3.4 Carbonates
could not follow the high-amplitude Quatemary sea-Ievel
changes, really drowned. In other cases, rapid tilting of
crustal blocks in combination with increased terrestrial influx sometimes caused locally or regionally reduced carbonate production and its replacement by deeper-water facies (e.g. in the Early Cretaceous ofOman, Arabian peninsula; Pratt and Smewing 1993).
On the other hand, reef growth and carbonate buildups ended more or less abruptly in large regions or
even globally several times during nonglacial periods. The cause of such global crises in carbonate
growth is not yet clear. Most authors assume that the
decline of reefs reflects some environmental deterioration, such as changes in salinity, supply of food
and nutrients, muddy waters, oxygen deficiency of
sea water, etc. In some cases, a coincidence of worldwide "reef drowning" and oceanic anoxia was
pointed out.
One of these reef crises occurred in the middle Cretaceous
(Hallock and Schlager 1986; see also Schlager and Philip
1990; Schlager 1998). Peritidal carbonate cycles, caused by
high-frequency, low-amplitude sea-Ievel changes are described in Sect. 7.9.
Pro- and Retrograding Carbonate Buildups
Due to the fact that reef fronts grow fast and are able
to migrate seaward on top of their talus and subsiding
basin floor, prograding carbonate buildups are common in settings of slow relative sea-Ievel rise (Fig.
3.28a). In this case, the different facies zones only
reach limited thicknesses in vertical sequences. Typical successions below the reef front show coarsening- and shallowing-upward trends. Farther landward, where lagoonal and tidal sediments cover the
rcef core, the vertica"l sections on top of the reef core
exhibit a fining- and deepening-upward trend toward
the central lagoonal deposits, thereafter some coarsening.
A classic example of a large, prograding carbonate buildup
is the Perrnian reef complex of the Guadalupe Mountains
region in Texas and New Mexico (NeweIl et al. 1953;
Ward et al. 1986; Osleger 1998). In times of reduced
aggradation, the reef front prograded 3 to 10 mika
(Mazullo 1995); the maximum rate of vertical upbui1ding
was 0.3 to 0.35 mika. This reef complex is also known for
its rich oil and gas fields. Hydrocarbons have accumulated
mainly at the contact of lagoonal sediments (here
dolomites) with coastal evaporites as weil as in the basinal
channel-fill clastic carbonates. The cavemous pore space of
the marginal reef facies is commonly filled with water.
Rapid relative sea-Ievel rise may lead to a retrograding carbonate system with landward migrating carbonate facies zones (Fig. 3.28b). Under these conditions, vertical carbonate buildup is reduced or terminated by the transgressive sea. Some time lag be147
tween sea-Ievel rise and reef response to the changed
environmental conditions may aid to achieve this
successive "reef drowning" (Enos 1991). The vertical
sections of the former carbonate buildup vary according to their location far off or close to the paleocoastline, but fining and deepening-upward sequences prevail (Locations 1 to 3 in Fig. 3 .28b).
More about sequence stratigraphy of carbonates is
discussed in Sect. 7.5.
3.4.7 Diagenesis of Reefs and
Carbonate Buildups
General Processes of Carbonate Diagenesis
It appears appropriate to add here a brief summary on
carbonate diagenesis. (For a more general, broader
discussion about the processes operating in
diagenesis see Chap. 13.) As previously mentioned,
reefs and other carbonate sediments often undergo
marked changes immediately after their deposition
(early pre-burial diagenesis ) and also later on when
they are buried under younger sediments (late or
burial diagenesis). The most important processes
involved are:
Dissolution of pre-exlstmg carbonate minerals
creating void space, for example, in the unsaturated vadose zone of emerged platforrns (cf. Fig.
3.30c).
- Cementation, i.e., precipitation and growth of new
carbonate crystals in the void space, thus leading
to lithification.
- Replacement of single crystals or mosaics of fine
crystals by other, often larger carbonate minerals
(neomorphism).
Numerous articles have been written on this topic, and several textbooks describe the various aspects of this problem
in detail (e.g., Bathurst 1975; Purser and Schroeder 1986;
Füchtbauer 1988; Moore 1988; Scholle et al. 1989; Tucker
and Wright 1990; Heydari 1997). It seems that carbonate
diagenesis, including carbonate-bearing mixed rock types,
is a non-ending objective of study. The following text mentions some problems of do10mitization, but physico-chemical and minera10gical aspects as weil as the isotopic composition of the products of diagenesis are largely omitted.
Rapid and pervasive diagenetic transformation of
most of the primary material of a carbonate buildup
is favored by:
- Relatively good solubility of skeletal carbonate and
thermodynamic instability of some carbonate minerals (aragonite and Mg-calcite) under changing
physico-chemical conditions.
- High porosity and permeability of many carbonate
buildups and reef structures, reef talus, and skeletal
sands.
could not follow the high-amplitude Quatemary sea-Ievel
changes, really drowned. In other cases, rapid tilting of
crustal blocks in combination with increased terrestrial influx sometimes caused locally or regionally reduced carbonate production and its replacement by deeper-water facies (e.g. in the Early Cretaceous ofOman, Arabian peninsula; Pratt and Smewing 1993).
On the other hand, reef growth and carbonate buildups ended more or less abruptly in large regions or
even globally several times during nonglacial periods. The cause of such global crises in carbonate
growth is not yet clear. Most authors assume that the
decline of reefs reflects some environmental deterioration, such as changes in salinity, supply of food
and nutrients, muddy waters, oxygen deficiency of
sea water, etc. In some cases, a coincidence of worldwide "reef drowning" and oceanic anoxia was
pointed out.
One of these reef crises occurred in the middle Cretaceous
(Hallock and Schlager 1986; see also Schlager and Philip
1990; Schlager 1998). Peritidal carbonate cycles, caused by
high-frequency, low-amplitude sea-Ievel changes are described in Sect. 7.9.
Pro- and Retrograding Carbonate Buildups
Due to the fact that reef fronts grow fast and are able
to migrate seaward on top of their talus and subsiding
basin floor, prograding carbonate buildups are common in settings of slow relative sea-Ievel rise (Fig.
3.28a). In this case, the different facies zones only
reach limited thicknesses in vertical sequences. Typical successions below the reef front show coarsening- and shallowing-upward trends. Farther landward, where lagoonal and tidal sediments cover the
rcef core, the vertica"l sections on top of the reef core
exhibit a fining- and deepening-upward trend toward
the central lagoonal deposits, thereafter some coarsening.
A classic example of a large, prograding carbonate buildup
is the Perrnian reef complex of the Guadalupe Mountains
region in Texas and New Mexico (NeweIl et al. 1953;
Ward et al. 1986; Osleger 1998). In times of reduced
aggradation, the reef front prograded 3 to 10 mika
(Mazullo 1995); the maximum rate of vertical upbui1ding
was 0.3 to 0.35 mika. This reef complex is also known for
its rich oil and gas fields. Hydrocarbons have accumulated
mainly at the contact of lagoonal sediments (here
dolomites) with coastal evaporites as weil as in the basinal
channel-fill clastic carbonates. The cavemous pore space of
the marginal reef facies is commonly filled with water.
Rapid relative sea-Ievel rise may lead to a retrograding carbonate system with landward migrating carbonate facies zones (Fig. 3.28b). Under these conditions, vertical carbonate buildup is reduced or terminated by the transgressive sea. Some time lag be147
tween sea-Ievel rise and reef response to the changed
environmental conditions may aid to achieve this
successive "reef drowning" (Enos 1991). The vertical
sections of the former carbonate buildup vary according to their location far off or close to the paleocoastline, but fining and deepening-upward sequences prevail (Locations 1 to 3 in Fig. 3 .28b).
More about sequence stratigraphy of carbonates is
discussed in Sect. 7.5.
3.4.7 Diagenesis of Reefs and
Carbonate Buildups
General Processes of Carbonate Diagenesis
It appears appropriate to add here a brief summary on
carbonate diagenesis. (For a more general, broader
discussion about the processes operating in
diagenesis see Chap. 13.) As previously mentioned,
reefs and other carbonate sediments often undergo
marked changes immediately after their deposition
(early pre-burial diagenesis ) and also later on when
they are buried under younger sediments (late or
burial diagenesis). The most important processes
involved are:
Dissolution of pre-exlstmg carbonate minerals
creating void space, for example, in the unsaturated vadose zone of emerged platforrns (cf. Fig.
3.30c).
- Cementation, i.e., precipitation and growth of new
carbonate crystals in the void space, thus leading
to lithification.
- Replacement of single crystals or mosaics of fine
crystals by other, often larger carbonate minerals
(neomorphism).
Numerous articles have been written on this topic, and several textbooks describe the various aspects of this problem
in detail (e.g., Bathurst 1975; Purser and Schroeder 1986;
Füchtbauer 1988; Moore 1988; Scholle et al. 1989; Tucker
and Wright 1990; Heydari 1997). It seems that carbonate
diagenesis, including carbonate-bearing mixed rock types,
is a non-ending objective of study. The following text mentions some problems of do10mitization, but physico-chemical and minera10gical aspects as weil as the isotopic composition of the products of diagenesis are largely omitted.
Rapid and pervasive diagenetic transformation of
most of the primary material of a carbonate buildup
is favored by:
- Relatively good solubility of skeletal carbonate and
thermodynamic instability of some carbonate minerals (aragonite and Mg-calcite) under changing
physico-chemical conditions.
- High porosity and permeability of many carbonate
buildups and reef structures, reef talus, and skeletal
sands.