Page 377 - Carbonate Facies in Geologic History

364 Summary

monly such margins are fronted by open oceans. Theoretically, if tectonic down-
warping causes steep enough slopes, an organic barrier might not develop at the
shelf margin and a narrow ramp facies, with high energy belts along the littoral
zone, might be formed (Ahr, 1973). Such narrow fringes at cratonic margins are
rare but recognized as a special category in the classification of tectonic settings
which follows. In most instances, high wave energy and upwelling in turn produce
optimum growth of organic framework even on steep slopes in areas of consider-
able subsidence. Under these circumstances Type III profiles develop, particularly
in the Holocene when the coral, Acropora dominates. When factors inhibit or-
ganic growth at the shelf margins (e.g., overflooding of saline brines from shelf
lagoons), Type I profiles may develop below the narrow fringes at the shelf margin.
Conversely, it appears that in many cases where tectonic downwarping has
been slight, shelf margins display ramps or platforms with Type II knoll reefs. In
areas of moderate subsidence in shelf lagoons the standard belts of carbonate
facies develop ramps with Type II knoll reefs built out in irregular patterns from
margins of intracratonic basins (category 3 of the tectonic classification) or as
fringes around mildly positive areas (category 4).

Carbonate Mounds and Associated Ecologic Reefs

Origin and Orientation of Mounds

Mounds and banks are typical forms of carbonate accumulation in a quiet-water
environment. They usually consist of detrital, poorly sorted, bioclastic micrite
with only minor amounts of organic boundstone. These accumulations formed
either well below wave base or in water so shallow that effective wave action does
not exist. In many places such bodies form the base upon which ecologic reefs
grow. The numerous examples described in Chapters IV -XI show three preferred
geographical positions for such carbonate bodies: distributed in various patterns
in deeper basins, arranged just downslope along Type I shelf margins, and spread
widely in shelf lagoons or shallow basinal areas. The latter occurrence is most
common. The mound shape results from accretion of locally produced carbonate
sediment up to the wave base and under conditions of stable or rising sea level.
These shapes vary from flat lenses to steep, conical piles with slopes of 30-40
degrees. Studies of Holocene mounds and banks in Florida Bay, the Florida
Straits, Yucatan and British Honduras lagoons, and in western Australia have
been instructive in understanding the processes originating and perpetuating the
mound phenomenon.
Modern mounds occur in several orientations-even when distributed across
vast shelf areas. These mounds may occur at random or in crude lineations
encircling shallow ponds or bays. These forms are believed to be responses to very
sluggish currents or gyres set up in shallow water bodies by persistent moderate
winds. Large, elongate, irregular banks are known at the open ends of large
shallow bays in several modern areas: western Florida Bay, Cayo Sucio at the