Page 375 - Carbonate Facies in Geologic History

362 Summary

3. Waulsortian (Lower Carboniferous) mounds in Belgium, England, Ireland, and North
America (Hudson, 1930; Pray, 1958; Lees, 1961, 1964; Troell, 1962; Cotter, 1965.
4. Sponge-algal reefs of the Weiss Jura (Upper Jurassic) of Schwabia (Gwinner, 1962,
1971; Hiller, 1964).
5. Stromatoporoid-tabulate coral lime mud mounds of the Late Devonian (Frasnian) age
in the Dinant basin, Belgium (Lecompte, 1956). Growth upward into wave base results in cap
of stromatoporoid boundstone and development of Type II.
No exact modern analogs of these downslope mud mounds have been de-
scribed, perhaps because we have not searched in the correct places for them.
Such localities might exist north of the Great Pearl Bank on the gentle slope into
the axis of the Persian Gulf.
Interestingly, dives in submersibles in the Florida straits off Bimini have en-
countered loaf-shaped current-oriented piles of fine and coarse lime sediment
supporting growth of abundant organisms (Neumann et al., 1972). These are now
at depths of about 700 m and may be analogous to some downslope bioherms
seen in the geologic record. For example, small mounds of lime mud are seen at
the foot of the Capitan depositional slope in McKittrick Canyon. The actual
depth of the formation of these is dependent on the amount of sea-level drop
during Late Permian time. Their position low on the slope would indicate a
possibility of depth of at least 100-200 m.

Type II. Knoll Reef Ramps

These ramps consist of linear belts of ecologic (framebuilt) knoll reefs on gentle
slopes at the outer edge of the shelf margin. These apparently began growth at the
normal wave base or at positions a little farther downslope at depths of a few tens
of meters. In the absence of strong waves or currents, there was little massive
framebuilding but many sessile and encrusting organisms flourished. The frame-
builders form chiefly branching and fasciculate colonies. Vertical ecologic zoning
is common within the knolls, the growth form changing upward generally to
massive encrusting forms.
The buildups are produced fully as much by organic productivity, binding,
trapping, and encrusting, and by lack of removal of debris as by organic in situ
frame construction. Interreef material may be volumetrically much greater than
the patches of framebuilding organisms. The vast quantity of debris is mostly
bioclastic and was derived from prolific growth on the tops of the reef knolls and
not necessarily from destruction of the organic framework or previously lithified
material. Water energy was sufficient to remove only the finest debris; hence
mostly lime sand accumulated in interreef areas. Cores of the reef knolls com-
monly contain lime mud because in this realm of only moderate wave action, the
framebuilders offered enough protection to prevent its removal. Moderate surge
over the cavernous, vuggy knolls may have provided a suction or inward pump-
ing which helps trap the finer sediment. In most of the examples studied the
shallow ramp has a gentle seaward slope (from a few degrees up to about
15 degrees). From this one might infer that the ramps are a response to mild
tectonic subsidence. Such gentle slopes effectively dampen the most violent wave