Page 29 - Carbonate Facies in Geologic History

16 Principles of Carbonate Sedimentation

sediment in a very few million years. Considerable discrepancy exists when such
rates obtained from deposition on modern tidal flats and reefs are applied to
thicknesses of ancient neritic strata. For example, the Great Bahama Bank should
have 35000-50000m of post-Cretaceous sediment instead of 4500+ (Goodell
and Garman, 1969, p. 528). Since these rates do not jibe with the rates of deposi-
tion of even thickest known ancient carbonate deposits of comparable environ-
ment, we assume that the carbonate producing system operates intermittently and
is very sensitive. That is, it may cease abruptly and start again when conditions
are favorable. Carbonate sedimentation resembles a powerful Cadillac with a
defective carburetor.
This principle, of fast but intermittent deposition, is important in interpreting
thickness and stratigraphic relations in carbonate rocks. These are discussed in
Chapter II, but some of these effects are listed below:
1. When conditions remain favorable, carbonate production can keep up with
almost any amount of tectonic subsidence or eustatic sea level rise. Carbonate
sedimentation may be intermittent, is also commonly diachronous and generally
regressIve.
2. One may expect great irregularities in thickness of carbonate deposits at
certain places, particularly along shelf margins where conditions become opti-
mum for production and accumulation and where subsidence is more continuous.
Commonly, sudden thickening occurs down depositional dip in limestones which,
in thinner beds, have followed for many miles across shelves with no consistent
thickness change. Carbonates may build out and up to great "pods" of sediment
in miogeosynclines. In the Lower Ordovician of North America, great local thick-
nesses occur in the Central Appalachian and Anadarko basins (Arbuckle Moun-
tains of Oklahoma) (Fig. II -8). Similar areas of great local thickness are seen in the
Middle Cretaceous of the Mexican geosyncline .
. 3. The vagaries of carbonate sedimentation make isopach maps of carbonate
units difficult to interpret unless facies information is available. Thick carbonate
accretion can occur over or downflank from positive areas. Very thin carbonate
deposits may occur over strongly positive areas. Slowly subsiding basins may
contain a great thickness of carbonates, while strongly subsiding troughs contain
water too deep for carbonate accumulation and become sediment-starved.
4. Rapid, but intermittent accretion of carbonate banks may be responsible
for much pervasive meteoric diagenesis to which such rocks are subjected. When
subsidence ceases, carbonate sedimentation can build a wide shelf or bank rapidly
to sea level and above; the recently formed sediment is affected by fresh or
hypersaline water depending on climate and geography.

Carbonate Sediments and Rock Are Peculiarly Subject
to Many Stages of Diagenesis

Puzzling and confusing differences appear when a microscope view of Holocene
carbonate sediment is compared with a thin section cut from ancient limestone.
Pure lime mud from a modern lagoon is a creamy, stiff gel of organic slime,