12
Reef Lime Constructions
could be mentioned: (1) the size and the geomorphology of the Pleistocene
reef platform or other bottom elements which started their growth in the
middle phase of the Holocene transgression; (2) the rate of rise of the sea
level in juxtaposition to the shoreline during the Holocene transgression; (3)
the absolute time when the growing reef reached the stabilized sea surface at
the end of the transgression, i.e., the time when their vertical growth
stopped (Davies 1983). All these factors varied in different regions, thus
resulting in a great variety of geomorphological features of modern reefs
(Thorn and Chappell 1975; Davies 1983; Guilcher 1988). They are
represented by several types, such as atolls, fringing and barrier reefs,
ribbon and planular reefs, sand cays and coral banks. Their genetical
classification was developed by Maxwell (1968). But it has not been
confirmed by subsequent geological data (Purdy 1974; Thorn et al. 1978;
Hopley et al. 1978). Later more adequate classification was presented by
Hopley (1982, 1983). This classification accounts for the interaction of the
main factors in the genesis of Holocene reefs, such as the geological (size
and geomorphology of Pleistocene platform bases on which they grew, the
rate of transgression), the biological (conditions of coral growth and its rate)
and the physical (mechanical erosion, transport of sediments, etc.) factors.
This classification concerns only the shelf reefs developing in the Great
Barrier Reef region. But it contains the nesessary background for
developing a universal genetic classification of modern coral reefs. By this
classification all modern reefs are divided into four main groups depending
on size and structure of the base platforms on which they have grown.
The first group includes the ring reefs, which have grown up on large
Pleistocene platforms more than 3.25 km wide. Their surface had a complex
relief which included several Karst depressions. On these platforms large
crescent or ring reefs (atolls) grew up which had several lagoons mimicking
the former depressions of their ancient bases (Fig. 1.5). Later their lagoons
were filled with rubble and sand, and overgrown with patch reefs. Finally, at
the latest stage of their evolution they transformed into flat planar reefs.
The second group of average-sized ring reefs grew up on ancient platforms
less than 3.25 km wide. They underwent actually the same evolution, but
had only one central lagoon. To the third group belong the small round or
ribbon planar reefs which have grown up on the small platforms less than
1.75 km wide or on narrow ridges left over from ancient reefs after their
long exposure. Their peculiar feature is the absence of lagoons. As they
were reaching the stabilized sea-level during their growth, the depressions
on the surface of their bases were also filled with sediments. Finally, the
fourth group of reefs had no Pleistocene base at all and grew up on hard
rocky bottom elements in the tropical shelf. They usually do not form
consolidated structures, often being represented by banks or cays. Some of
these reefs resemble in their geomorphology and evolution the planer reefs
of the third group. The reefs of this group are relatively rare in the IndoPacific, but are rather widespread in the Western Atlantic region. The
Reef Lime Constructions
could be mentioned: (1) the size and the geomorphology of the Pleistocene
reef platform or other bottom elements which started their growth in the
middle phase of the Holocene transgression; (2) the rate of rise of the sea
level in juxtaposition to the shoreline during the Holocene transgression; (3)
the absolute time when the growing reef reached the stabilized sea surface at
the end of the transgression, i.e., the time when their vertical growth
stopped (Davies 1983). All these factors varied in different regions, thus
resulting in a great variety of geomorphological features of modern reefs
(Thorn and Chappell 1975; Davies 1983; Guilcher 1988). They are
represented by several types, such as atolls, fringing and barrier reefs,
ribbon and planular reefs, sand cays and coral banks. Their genetical
classification was developed by Maxwell (1968). But it has not been
confirmed by subsequent geological data (Purdy 1974; Thorn et al. 1978;
Hopley et al. 1978). Later more adequate classification was presented by
Hopley (1982, 1983). This classification accounts for the interaction of the
main factors in the genesis of Holocene reefs, such as the geological (size
and geomorphology of Pleistocene platform bases on which they grew, the
rate of transgression), the biological (conditions of coral growth and its rate)
and the physical (mechanical erosion, transport of sediments, etc.) factors.
This classification concerns only the shelf reefs developing in the Great
Barrier Reef region. But it contains the nesessary background for
developing a universal genetic classification of modern coral reefs. By this
classification all modern reefs are divided into four main groups depending
on size and structure of the base platforms on which they have grown.
The first group includes the ring reefs, which have grown up on large
Pleistocene platforms more than 3.25 km wide. Their surface had a complex
relief which included several Karst depressions. On these platforms large
crescent or ring reefs (atolls) grew up which had several lagoons mimicking
the former depressions of their ancient bases (Fig. 1.5). Later their lagoons
were filled with rubble and sand, and overgrown with patch reefs. Finally, at
the latest stage of their evolution they transformed into flat planar reefs.
The second group of average-sized ring reefs grew up on ancient platforms
less than 3.25 km wide. They underwent actually the same evolution, but
had only one central lagoon. To the third group belong the small round or
ribbon planar reefs which have grown up on the small platforms less than
1.75 km wide or on narrow ridges left over from ancient reefs after their
long exposure. Their peculiar feature is the absence of lagoons. As they
were reaching the stabilized sea-level during their growth, the depressions
on the surface of their bases were also filled with sediments. Finally, the
fourth group of reefs had no Pleistocene base at all and grew up on hard
rocky bottom elements in the tropical shelf. They usually do not form
consolidated structures, often being represented by banks or cays. Some of
these reefs resemble in their geomorphology and evolution the planer reefs
of the third group. The reefs of this group are relatively rare in the IndoPacific, but are rather widespread in the Western Atlantic region. The