Diagenesis, Porosity, and Dolomitization - Microfacies of Carbonate Rocks - page 272

Dolomitization is a process whereby limestone or
its precursor sediment is completely or partly converted
to dolomite by the replacement of the original CaCO 3
by magnesium carbonate, through the action of Mg-
bearing water (Sect. 7.8).

transformation of deep-marine calcareous ooze to chalk
and subsequently to limestones is associated with a loss
of porosity with increasing age and depth of burial
(Schlanger and Douglas 1974).
The source of the calcium carbonate necessary for
burial lithification is seen in the dissolution and re-
precipitation of CaCO 3 by pressure solution, or disso-
lution promoted by increasing temperature with increas-
ing depths and overburden, or dissolution/ precipita-
tion of subsurface carbonate rocks by freshwater mov-
ing down an aquifer into the basin.

Major controls on carbonate diagenesis are miner-
alogy and crystal chemistry, the chemistry of pore wa-
ters, water movement, dissolution and precipitation
rates, grain size, and the interaction with organic sub-
stances. The extent and path of diagenetic reactions are
determined by the thermodynamic stability of carbon-
ate minerals being dissolved or precipitated (MacInnys
and Brantley 1992), the saturation state of the diage-
netic fluid and the available surface area for reaction.
Dissolution experiments show that differences in the
surface area (corresponding to the microstructure of
sedimentary grains) appear to be more significant in
controlling relative dissolution rates than mineralogic
stability (Walter 1985). The effect of the parameters
summarized above depends on the saturation stage and
flow rates of the diagenetic fluids (Gonzalez et al. 1992;
Lighty 1985).

7.1.5 Oscillating Trends in Phanerozoic
Carbonate Mineralogy

Dominating mineralogies of marine non-skeletal and
skeletal carbonate constituents have varied during earth
history. Since the primary mineralogy governs early di-
agenetic processes, these long-term variations must be
considered in interpreting carbonate cements, dissolu-
tion patterns and porosity development.

7.1.5.1 Secular Variations

7.1.4 From Soft Sediments to Hard Rocks

Based on ooid and cement composition, Sandberg
(1975, 1983, 1985) presented strong evidence for secu-
lar variations in Phanerozoic non-skeletal carbonate
mineralogy. The concept was discussed and underwent
various modifications (Mackenzie and Pigot 1981;
Wilkinson et al. 1985; Wilkinson and Given 1986; Bur-
ton and Walter 1987; Opdyke and Wilkinson 1990;
Bates and Brand 1990), but most scientists agree on
the inferred existence of calcitic seas typified by the
dominance of calcite mineralogies during greenhouse
times and aragonitic seas typified by aragonite and
High-Mg calcite mineralogies during icehouse times.
The Sandberg curve divides the past 600 million years
into two greenhouse and three icehouse eras (Fig. 7.1A).

To transform soft and friable carbonate muds and sands
into micritic limestones and carbonate sands into grainy
limestones pore spaces must be filled with carbonate
cement. The occlusion of primary depositional porosi-
ties up to 60% and 80%, respectively, requires tremen-
dous amounts of carbonate material. But where does
all this material come from and where does lithifica-
tion take place?
For a certain time, extensive lithification was thought
to be limited to near-surface and subaerial settings
(Friedman 1964; Land et al. 1967; see James and Cho-
quette 1990 for a review).
The comparison of recent and Pleistocene carbon-
ates seems to indicate that subaerial and near-surface
lithification is of overwhelming importance and that
most shallow-marine carbonates were lithified as a re-
sult of dissolution-precipitation effects. At the same time
submarine lithification on or close to the sea bottom
was demonstrated for shallow-marine environments
(tidal flats, reefs) and deep-water settings (Fischer and
Garrison 1967; Ginsburg 1971; see James and Cho-
quette 1983 for overviews). The study of burial lithifi-
cation started in the late 70s and in the 80s of the last
century along with the development of new geochemi-
cal techniques and the investigation of pelagic chalks
(Garrison 1981; Scholle and Halley 1985; see Choquette
and James 1990 and Munnecke 1997 for reviews). The

7.1.5.2 How to Recognize Former Aragonite
and Mg-Calcite Mineralogy in Ancient Low-
Calcite Limestones?

Estimating the starting mineralogy of matrix, carbon-
ate cements, and grains of limestones assists in under-
standing early diagenetic processes and porosity evo-
lution.

Recognizing former aragonite in skeletal grains :
The diagenetic behavior of aragonite shows a surpris-
ing uniformity among skeletons of diverse taxa. Re-