Geological History, Features of Geomorphology
7
The earliest taxa of these had small colonies (1O-30cm crosswise).
Nevertheless, they soon became basic elements of the frameworks in reefs
which grew in that period (Hill and Stumm 1956; Koroljuk and Mikhailova
1986). Stromatoporide - tabulate reef rocks - are common in SilurianDevonian sedimental facies (470-340 x 10 6 years B.P.). Their size runs to
several kilometers with a thickness of 50 to 100 m (Lecompte 1956; Sokolov
1986). Rugose corals had firm colonies withstanding the wave stress
(Sokolov 1955). With their appearance the processes of reef formation and
growth expanded and large reef constructions appeared extending hundreds
of kilometers. Where tectonic subsidence look place the growth of these
Devonian rugose-based reefs resulted in the construction of 1000 m and
more of thick reefal massifs (Nalivkin 1967). At the end of the Devonian
corals became extinct due to the start of the cold period followed by
glaciations and the drop of the sea level (Copper 1974). The growth of reefs
then started anew only in the Triassic, but initially they were again structured like the biostromes with calcareous sponges as basis for their frameworks (Wilkinson 1983b), and with calcareous algae as main producers of
carbonaceous filling material. Then in the Middle Triassic the hermatypic
scleractinian corals appeared at the tropical coasts. Among them there were
colonial as well as solitary forms. The most common of these were the
families Thamnasteriidae, Astrocoeniidae, Conophyllidae, Montlivaltiidae,
and Actinacididae (Sokolov 1955; Krasnov 1976; Sokolov 1986). It was
supposed that they had descended from various taxa of rugoses. But the
contention that all basic branches of hard corals, be it tabulates, rugoses or
scleractinians, represented independent evolutionary lines of the ancient
polypoid actinians (Wells 1956) seems to be more acceptable. They became
extremely widespread among neritic fauna of the Thetis and then in all
tropical zones (Potts 1983).
At mid-Late Jurassic scleractinians created large reef constructions on
vast areas of tropical shelves. But in some places the thick reefs (up to 50m)
were built up also by sabellarid polychaetes and plecypodes. Reefs grew
extensively also during the Cretaceous when the fast evolution of
scleractinians simultaneously proceeded. At the end of this period many of
their modern families appeared. A significant role in the formation of these
reefs was played also by hydrocorals like milleporides and stylasterids,
which flourished on the reef of the Late Cretaceous (Boshma 1956). In the
Paleogene-Eocene many new modern families of scleractinians appeared.
After some interruption, which took place during that period, .the active
growth of reefs started again and acquired the features of modern reefs.
Their thickness in some places attained 600 m. In their formation, besides
corals, also corallines, calcareous green algae and foraminiferans
participated, forming the basic mass of carbonaceous material. The growth
of reefs proceeded also in the Pleistocene, but with interruptions caused by
large oscillations of the sea level in this period. In Fig. 1.2 the scheme of the
geological history of the main reef-building is shown.
7
The earliest taxa of these had small colonies (1O-30cm crosswise).
Nevertheless, they soon became basic elements of the frameworks in reefs
which grew in that period (Hill and Stumm 1956; Koroljuk and Mikhailova
1986). Stromatoporide - tabulate reef rocks - are common in SilurianDevonian sedimental facies (470-340 x 10 6 years B.P.). Their size runs to
several kilometers with a thickness of 50 to 100 m (Lecompte 1956; Sokolov
1986). Rugose corals had firm colonies withstanding the wave stress
(Sokolov 1955). With their appearance the processes of reef formation and
growth expanded and large reef constructions appeared extending hundreds
of kilometers. Where tectonic subsidence look place the growth of these
Devonian rugose-based reefs resulted in the construction of 1000 m and
more of thick reefal massifs (Nalivkin 1967). At the end of the Devonian
corals became extinct due to the start of the cold period followed by
glaciations and the drop of the sea level (Copper 1974). The growth of reefs
then started anew only in the Triassic, but initially they were again structured like the biostromes with calcareous sponges as basis for their frameworks (Wilkinson 1983b), and with calcareous algae as main producers of
carbonaceous filling material. Then in the Middle Triassic the hermatypic
scleractinian corals appeared at the tropical coasts. Among them there were
colonial as well as solitary forms. The most common of these were the
families Thamnasteriidae, Astrocoeniidae, Conophyllidae, Montlivaltiidae,
and Actinacididae (Sokolov 1955; Krasnov 1976; Sokolov 1986). It was
supposed that they had descended from various taxa of rugoses. But the
contention that all basic branches of hard corals, be it tabulates, rugoses or
scleractinians, represented independent evolutionary lines of the ancient
polypoid actinians (Wells 1956) seems to be more acceptable. They became
extremely widespread among neritic fauna of the Thetis and then in all
tropical zones (Potts 1983).
At mid-Late Jurassic scleractinians created large reef constructions on
vast areas of tropical shelves. But in some places the thick reefs (up to 50m)
were built up also by sabellarid polychaetes and plecypodes. Reefs grew
extensively also during the Cretaceous when the fast evolution of
scleractinians simultaneously proceeded. At the end of this period many of
their modern families appeared. A significant role in the formation of these
reefs was played also by hydrocorals like milleporides and stylasterids,
which flourished on the reef of the Late Cretaceous (Boshma 1956). In the
Paleogene-Eocene many new modern families of scleractinians appeared.
After some interruption, which took place during that period, .the active
growth of reefs started again and acquired the features of modern reefs.
Their thickness in some places attained 600 m. In their formation, besides
corals, also corallines, calcareous green algae and foraminiferans
participated, forming the basic mass of carbonaceous material. The growth
of reefs proceeded also in the Pleistocene, but with interruptions caused by
large oscillations of the sea level in this period. In Fig. 1.2 the scheme of the
geological history of the main reef-building is shown.