10
Reef Lime Constructions
sea-level was 20-30 m below the present one. Therefore, all the modern
Holocene reefs grown on the drowned ones during the transgression of the
ancient platforms have a thickness of only 5-20 m as an average (Adey.
1978). At the GBR (Great Barrier Reef), the longest discovered, their
thickness was 23m (Hopley 1983; Davies 1983), and in the Caribbean 33m
(reef Alearan, Macintyre et al. 1977). Thus, the modern coral reefs form
only a thin veneer over the hundreds of meters thickness of old Pleistocene
platforms. The rocks at the base of modern reefs at a depth of 10-20 m
under their surface have an age of 5-10 x 10 3 years shown by isotopic
dating. Several meters below, the rocks of their Pleistocene base have an
age of 1-2 x 10 5 years (Thurber et al. 1965).
The upper layer of these ancient reef platforms, having been exposed for
about 1 x 10 5 years, was subjected to various kinds of aerial and karst
erosion (Purdy 1974). The growth of Holocene reefs started and proceeded
most actively at the elevated places of their relief. In depressions and
funnels, where the sediments were accumulating, the growth of corals was
inhibited by siltation. Later on, in depressions the lagoons were formed
(Davies and Kinsey 1977; Hopley 1983). When the growing reefs were
approaching the stabilized sea-level at 5-4 x 10 3 years B.P. their growth
slowed down from 5-1Om in 10- 3 years to 1-4m in 10- 3 years. Upon
reaching the surface, growth stopped and the erosion of their surface,
exposed during ebb tides, started. This resulted in the formation of
consolidated reef flats. The rate of reef growth in the depressions was slow:
0.5-1.5 m in 10 3 years. Therefore, in the initial phase of growth in the
Holocene in these places, usually deep lagoons were forming (Fig. 1.4).
After the elevated edges of the reefs reached the sea surface and stopped
growing, the lagoons gradually filled with sediment. At a later phase in the
evolution of such reefs these shallow lagoons became covered by patch reefs
and the edges of their reef flat gradually spread over the lagoon. This
process is going on now on numerous modern reefs in the Indo-Pacific.
Later, such a reef will reach the phase of an old platform or table reef (Fig.
1.5). Thus, among the main geological factors which controlled the growth
rate of modern reefs and determined their future forms and shapes there
Fig. 1.5. Morphology of some Holocene reefs of the GBR region at different phases of
their evolution; designations on figures are given as combinations of numbers (1 to 3) and
letters (A, B, C); the numbers correspond to size or type of ancient Pleistocene platform
bases of modern reefs: 1 medium-sized platform (1.75-3.25km); 2 small platform
«1.75 km); 3 stretched ribbon-like platform; the letters correspond to the evolution
phases of modern reefs: A Pleistocene platform at the end of the last decrease in sea level
(SL); B phase of mature crescent or ring reef (atoll); C phase of old platform reef. Facies
of sedimented rocks: 1 Pleistocene platform; 2 submerged part of reef or lagoon; 3 corals;
4 soft sediments; 5 algal flat (coralline algae); 6 sandy flat; 7 rubble; 8 ridges of gravel; 9
karst hillocks; 10 karst depressions on the Pleistocene platform; 11 sand cays. (After
Hopley 1983)
Reef Lime Constructions
sea-level was 20-30 m below the present one. Therefore, all the modern
Holocene reefs grown on the drowned ones during the transgression of the
ancient platforms have a thickness of only 5-20 m as an average (Adey.
1978). At the GBR (Great Barrier Reef), the longest discovered, their
thickness was 23m (Hopley 1983; Davies 1983), and in the Caribbean 33m
(reef Alearan, Macintyre et al. 1977). Thus, the modern coral reefs form
only a thin veneer over the hundreds of meters thickness of old Pleistocene
platforms. The rocks at the base of modern reefs at a depth of 10-20 m
under their surface have an age of 5-10 x 10 3 years shown by isotopic
dating. Several meters below, the rocks of their Pleistocene base have an
age of 1-2 x 10 5 years (Thurber et al. 1965).
The upper layer of these ancient reef platforms, having been exposed for
about 1 x 10 5 years, was subjected to various kinds of aerial and karst
erosion (Purdy 1974). The growth of Holocene reefs started and proceeded
most actively at the elevated places of their relief. In depressions and
funnels, where the sediments were accumulating, the growth of corals was
inhibited by siltation. Later on, in depressions the lagoons were formed
(Davies and Kinsey 1977; Hopley 1983). When the growing reefs were
approaching the stabilized sea-level at 5-4 x 10 3 years B.P. their growth
slowed down from 5-1Om in 10- 3 years to 1-4m in 10- 3 years. Upon
reaching the surface, growth stopped and the erosion of their surface,
exposed during ebb tides, started. This resulted in the formation of
consolidated reef flats. The rate of reef growth in the depressions was slow:
0.5-1.5 m in 10 3 years. Therefore, in the initial phase of growth in the
Holocene in these places, usually deep lagoons were forming (Fig. 1.4).
After the elevated edges of the reefs reached the sea surface and stopped
growing, the lagoons gradually filled with sediment. At a later phase in the
evolution of such reefs these shallow lagoons became covered by patch reefs
and the edges of their reef flat gradually spread over the lagoon. This
process is going on now on numerous modern reefs in the Indo-Pacific.
Later, such a reef will reach the phase of an old platform or table reef (Fig.
1.5). Thus, among the main geological factors which controlled the growth
rate of modern reefs and determined their future forms and shapes there
Fig. 1.5. Morphology of some Holocene reefs of the GBR region at different phases of
their evolution; designations on figures are given as combinations of numbers (1 to 3) and
letters (A, B, C); the numbers correspond to size or type of ancient Pleistocene platform
bases of modern reefs: 1 medium-sized platform (1.75-3.25km); 2 small platform
«1.75 km); 3 stretched ribbon-like platform; the letters correspond to the evolution
phases of modern reefs: A Pleistocene platform at the end of the last decrease in sea level
(SL); B phase of mature crescent or ring reef (atoll); C phase of old platform reef. Facies
of sedimented rocks: 1 Pleistocene platform; 2 submerged part of reef or lagoon; 3 corals;
4 soft sediments; 5 algal flat (coralline algae); 6 sandy flat; 7 rubble; 8 ridges of gravel; 9
karst hillocks; 10 karst depressions on the Pleistocene platform; 11 sand cays. (After
Hopley 1983)