Page 38 - Petroleum Geology

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Fig. 1-6. Devonian reefs of western Canada. (Reproduced with permission from Barss et
al., 1970.)

more of the reef-building organisms, such a reef is transgressive by inference
even though the facies do not migrate.
Modern colonial corals, for example, are sensitive to light, water tempera-
ture and salinity; and their depth tolerance is to about 30-45 m (100-150
ft). It cannot be assumed that the same tolerances applied to ancient reef
organisms, and as yet there are no reliable means of determining them. It is
reasonable, however, to assume that their tendency to form wave-resistant
structures, to migrate, and to be exterminated by muddy sediment or
hypersalinity, all indicate a rather restricted range of favourable environ-
mental conditions in which light and salinity played an important part.
Ingels (1963) estimated the water depth around the Silurian Thornton reef
of north-east Illinois to be about 60 m (200 ft), and Terry and Williams
(1969) suggested a similar depth around the Paleocene Intisar “A” bioherm
in Libya (this bioherm was known as “Idris” in the literature from discovery
in 1967 until 1969, “Intisar” thereafter). But depth of water around a reef is
not a reliable indicator of depth of tolerance of the organisms. What is re-