28
Reef Lime Constructions
1.3 Bottom Sediments
Reef bottom sediments contain a larger part (ca. 60-80%) of total lime
material of modern and also of the Pleistocene reefs (Thomassin 1978;
Riddle 1988). The benthic biocenoses of the soft bottom are important
components of the coral reef ecosystems. Studies of composition, origin and
distribution of bottom sediments give a remarkable insight into the problems
of reef geology and biology (Purdy 1963; Goreau and Goreau 1973; Maxwell
1968, 1973a; Stoddart 1969). The main part of the lime material composing
the reef bottom sediments is of biological origin (Flood 1983; cf. Table 1.7).
The terrigenous clastic material, consisting of clay, sand, pebbles, and
ferrous oxides, comprises a significant part of sediments (10-15%) only in
lagoons and on the flat of some fringing and barrier reefs (Ginsburg 1956;
Morelock and Koenig 1967; Milliman 1967; Stoddart et al. 1969). As an
example, the reefs off the coasts of Florida, Cuba and GBR could be
mentioned (Stoddart 1969; Maxwell 1968; Riddle 1988). From Table 1.7, it
can be seen that the lime material of sediments has two main sources of
origin. One part (30 to 60%) is formed by the erosion of lime rocks of
modern and ancient reef plateaus and of lime skeletons of animals -
molluscan shells, corals, urchin's needles, etc. This erosion proceeds by the
action of the boring, scraping and grazing reef animals, such as boring
clams, sponges, worms, coral-eating and periphyton scraping fish and
urchins. Thesefore, this part of sediments is formed mainly by "chips" of
borers and by fecal material of coral feeders and scrapers (Emery 1956;
Bakus 1968; Randall 1974; Gygi 1975; Ogden 1977).
The second part of lime reef sediments is formed by small skeletal
elements of reef animals and sea weeds - calcite plates, globules or spicules.
The same elements are contained in sponges, soft corals, holothurians,
asteroids as well as in calcareous algae. The third part is formed by
calcareous microshells of benthic foraminifera and of planktonic
coccolithophorids (Mc Kee et al. 1959; Flood 1983). Reef sediments contain
fractions of rubble particles (> 10 mm), crude sand particles (2-10 mm), of
medium sands (0.5-2mm), and a fine sediment fraction (0.1-0.5mm). The
rubble fraction is formed from the dead coral colonies and from the crusts of
coralline algae eroded by boring animals and then crushed by surf. The
rubble and the sand material are sorted out by surf and currents. Its
fractions are resedimented in definite reef zones. The rubble is accumulating
mostly on the surface of flats where it forms the ramparts, banks and
islands. Large masses of rubble material accumulate in the moat on the
outer reef slope under the reef edge and also in shallows of the large lagoons
exposed to surf action. The most widespread fractions of reef sediments are
medium coral sands with particle sizes of 0.3-0.6mm (Fig. 1.7). The
medium coral sands are formed mainly from calcareous plates of Halimeda,
from the spiecules of soft corals, from shells of forams, and from lime
Reef Lime Constructions
1.3 Bottom Sediments
Reef bottom sediments contain a larger part (ca. 60-80%) of total lime
material of modern and also of the Pleistocene reefs (Thomassin 1978;
Riddle 1988). The benthic biocenoses of the soft bottom are important
components of the coral reef ecosystems. Studies of composition, origin and
distribution of bottom sediments give a remarkable insight into the problems
of reef geology and biology (Purdy 1963; Goreau and Goreau 1973; Maxwell
1968, 1973a; Stoddart 1969). The main part of the lime material composing
the reef bottom sediments is of biological origin (Flood 1983; cf. Table 1.7).
The terrigenous clastic material, consisting of clay, sand, pebbles, and
ferrous oxides, comprises a significant part of sediments (10-15%) only in
lagoons and on the flat of some fringing and barrier reefs (Ginsburg 1956;
Morelock and Koenig 1967; Milliman 1967; Stoddart et al. 1969). As an
example, the reefs off the coasts of Florida, Cuba and GBR could be
mentioned (Stoddart 1969; Maxwell 1968; Riddle 1988). From Table 1.7, it
can be seen that the lime material of sediments has two main sources of
origin. One part (30 to 60%) is formed by the erosion of lime rocks of
modern and ancient reef plateaus and of lime skeletons of animals -
molluscan shells, corals, urchin's needles, etc. This erosion proceeds by the
action of the boring, scraping and grazing reef animals, such as boring
clams, sponges, worms, coral-eating and periphyton scraping fish and
urchins. Thesefore, this part of sediments is formed mainly by "chips" of
borers and by fecal material of coral feeders and scrapers (Emery 1956;
Bakus 1968; Randall 1974; Gygi 1975; Ogden 1977).
The second part of lime reef sediments is formed by small skeletal
elements of reef animals and sea weeds - calcite plates, globules or spicules.
The same elements are contained in sponges, soft corals, holothurians,
asteroids as well as in calcareous algae. The third part is formed by
calcareous microshells of benthic foraminifera and of planktonic
coccolithophorids (Mc Kee et al. 1959; Flood 1983). Reef sediments contain
fractions of rubble particles (> 10 mm), crude sand particles (2-10 mm), of
medium sands (0.5-2mm), and a fine sediment fraction (0.1-0.5mm). The
rubble fraction is formed from the dead coral colonies and from the crusts of
coralline algae eroded by boring animals and then crushed by surf. The
rubble and the sand material are sorted out by surf and currents. Its
fractions are resedimented in definite reef zones. The rubble is accumulating
mostly on the surface of flats where it forms the ramparts, banks and
islands. Large masses of rubble material accumulate in the moat on the
outer reef slope under the reef edge and also in shallows of the large lagoons
exposed to surf action. The most widespread fractions of reef sediments are
medium coral sands with particle sizes of 0.3-0.6mm (Fig. 1.7). The
medium coral sands are formed mainly from calcareous plates of Halimeda,
from the spiecules of soft corals, from shells of forams, and from lime