Page 27 - Carbonate Facies in Geologic History

14 Principles of Carbonate Sedimentation

and widely-applied classification of Folk (1959, 1962) also designates class names
based partly on size. A biomicrite in "Folklore" or a bioclastic lime wackestone in
"Dunhamese" becomes, with coarser grains (e.g., oyster shells entombed in a mud
matrix), a biomicrudite or bioclast floatstone.
Grain shape is a useful parameter to describe, provided one recognizes grain
type. Grains from gastropods and crinoid stems are formed round and not subject
to the same consideration as originally angular mollusk shell fragments which
have been rounded and coated in agitated water.

5. Biogenically Precipitated Carbonate Masses

These form an additional textural category treated in all classifications. Folk's
term biolithite is essentially equivalent to boundstone of Dunham whose classifica-
tion scheme offers three signs of binding which may be used as evidence: visible
construction of organic framework, stromatolitic lamination contrary to gravity,
and the presence of roofed over, sediment-floored cavities which appear to have
been organically constructed. Such cavities may be recognized even when an
organic frame is not. They are of improper shape to be solutional and too large to
be normal interstices. Embry and Klovan (1971) added to Dunham's boundstone
category, attempting a genetic interpretation of the type of binding.
Bafflestone: Sediment with abundant stalk-shaped (dendroid) fossil remains
which are interpreted to form baffle for matrix accumulation; matrix is volumetri-
cally important; commonly ill-sorted.
Bindstone: Tabular-lamellar organisms binding and encrusting a large
amount of matrix. No self-supporting organic fabric.
Framestone: In situ massive fossils which construct a rigid framework; matrix,
cement, or void space fills in the framework.

Carbonate Deposition Is Rapid but Is Easily Inhibited
and Therefore Sporadic during Geologic Time

It is commonly stated that carbonate deposition is very slow compared to that of
terrigenous or evaporite sedimentation. Indeed, when one compares great deltaic
sections such as that within the south Louisiana Tertiary depocenter with maxi-
mum thickness of carbonate banks in the same marine basin, for example, the
Florida Shelf or Great Bahama Bank, one finds about twice as much terrigenous
clastic as carbonate sediment from earliest Cretaceous to the present day. But,
actually, rates of shallow neritic carbonate sedimentation derived from deposited
thicknesses in the last 5000 years are extremely fast. Table 1-1 compares these
from the Caribbean, Gulf of Mexico, Atlantic and Persian Gulf areas. The Persian
Gulf tidal flats are building out so fast that they should be 100-200 km wide in
100000 years (Kinsman, 1969, p. 839), filling in the whole Trucial Coast embay-
ment. Indeed, volumetric calculations show that the whole Persian Gulf, includ-
ing the 90 m deep axial trough, could well be buried by lagoonal and intertidal