Geological History, Features of Geomorphology
13
barrier reefs near the high islands within this classification correspond to the
mature ring reefs, and the fringing reefs to the late phase of their evolution
- e.g., to the planar reefs - which is just the opposite of Darwin's theory.
But in areas of active modern tectonic movements they are also an important factor in reef evolution and growth (Chappell 1983), as it was supposed
by Darwin. The transgression interacted in such areas with the tectonic
movements, and the direction of the latter was largely regulating the formation of the modern reefs which grew there (Thorn and Chappell 1978).
For example, because of transgression being simultaneous with tectonic
subsidence the modern stabilized sea-level in the Carribbean reef region was
reached at about 1000 years B.P., while in the Indo-Pacific it happened 5-4
x 10 3 years B.P.
The reefs grown up on ancient reef platforms were definitely developing
the topography of their base, which largely predicted their modern morphology. These reefs are classified as "structural." In fact, they have quite
regular elements of structure, such as reef edge, algal ridge, reef flat, and
patch-reef zone. These elements are disposed on structural reefs as stripes,
which are situated normally in the direction of the dominating wind and
wave stress (Stoddart 1969; Guilcher 1988). Each stripe or zone represents
also a definite local ecological unity, characterized by a definite selection of
key species in its biota. Actually, they are a kind of linear biocenoses or
biotopes formed within a definite geomorphological structure of the reef.
The zonal distribution of biocenoses on the structural reefs is created by
physical stress and light distribution in their interaction with the reef biota
(Graus and Macintyre 1989). The zonal structure makes an important background for describing the composition of reef biota and the distribution of
its main components, as well as of their functional roles within the reef
ecosystem (Yonge 1940; Stoddart 1969; Pichon and Morrissey 1981). The
description and classification of zones were developed by Ladd (1977) for
the Pacific atolls, by Maxwell (1968) for the GBR, by Pichon (1974) for the
Barrier reef of Tulear, Madagascar, by Goreau and Goreau (1973) for
Caribbean reefs. A general geomorphology of coral reefs was reviewed by
Guilcher (1988). In Fig. 1.6 schemes are shown of the profiles across several
main types of reef which demonstrate the position of zones and provide the
terminology used for their designation (Leontjev et al. 1979). Among these
profiles with an outer slope across the reef to the leeward of its side (or to
landward on the Barrier reef) the following main zones should be mentioned:
1. Zones of outer windward slope (the ranges of depths are given as
averages)
a) Fs - Reef slope zone, depth 200-80m. It is usually formed by the
steep rocky slope of the ancient reef base platform, cut by grooves and
ditches - signs of former aerial and then underwater erosion. The ridges
between grooves are overgrown mostly by ahermatypic corals and
sponges.
13
barrier reefs near the high islands within this classification correspond to the
mature ring reefs, and the fringing reefs to the late phase of their evolution
- e.g., to the planar reefs - which is just the opposite of Darwin's theory.
But in areas of active modern tectonic movements they are also an important factor in reef evolution and growth (Chappell 1983), as it was supposed
by Darwin. The transgression interacted in such areas with the tectonic
movements, and the direction of the latter was largely regulating the formation of the modern reefs which grew there (Thorn and Chappell 1978).
For example, because of transgression being simultaneous with tectonic
subsidence the modern stabilized sea-level in the Carribbean reef region was
reached at about 1000 years B.P., while in the Indo-Pacific it happened 5-4
x 10 3 years B.P.
The reefs grown up on ancient reef platforms were definitely developing
the topography of their base, which largely predicted their modern morphology. These reefs are classified as "structural." In fact, they have quite
regular elements of structure, such as reef edge, algal ridge, reef flat, and
patch-reef zone. These elements are disposed on structural reefs as stripes,
which are situated normally in the direction of the dominating wind and
wave stress (Stoddart 1969; Guilcher 1988). Each stripe or zone represents
also a definite local ecological unity, characterized by a definite selection of
key species in its biota. Actually, they are a kind of linear biocenoses or
biotopes formed within a definite geomorphological structure of the reef.
The zonal distribution of biocenoses on the structural reefs is created by
physical stress and light distribution in their interaction with the reef biota
(Graus and Macintyre 1989). The zonal structure makes an important background for describing the composition of reef biota and the distribution of
its main components, as well as of their functional roles within the reef
ecosystem (Yonge 1940; Stoddart 1969; Pichon and Morrissey 1981). The
description and classification of zones were developed by Ladd (1977) for
the Pacific atolls, by Maxwell (1968) for the GBR, by Pichon (1974) for the
Barrier reef of Tulear, Madagascar, by Goreau and Goreau (1973) for
Caribbean reefs. A general geomorphology of coral reefs was reviewed by
Guilcher (1988). In Fig. 1.6 schemes are shown of the profiles across several
main types of reef which demonstrate the position of zones and provide the
terminology used for their designation (Leontjev et al. 1979). Among these
profiles with an outer slope across the reef to the leeward of its side (or to
landward on the Barrier reef) the following main zones should be mentioned:
1. Zones of outer windward slope (the ranges of depths are given as
averages)
a) Fs - Reef slope zone, depth 200-80m. It is usually formed by the
steep rocky slope of the ancient reef base platform, cut by grooves and
ditches - signs of former aerial and then underwater erosion. The ridges
between grooves are overgrown mostly by ahermatypic corals and
sponges.