolite shoals and oolitic grapestone.
Reefs and rocky bottom along the platform edge.
These facies association can migrate landward or
seaward with time with the result that some of them
become more, others less important as contributors to
the total carbonate buildup. Nevertheless, both the
isolated and land-attached platforms may grow more
or less continually, regardless of the specific carbonate production of frame-building organisms and the
various reeftypes discussed below.
The Back-Reef Zone and Reef Lagoons
The salinity and other environmental characteristics
of the back-reef zone of land-attached platforms
(Figs. 3.25c and d) are strongly influenced by both
water exchange between the open sea and influx of
river water. Water exchange with the sea occurs via
inlets or deeper channels crossing the baITier reef. If
it is sufficient to maintain approximately normal marine conditions, carbonate production in the lagoons
belonging to the back reef zone behind a larger reef
barrier. Patch reefs are frequently associated with
skeletal and oolithic sand bars. In deep lagoons the
patch reefs may develop into isolated conical high
pinnacle reefs (Fig. 3.25d), surrounded largely by
pelagic carbonate.
Reef rims, fringing reefs, and barrier reefs. In
high-energy environments the frame-building organisms can produce narrow elongated structures referred to as reef rims. They grow up to or c10se to
mean sea level into the zone of greatest wave action.
This implies that their primary structure must be sufficiently solid to withstand permanent and occasionally very strong wave attack. As a result, their seaward slope tends to be very steep and their proximal
foreslope talus often consists of coarse reef debris
and fallen blocks from the active reef zone. Nevertheless, these structures are able to grow not only
upward, but also to prograde seaward toward their
food supply and on top of their foreslope talus (cf.
Fig. 3.28a).
Ifseaward outbuilding prevails and the carbonate
body is attached to land masses or islands, the resulting platforms are called fringing reefs (Fig. 3.25b).
Wide structures of this type develop when carbonate
production exceeds subsidence and, in addition, the
relative sea level remains cither constant or repeatedly reaches about the same level (cf. Sect. 7.5).
Conversely, rapidly rising sea level or strong subsidence often leads to a pronounced vertical buildup of
the reef front and thus to barrier reefs and atolls separating deepened and sometimes wide lagoons from
the open sea (Figs. 3.25c and d; see below).
Modem fringing reefs are commonly 0.5 to 1 km wide and
may include some very shallow lagoons of limited extent.
Well-known examples occur, for example, along the coasts
of the Red Sea and many tropical islands where only little
teITigenous material is swept into the sea. The most prominent modern example of a baITier reef is the 2000 km long
Great Barrier Reef along the eastern continental margin of
Australia (e.g. Davies 1989; Davies and Peerdeman 1998).
An outstanding fossil example are the Devonian baITier
reefs in the Canning basin, western Australia (e.g. Playford
et a1. 1989).
The many atolls of the present-day oceans (more than
320 examples in the Indian and Pacific oceans) are ringlike baITier reefs sUITounding a shallow lagoon in which a
central land mass is lacking (Fig. 3.25c). As already described by Darwin, most of these atolls are associated with
submerged, isolated volcanic seamounts, which previously
formed islands. These reef structures can be explained by
alternating outbuilding of fringing reefs and upbuilding of
baITier reefs related to the rapid Quaternary and slower earlier sea-Ievel fluctuations. In addition, a general tendency
of the seamounts to subside plays a role. The carbonate
buildups of some atolls reach thicknesses in excess of 1000
m and represent time spans of some tens of million years.
Their central lagoons collect reef debris and sediment produced in the lagoon itself, mainly sand- and silt-sized skeletal carbonate.
141
Composite Carbonate Buildups
Carbonate shelves and carbonate platforms generally
are composite structures, consisting of the reef front
or reef belt, the foreslope proximal and distal reef
talus (fore reet), and the back reef, inc1uding reef
flats and lagoons (Figs. 3.25a and 3.26a). In the case
of carbonate shelves, the coastal mud belt and tidal
flats form an additional part of the total carbonate
structure (cf. Sect. 3.2.2).
Carbonate buildups have received much attention by
sedimentologists in both academia and oil exploration. A
number of special volumes on this subject has been published, for example about the widespread Cretaceous platforms in the Tethys region (Simo et al. 1993), where about
16% of the world's hydrocarbon reserves are located. The
facies zones and architecture of carbonate buildups also
considerably varied through geologie time as a result ofthe
evolution of reef-building organism as mentioned earlier
(summary in James and Bourque 1992). In addition, shifts
of climate zones and long-term changes in relative sea level
contributed to this remarkable variations in the products of
reef-building organisms. The response of carbonate buildups to third order and higher-frequency sea-Ievel changes,
including fore-reef slopes, will be discussed in Sect. 7.5.
These processes largely control the formation of reservoirs
for hydrocarbons.
The lateral sediment distribution on the tops of carbonate platforms, as for example observed on the
Great Bahama Bank, commonly inc1udes (from land
to sea):
- Calcareous mud (biofacies adjusted to normal to
hypersaline conditions).
Pellet mud.
- Oolite shoals and oolitic grapestone.
Reefs and rocky bottom along the platform edge.
These facies association can migrate landward or
seaward with time with the result that some of them
become more, others less important as contributors to
the total carbonate buildup. Nevertheless, both the
isolated and land-attached platforms may grow more
or less continually, regardless of the specific carbonate production of frame-building organisms and the
various reeftypes discussed below.
The Back-Reef Zone and Reef Lagoons
The salinity and other environmental characteristics
of the back-reef zone of land-attached platforms
(Figs. 3.25c and d) are strongly influenced by both
water exchange between the open sea and influx of
river water. Water exchange with the sea occurs via
inlets or deeper channels crossing the baITier reef. If
it is sufficient to maintain approximately normal marine conditions, carbonate production in the lagoons