Page 83 - Carbonate Facies in Geologic History

70 Outline of Carbonate Petrography

centripetally within the grain and may completely convert it to micrite. The process
is not fully understood; it is probably biochemical and both aragonite and high Mg
calcite minerals are known to participate in it.
c) Reef cavities contain linings and fillings of both aragonite and high Mg calcite down to
depths of at least 70 m. Layers of coarse druse as well as micritic linings are known. To
what extent such carbonate is organic, inorganic or fully marine is now debated.
d) Cementation of sands just below sediment-marine water interface and creation of hard
grounds has been extensively described both from ancient and recent deposits (Purser,
1969). Many of these instances must be completely marine and represent stillstands in
deposition.
In the Recent Persian Gulf marine hard grounds (Shinn, 1969) the processes of arago-
nite and Mg calcite cementation, mutual mineralogical replacements, and detrital infill-
ing by mud may proceed from the surface to at least half a meter below the surface.
2. Diagenesis in deep marine water.
a) Bathurst lists (1971, Table XVI, p.376) fifteen occurrences of modern to Tertiary ce-
mented globigerinid oozes from the deep sea (from 90 to 3300 m). In most of these
samples the mineral is high Mg calcite but some low Mg calcite is also present.
b) Red nodular limestone with encrusting Mn and Fe minerals. A process of aragonite
and/or calcite subsolution on deep sea bottoms is proposed for creation of these nod-
ules (Garrison and Fischer, 1969). They are common sediment deposited as bathyal
deposits on geosynclinal swells in many Mediterranean-Alpine Jurassic strata. Partial
aragonitic solution and recementation in deep water has also been described from the
Holocene.
3. Diagenesis by meteoric water in the vadose and phreatic (water table) zones.
a) Solution and void-creating mechanisms are as follows:
(1) Solution of aragonite grains above the water table or solution of anhydrite replace-
ments of carbonate grains occurs.
(2) Collapse brecciation occurs if much solution takes place.
(3) In lime muds some flowage of material occurs to form openings such as stromatac-
toid structures.
(4) In peloidal muds fenestral fabric (birdseye structure) occurs through gas escaping
from organic decay concomitant with desiccation.
(5) Solution compaction of lime sands occurs when fresh water percolates through and
dissolves grains in uncemented, loose calcarenites.
b) Void-filling cement types (all low Mg calcite) are varied:
(1) In lime sands: needle fiber Gackstraw) cement, meniscus cement, pendent or micro-
stalactitic cement, coarse to fine blocky (pervasive in phreatic zone), geopetal fillings
of vadose silt, rim cement on echinoderms.
(2) In lime muds: alteration to low Mg calcite and growth of blocky calcite crystals
from aragonite needles. Preservation of some of the original porosity may occur.
There is generally a decrease from 50% (normal original packing porosity) to 25 or
30% when chalky textured rock is formed.
(3) Calcite veins, fissure fillings following joint patterns or lines of weakness in collapse
breccias.
c) Calichification: development of fine granular perhaps microsparry micrite, often
vaguely pelleted or grumelous (clotted) and with wavy lamellar structure or with con-
cretionary pisoids. Micritized grains and faint remnants of incorporated bioclasts are
seen. Circumgranular cracking forms breccias of various sizes. Also coarse blocky
calcite patches or poikilitic crystals occur. Color splotches exist around root hairs and
casts and needle-fibre cement perhaps follows outlines offungi.
d) Mineralogical changes and metasomatic replacements:
(1) The exsolution ofMg from calcite lattices.
(2) Formation of siliceous nodules at old water tables.
(3) Dolomite crusts formed by replacement of aragonitic mud at subaerial exposure
surfaces through processes of evaporation and capillary attraction.
(4) Dedolomitization by CaS0 4 -enriched meteoric water which percolates through
sediment above water table.