3.4 Carbonates
elevated crustal blocks, oceanic plateaus and islands
when carbonate production can keep up with relative
sea-level rise but is not sufficient to fill the space
around the initial carbonate bank (e.g. Wilson 1975;
Read 1985). Prograding of the platform rim is then
hardly possible (see below). Under these conditions,
the platform carbonates can theoretically form very
thick successions of the same facies, for example
superimposed knoll reefs or bioherms, fore-slope
reef talus, lagoonal muds, etc. At the same time,
interreef basinal muds or even deep-sea pelagic sediments can accumulate close to the carbonate platform. If the platform is subdivided by large and deep
channels or troughs, shallow water carbonates can be
bordered by carbonate turbidites and other deposits
transported by gravitiy mass movements.
A well-known and frequently described modem example of
this type of carbonate buildup is the Bahama Platform (e.g.,
Purdy 1963; summaries in Bathurst 1975; Sellwood 1986,
and Sect. 12.2.2). This platform is divided into two parts
by the deep Tongue of the Ocean (cf. Fig. 12.12d). The
platform represents a carbonate massif of more than 4 km
in thickness which has been built up since the Cretaceous.
Ancient carbonate platforms are of similar extent (e.g., in
the Alps, in the southem Apeninnes, and in the Devonian
of Canada), but they normally do not reach such great
thicknesses. Famous examples of isolated platforms are
Triassie carbonate buildups of the Dolomites in the Southem Alps, Italy (e.g. Bosellini 1989).
The slopes of these platforms are mainly controlled
by the angle of repose of reef talus as weIl as by erosional processes (rock falls, avalanching; current action; Kenter 1990; Harris 1994). The architecture of
isolated platforms depends to some extent on their
size and mud production (Fig. 3.27). Large platforms
with wide areas of bank tops allow more lime mud
production than smaller platforms. Due to matrixsupported gravity flows, the large, mud-rich platforms develop wider and gentler slopes in their early
stages of development than do mud-poor platforms.
In the latter case, rock falls and grain flows control
the slope angle.
This is demonstrated by comparing size and architeeture of
two Triassie platforms: the Latemar buildup in the Italian
Alps and the larger Great Bank of Guizhou in South China
(Fig. 3.27; Lehrmann et al. 1998). The mud-poor, steeply
sloped Latemar buildup is of "pinnacle" type, 5 km in diFig. 3.26. a Topography and facies zones of shoalrimmed (upper part of block diagram) and reefrimmed carbonate shelves or platforms (lower half).
Low-energy (LE) waves produce a gentler foreslope
with mud mounds, sandy shoals, and islands at the
shelf break. High-energy ( HE) conditions can only be
tolerated by solid reef structures that have steep
foreslopes. The mud belt facies of the inner shelf is
influenced significantly by climate. b Reef mosaic,
145
ameter and 700 m high. The Great Bank ofGuizhou (GBG)
formed on the outer margin of the huge Permian Yangtze
platform from the latest Permian to the Middle Triassie in
~10 Ma. It covers an area of 15 x 70 km and reaches up to
2.5 km in height. It evolved from a low-relief bank with
some patch reefs to a reef-rimmed platform and finally to a
progressively steepening escarpment stage. The flat top of
the platform interior was most of the time characterized by
muddy tidal flats (peritidal cyc1ic limestones and
dolomites; cf. Sect. 7.9) or a deeper lagoon creating an
atoll-like morphology. Framework builders and
boundstones, reinforced by microbial ernsts (Tubiphytes)
allowed slightly prograding of the platform rim during its
intermediate stage. During the escarpment stage the steep
slopes became sediment-starved and were partially affected
by current erosion. Finally, also the platform top drowned
as indicated by oolithic skeletal grainstones and packstones
grading upward to nodular-bedded oncolitic wackestones
with open-marine biota. Due to continued deepening, the
total strncture was buried by a thick pile of shales and
siliciclastic turbidites. Pervasive dolomitization partially
obliterated the depositional fabrics.
A main difference between the two types (end members)
of platforms is the percentage of redeposited material at the
platform margin. This seems to make up about 50% of the
total buildup volume of mud-poor isolated platforms of
limited size (Latemar example, Fig. 3.27c). However, a
considerable part of the slope sediments may have been
produced by in-situ-growing reef organisms (Blendinger
1994). In the case ofvery large, mud-rich platforms (Great
Bank of Guizhou example), debris flow breccias, turbidite
grainstones and pelagic limestones and mudstones only
account for about 10% ofthe total buildup.
In many cases the relationship between relative sealevel rise and the growth potential of carbonate
buildups leads to more complicate structures for the
following reasons: (1) the sea level changed frequently and at differing rates (cf. Sects. 7.2 and 7.5),
and (2) the growth rates of carbonate buildups varied
with the type of reef builders and the depositional
environment (e.g. Homewood 1996). One can distinguish several situations (Kendall and Schlager 1981):
- Start-up phase. Carbonate accumulation has commenced but lags behind the rapidly rising sea.
- Catch-up phase (after initial drowning). Carbonate
accumulation exceeds the rate of coeval sea-level
rise. Then two cases are possible: (a) The fast-growing rim and patches of the interior platform are able
to catch up with sea-level rise and survive, whereas
the platform interior is transformed into a deep lashowing details of composite reef buildup. c Stages
of reef growth; high species diversity with domal,
massive, lamellar, branching and encrusting reef
builders is realized only in the diversification stage.
d Simplified nomenclature of autochthonous and
allochthonous reef limestones. (Based on different
sources, e.g., Zankl 1971; Wilson 1975; James 1983;
Sellwood 1986, modified)
elevated crustal blocks, oceanic plateaus and islands
when carbonate production can keep up with relative
sea-level rise but is not sufficient to fill the space
around the initial carbonate bank (e.g. Wilson 1975;
Read 1985). Prograding of the platform rim is then
hardly possible (see below). Under these conditions,
the platform carbonates can theoretically form very
thick successions of the same facies, for example
superimposed knoll reefs or bioherms, fore-slope
reef talus, lagoonal muds, etc. At the same time,
interreef basinal muds or even deep-sea pelagic sediments can accumulate close to the carbonate platform. If the platform is subdivided by large and deep
channels or troughs, shallow water carbonates can be
bordered by carbonate turbidites and other deposits
transported by gravitiy mass movements.
A well-known and frequently described modem example of
this type of carbonate buildup is the Bahama Platform (e.g.,
Purdy 1963; summaries in Bathurst 1975; Sellwood 1986,
and Sect. 12.2.2). This platform is divided into two parts
by the deep Tongue of the Ocean (cf. Fig. 12.12d). The
platform represents a carbonate massif of more than 4 km
in thickness which has been built up since the Cretaceous.
Ancient carbonate platforms are of similar extent (e.g., in
the Alps, in the southem Apeninnes, and in the Devonian
of Canada), but they normally do not reach such great
thicknesses. Famous examples of isolated platforms are
Triassie carbonate buildups of the Dolomites in the Southem Alps, Italy (e.g. Bosellini 1989).
The slopes of these platforms are mainly controlled
by the angle of repose of reef talus as weIl as by erosional processes (rock falls, avalanching; current action; Kenter 1990; Harris 1994). The architecture of
isolated platforms depends to some extent on their
size and mud production (Fig. 3.27). Large platforms
with wide areas of bank tops allow more lime mud
production than smaller platforms. Due to matrixsupported gravity flows, the large, mud-rich platforms develop wider and gentler slopes in their early
stages of development than do mud-poor platforms.
In the latter case, rock falls and grain flows control
the slope angle.
This is demonstrated by comparing size and architeeture of
two Triassie platforms: the Latemar buildup in the Italian
Alps and the larger Great Bank of Guizhou in South China
(Fig. 3.27; Lehrmann et al. 1998). The mud-poor, steeply
sloped Latemar buildup is of "pinnacle" type, 5 km in diFig. 3.26. a Topography and facies zones of shoalrimmed (upper part of block diagram) and reefrimmed carbonate shelves or platforms (lower half).
Low-energy (LE) waves produce a gentler foreslope
with mud mounds, sandy shoals, and islands at the
shelf break. High-energy ( HE) conditions can only be
tolerated by solid reef structures that have steep
foreslopes. The mud belt facies of the inner shelf is
influenced significantly by climate. b Reef mosaic,
145
ameter and 700 m high. The Great Bank ofGuizhou (GBG)
formed on the outer margin of the huge Permian Yangtze
platform from the latest Permian to the Middle Triassie in
~10 Ma. It covers an area of 15 x 70 km and reaches up to
2.5 km in height. It evolved from a low-relief bank with
some patch reefs to a reef-rimmed platform and finally to a
progressively steepening escarpment stage. The flat top of
the platform interior was most of the time characterized by
muddy tidal flats (peritidal cyc1ic limestones and
dolomites; cf. Sect. 7.9) or a deeper lagoon creating an
atoll-like morphology. Framework builders and
boundstones, reinforced by microbial ernsts (Tubiphytes)
allowed slightly prograding of the platform rim during its
intermediate stage. During the escarpment stage the steep
slopes became sediment-starved and were partially affected
by current erosion. Finally, also the platform top drowned
as indicated by oolithic skeletal grainstones and packstones
grading upward to nodular-bedded oncolitic wackestones
with open-marine biota. Due to continued deepening, the
total strncture was buried by a thick pile of shales and
siliciclastic turbidites. Pervasive dolomitization partially
obliterated the depositional fabrics.
A main difference between the two types (end members)
of platforms is the percentage of redeposited material at the
platform margin. This seems to make up about 50% of the
total buildup volume of mud-poor isolated platforms of
limited size (Latemar example, Fig. 3.27c). However, a
considerable part of the slope sediments may have been
produced by in-situ-growing reef organisms (Blendinger
1994). In the case ofvery large, mud-rich platforms (Great
Bank of Guizhou example), debris flow breccias, turbidite
grainstones and pelagic limestones and mudstones only
account for about 10% ofthe total buildup.
In many cases the relationship between relative sealevel rise and the growth potential of carbonate
buildups leads to more complicate structures for the
following reasons: (1) the sea level changed frequently and at differing rates (cf. Sects. 7.2 and 7.5),
and (2) the growth rates of carbonate buildups varied
with the type of reef builders and the depositional
environment (e.g. Homewood 1996). One can distinguish several situations (Kendall and Schlager 1981):
- Start-up phase. Carbonate accumulation has commenced but lags behind the rapidly rising sea.
- Catch-up phase (after initial drowning). Carbonate
accumulation exceeds the rate of coeval sea-level
rise. Then two cases are possible: (a) The fast-growing rim and patches of the interior platform are able
to catch up with sea-level rise and survive, whereas
the platform interior is transformed into a deep lashowing details of composite reef buildup. c Stages
of reef growth; high species diversity with domal,
massive, lamellar, branching and encrusting reef
builders is realized only in the diversification stage.
d Simplified nomenclature of autochthonous and
allochthonous reef limestones. (Based on different
sources, e.g., Zankl 1971; Wilson 1975; James 1983;
Sellwood 1986, modified)