196 Imprint of Climatic Zonation on Marine Sediments
rate of upward growth over considerable areas. The fill between the framework
consists of reef debris and the hard parts of various other reef inhabitants. The fill can
provide up to nine tenths of the reef mass.
The reef dwellers are legion, for the reef offers an extraordinary variety of life
styles (Fig. 7.7). Solid substrate favors epibionts such as bryozoans and sessile foraminifera. Cavities offer shelter for fish and crustaceans. The rich and colorful mollusk fauna is familiar from underwater photography and (rather unfortunately) from
souvenir shops.
The tropical coral reef is the shallow-water habitat with the greatest number of
species, that is, with the greatest diversity. More than 3000 species live in Indo-Pacific reefs. The skeletal remains produced in rich variety are broken up in various
ways, by boring algae, sponges, and sea urchins. Much is crushed by fishes and
crustaceans. In many skeletons, calcite crystals are held together by organic material.
When it decays, such skeleton disintegrate to lime mud (micrite). Finally, waves and
tidal currents grind up the relatively soft calcareous matter. The debris fills the interstices within the reef structure, collects in the lagoons behind the reefs, and washes
up on the beach. Also, debris trickles down the sides of the reef into greater depths,
building the reef flanks. Dissolution of aragonite, recrystallization and cementation of
the debris start very soon after deposition. These processes (important in the investigation of fossil reefs as oil reservoir rocks) are influenced by microbial activity as
well as interaction with rainwater. Already a slight drop in sea level would expose
large areas to rain water. Thus, phreatic processes (karstification, cementation) were
extremely important throughout the Pleistocene.
There are other concentrations of skeletal carbonates sometimes called "reefs"
even in arctic waters (red algae, mollusks in shallow waters or corals of the genus
Lophe/ia in deeper waters). These are quite distinct from tropical reefs, however.
7.4.3 Atolls. The various processes of reef building have their classic representation
in atoll development. Palm-studded rings of coral and shell debris, enclosing a lagoon
and sheltering it from the pounding surf, are quite common in the tropical South
Pacific. Charles Darwin first explained their origin (Fig. 7.8). His theory was built on
the idea of a sequence going from fringing reef to barrier reef to atoll. The fringing
reef grows immediately next to the land. The barrier reef is separated from the land
by a shallow lagoon. The atoll, finally, is an isolated quasi-circular ring of reefs, with
an emergent portion of reef debris, the atoll island. In the center of the ring is a
lagoon. Dimensions vary; a large atoll can be up to 40 km in diameter. Since the
algae, both as carbonate producers and as symbionts of corals, need light, the coral
reefs can grow only in very shallow water. An atoll, therefore, cannot have grown up
from deep waters. This is the reasoning that led Darwin to propose a combination of
slowly sinking sea floor and upbuilding of reef.
Darwin's theory has since been proven correct by drilling into several atolls. In the
drillhole on Enewetak Atoll (Marshall Islands of the tropical Pacific) the basaltic
basement, of Eocene age, was hit at about 1400 m depth. Detailed investigation of the
cores recovered from the Enewetak drill hole indicated that the reef had on occasion
emerged from the sea: land snails, pollen, and spores from terrestrial plants are
rate of upward growth over considerable areas. The fill between the framework
consists of reef debris and the hard parts of various other reef inhabitants. The fill can
provide up to nine tenths of the reef mass.
The reef dwellers are legion, for the reef offers an extraordinary variety of life
styles (Fig. 7.7). Solid substrate favors epibionts such as bryozoans and sessile foraminifera. Cavities offer shelter for fish and crustaceans. The rich and colorful mollusk fauna is familiar from underwater photography and (rather unfortunately) from
souvenir shops.
The tropical coral reef is the shallow-water habitat with the greatest number of
species, that is, with the greatest diversity. More than 3000 species live in Indo-Pacific reefs. The skeletal remains produced in rich variety are broken up in various
ways, by boring algae, sponges, and sea urchins. Much is crushed by fishes and
crustaceans. In many skeletons, calcite crystals are held together by organic material.
When it decays, such skeleton disintegrate to lime mud (micrite). Finally, waves and
tidal currents grind up the relatively soft calcareous matter. The debris fills the interstices within the reef structure, collects in the lagoons behind the reefs, and washes
up on the beach. Also, debris trickles down the sides of the reef into greater depths,
building the reef flanks. Dissolution of aragonite, recrystallization and cementation of
the debris start very soon after deposition. These processes (important in the investigation of fossil reefs as oil reservoir rocks) are influenced by microbial activity as
well as interaction with rainwater. Already a slight drop in sea level would expose
large areas to rain water. Thus, phreatic processes (karstification, cementation) were
extremely important throughout the Pleistocene.
There are other concentrations of skeletal carbonates sometimes called "reefs"
even in arctic waters (red algae, mollusks in shallow waters or corals of the genus
Lophe/ia in deeper waters). These are quite distinct from tropical reefs, however.
7.4.3 Atolls. The various processes of reef building have their classic representation
in atoll development. Palm-studded rings of coral and shell debris, enclosing a lagoon
and sheltering it from the pounding surf, are quite common in the tropical South
Pacific. Charles Darwin first explained their origin (Fig. 7.8). His theory was built on
the idea of a sequence going from fringing reef to barrier reef to atoll. The fringing
reef grows immediately next to the land. The barrier reef is separated from the land
by a shallow lagoon. The atoll, finally, is an isolated quasi-circular ring of reefs, with
an emergent portion of reef debris, the atoll island. In the center of the ring is a
lagoon. Dimensions vary; a large atoll can be up to 40 km in diameter. Since the
algae, both as carbonate producers and as symbionts of corals, need light, the coral
reefs can grow only in very shallow water. An atoll, therefore, cannot have grown up
from deep waters. This is the reasoning that led Darwin to propose a combination of
slowly sinking sea floor and upbuilding of reef.
Darwin's theory has since been proven correct by drilling into several atolls. In the
drillhole on Enewetak Atoll (Marshall Islands of the tropical Pacific) the basaltic
basement, of Eocene age, was hit at about 1400 m depth. Detailed investigation of the
cores recovered from the Enewetak drill hole indicated that the reef had on occasion
emerged from the sea: land snails, pollen, and spores from terrestrial plants are