Page 376 - Carbonate Facies in Geologic History

Speculative Relationships between Types of Shelf Margins and Tectonics 363

action. Shoals and islands of laminated, tidal flat lime muds and sands generally
lie behind such reef flats. Examples are:
1. Modern ledge-flat reefs of Bermuda, located atop the Bermuda platform (Verril, 1907).
2. Rudist reefs at the shelf margins in the Middle Cretaceous of Mexico, subsurface of south
Texas, and the Middle East (Bonet, 1952; Nelson, 1959; Griffith et aI., 1969; Harris et aI.,
1968.
3. Maim reefs of Swiss and French Jura Mountains developed in places into Type III frame-
built rims (P. Ziegler, 1956; M. Ziegler, 1962; Bolliger and Burri, 1970).
4. 1hecosmilia-sponge-spongiomorph Late Triassic reefs of the Northern Limestone Alps of
Austria and Bavaria (Fabricius, 1966, Zankl, 1969). In places these developed into Type III
framebuilt rims with slopes up to 20 degrees.
5. Stromatoporoid-tabulate coral reefs of Upper and Middle Devonian age in western Can-
ada (Klovan, 1964; Dooge, 1966; Fischbuch, 1968).

Type III. Framebuilt Reef Rims

This is a linear belt of organic reef frame growing up to sea level or into the zone
of turbulence. Submarine bars and shoals of lime sand exist behind the reef, filling
the lagoon and in places even forming islands. These may be barrier or fringing
reefs and are zoned ecologically in parallel belts according to their coral growth
form. They are essentially due to the growth of hexacorals and associated stabiliz-
ing red algae and are chiefly Mesozoic to Holocene in age. Such reefs commonly
possess steep slopes (more than 45 degrees or even vertical scarps) and much talus
debris. Examples are known of transition from a complex of reef knolls in water of
lower energy (Type II) to an active framebuilt reef (Type III).
Examples are:
1. Modern hexacoral reefs. Zonation exists from large rounded or sheety colonial corals in
water from 70 to 10 m deep to Acropora species building into the zone of the wave-action,
to a coralline algae flat of cemented rubble behind the reef front. Usually a sand shoal area
exists behind the "Lithothamnion" flat.
2. Some Maim reefs of Swiss and French Jura Mountains developed from Type II.
3. Steinpiatte, a Late Triassic 1hecosmilia and sponge reef of the Northern Limestone Alps in
Austria and Bavaria (Ohlen, 1959; Fabricius, 1966; Zankl, 1971). This reef growth devel-
oped in places over a complex of knoll reefs of Type II.
4. Kirkuk fringing reef of Middle Tertiary age in Iraq (Henson, 1950; Van Bellen, 1956;
Dunnington, 1958).
5. Canning Basin-Late Devonian reef complexes, western Australia (Playford and Lowry,
1966). These reefs possess a 30-35 degree slope and well developed talus. In Alberta,
Canada, beds of exactly the same age and bearing the same organisms develop shelf
margins of very low angle and are generally of Type II.

Speculative Relationships between Types of Shelf Margins and Tectonics

Given a reasonable amount of subsidence, hydrologic and climatic controls ap-
pear more important than tectonics in determining which type of shelf margin
develops. Buildups which are formed in areas of more or less major and continued
subsidence, display all three types of shelf margins. These include margins of great
offshore banks and edges of major platforms built out into miogeosynclines. The
seaward slopes are steep in these cases, owing to basement faulting, and com-