3.4 Carbonates
remains predominantly biogenie. However, it often
lags behind the carbonate buildup along the reef
front, with the exception of isolated patch and pinnaele reefs or along inshore banks (consisting, e.g., of
branching corals and algae; Fig. 3.26a). Consequently, lagoons associated with barrier reefs tend to
become deeper during times of rapid relative sealevel rise.
Inner parts of lagoons are frequently occupied by
tidal flats. In humid regions with influx of fresh water and sediment, brackish and fresh water ponds
evolve elose to the coast line (Fig. 3.26a). Farther
seaward the lagoonal water body rnay become stratified if water exchange with the open sea is limited
(cf. Sect. 4.2). Then bituminous lagoonal muds with
scarce or missing bioturbation may accumulate. In
arid regions, hypersaline conditions often promote
chemical and biochemical carbonate precipitation,
early transformation of calcium carbonate to dolomite, and the generation of evaporitic layers in the
supratidal zone (Sects. 3.2.2 and 6.4.1). Generally,
lagoonal sediments are sensitive to seasonal and longer term climatic changes and record such variations
in their sediments.
"Pelagic" Carbonate Platforms
Drowned or subsided carbonate platforrns below the
depth favorable for reef growth rnay be referred to as
"pelagic" carbonate platforms (Franke and Walliser
1983; cf. Sects. 5.3.7 and 12.2.2). At an intermediate
depth between shallow-water and true pelagic carbonate deposition, the platforrns are still exposed to
current action. The sediments overlying older carbonates or other rocks therefore tend to become discontinuous and interrupted by intervals of omission and
erosion. They are characterized by irregular bedding
and a nodular appearance, red color, ferromanganese
nodules, corrosion surfaces and hardgrounds with
sessil fauna. Accumulations of certain fossils (e.g.
cephalopods and crinoids) are common.
This type of a condensed carbonate section is common in
the geologic record and was described from many regions,
for example from the Jurassic and Cretaceous (e.g.,
Bemoulli and Jenkyns 1974; Fürsich 1979; Bergner et al.
1982; Jenkyns 1986) and from late Devonian and early
Carboniferous strata in various European countries and
North Africa (Tucker 1974; Wendt and Aigner 1985;
Wendt 1988).
Average sedimentation rates on such platforms are very
low (0.1 to 1 mlMa), while adjacent basins are simultaneously filled at rates between 20 and 100 mlMa. Laterally,
the condensed horizons of platforms may show transitions
to clastic shelf sediments or to brecciated, gravity-deformed resedimented slope deposits.
Sediments similar to those of ancient "pelagie" platforms have been observed on some deep marginal and oceanic plateaus ofthe modem oceans (Sect. 5.3.7). If current
143
action is weak pelagic oozes accumulate at higher rates on
bank tops, for example on the huge Ontong Java Plateau in
the southwestem Pacific where Neogen sedimentation rates
range from 20 to 60 mlMa at 2500 to 3800 m water depth
(Berger et al. 1993; cf. Sect. 5.3.2).
Types of Reef Limestone
The different facies zones of a reef complex are represented by various types of "reer' limestones (Fig.
3.26d). Apart from lagoonal and tidal sediments, reef
limestones can be subdivided into two groups (James
1983):
(1) Autochthonous reef limestones displaying inplace fossils, which support the framework of the
reef. We can distinguish:
- Framestones, consisting largely of frame-building
skeletons.
- Bindstones, showing in-place tabular or lamellar
fossils, which encrust or bind the sediment together during deposition.
- Bafflestones, exhibiting in-place stalked fossils,
which trap sediment by baffling.
(2) Allochthonous reef limestones, consisting of material delivered from nearby or more distal sources:
- Rudstones, c1ast-supported limestones, containing
a high proportion of coarse-grained partic1es.
- Floatstones, containing more than 10% particles
with diameters greater than 2 mm which, in contrast to the rudstones, are matrix-supported.
The occurrence ofthese limestone types within a reef complex is indicated by numbers in Figure 3.26a. All of these
rock types contain biomicrite and allochthonous rnicrite in
significant amounts. Detailed microfacies descriptions have
been surnmarized by several authors (e.g. Rezak and
Lavoie 1993).
3.4.6 Response of Carbonate Buildups
to Subsidence and Sea-Level Rise
Vertical Buildup and "Drowning"
Carbonate shelves and platforms are sensible gauges
of sea level changes and therefore in turn strongly
affected by relative rise and fall of sea level. In this
section, only some general points are addressed in
relation to subsidence and sea-level rise. More about
carbonate sequence stratigraphy inc1uding the effects
of relative sea-level fall will be discussed in Sect.
7.5.
A relatively simple case is the vertical buildup of
isolated carbonate platforrns with long-persisting sealevel rise (or subsidence of the basin floor; Fig.
3.25e). Such platforms usually develop on relatively
remains predominantly biogenie. However, it often
lags behind the carbonate buildup along the reef
front, with the exception of isolated patch and pinnaele reefs or along inshore banks (consisting, e.g., of
branching corals and algae; Fig. 3.26a). Consequently, lagoons associated with barrier reefs tend to
become deeper during times of rapid relative sealevel rise.
Inner parts of lagoons are frequently occupied by
tidal flats. In humid regions with influx of fresh water and sediment, brackish and fresh water ponds
evolve elose to the coast line (Fig. 3.26a). Farther
seaward the lagoonal water body rnay become stratified if water exchange with the open sea is limited
(cf. Sect. 4.2). Then bituminous lagoonal muds with
scarce or missing bioturbation may accumulate. In
arid regions, hypersaline conditions often promote
chemical and biochemical carbonate precipitation,
early transformation of calcium carbonate to dolomite, and the generation of evaporitic layers in the
supratidal zone (Sects. 3.2.2 and 6.4.1). Generally,
lagoonal sediments are sensitive to seasonal and longer term climatic changes and record such variations
in their sediments.
"Pelagic" Carbonate Platforms
Drowned or subsided carbonate platforrns below the
depth favorable for reef growth rnay be referred to as
"pelagic" carbonate platforms (Franke and Walliser
1983; cf. Sects. 5.3.7 and 12.2.2). At an intermediate
depth between shallow-water and true pelagic carbonate deposition, the platforrns are still exposed to
current action. The sediments overlying older carbonates or other rocks therefore tend to become discontinuous and interrupted by intervals of omission and
erosion. They are characterized by irregular bedding
and a nodular appearance, red color, ferromanganese
nodules, corrosion surfaces and hardgrounds with
sessil fauna. Accumulations of certain fossils (e.g.
cephalopods and crinoids) are common.
This type of a condensed carbonate section is common in
the geologic record and was described from many regions,
for example from the Jurassic and Cretaceous (e.g.,
Bemoulli and Jenkyns 1974; Fürsich 1979; Bergner et al.
1982; Jenkyns 1986) and from late Devonian and early
Carboniferous strata in various European countries and
North Africa (Tucker 1974; Wendt and Aigner 1985;
Wendt 1988).
Average sedimentation rates on such platforms are very
low (0.1 to 1 mlMa), while adjacent basins are simultaneously filled at rates between 20 and 100 mlMa. Laterally,
the condensed horizons of platforms may show transitions
to clastic shelf sediments or to brecciated, gravity-deformed resedimented slope deposits.
Sediments similar to those of ancient "pelagie" platforms have been observed on some deep marginal and oceanic plateaus ofthe modem oceans (Sect. 5.3.7). If current
143
action is weak pelagic oozes accumulate at higher rates on
bank tops, for example on the huge Ontong Java Plateau in
the southwestem Pacific where Neogen sedimentation rates
range from 20 to 60 mlMa at 2500 to 3800 m water depth
(Berger et al. 1993; cf. Sect. 5.3.2).
Types of Reef Limestone
The different facies zones of a reef complex are represented by various types of "reer' limestones (Fig.
3.26d). Apart from lagoonal and tidal sediments, reef
limestones can be subdivided into two groups (James
1983):
(1) Autochthonous reef limestones displaying inplace fossils, which support the framework of the
reef. We can distinguish:
- Framestones, consisting largely of frame-building
skeletons.
- Bindstones, showing in-place tabular or lamellar
fossils, which encrust or bind the sediment together during deposition.
- Bafflestones, exhibiting in-place stalked fossils,
which trap sediment by baffling.
(2) Allochthonous reef limestones, consisting of material delivered from nearby or more distal sources:
- Rudstones, c1ast-supported limestones, containing
a high proportion of coarse-grained partic1es.
- Floatstones, containing more than 10% particles
with diameters greater than 2 mm which, in contrast to the rudstones, are matrix-supported.
The occurrence ofthese limestone types within a reef complex is indicated by numbers in Figure 3.26a. All of these
rock types contain biomicrite and allochthonous rnicrite in
significant amounts. Detailed microfacies descriptions have
been surnmarized by several authors (e.g. Rezak and
Lavoie 1993).
3.4.6 Response of Carbonate Buildups
to Subsidence and Sea-Level Rise
Vertical Buildup and "Drowning"
Carbonate shelves and platforms are sensible gauges
of sea level changes and therefore in turn strongly
affected by relative rise and fall of sea level. In this
section, only some general points are addressed in
relation to subsidence and sea-level rise. More about
carbonate sequence stratigraphy inc1uding the effects
of relative sea-level fall will be discussed in Sect.
7.5.
A relatively simple case is the vertical buildup of
isolated carbonate platforrns with long-persisting sealevel rise (or subsidence of the basin floor; Fig.
3.25e). Such platforms usually develop on relatively