Geology Reference
In-Depth Information
• equant spar forming drusy or intergranular mosaics,
• bladed prismatic calcite spar, dark cathodolumines-
cence. Larger and clearer as compared to first gen-
eration bladed marine cement,
• poikilotopic fabric, characterized by large crystals,
• syntaxial overgrowth cement, most commonly with
echinoderm grains,
• coarse dolomite cement (saddle dolomite and xeno-
topic, characterized by a mosaic of 0.1 to 1 mm-sized
crystals),
• coarse anhydrite cement.
ganic matter; (4) stable carbonate minerals; (5) low di-
agenetic and compaction potential; (6) no possibility
of influx of meteoric water; (7) carbonate source for
cementation provided by pressure solution; (8) expo-
nential decrease with depth and overburden.
Very deep burial diagenesis
Studies of very deeply buried carbonates have been
conducted along two parallel lines: (1) Experimental
compaction of natural carbonate sediments (Bhatta-
charyya and Friedman 1984) and (2) investigation of
carbonate rocks from boreholes using fluid inclusions
and thin-section petrography (Friedman 1987).
Different burial criteria have been found in Paleo-
zoic carbonates studied in deep wells (6 000 to 9 000 m;
temperature around 210°; hydrostatic pressure around
2.5 kilobar: Friedman et al. 1984):
• Grainstones exhibit twinning and cleavage of cal-
cite spar crystals. Mechanical displacement along cleav-
age planes occurs within single echinoderm crystals
causing grain elongation. Twin lamellae appear more
narrowly set in deep burial than in shallow burial set-
tings. Multiple displacement of differently oriented twin
lamellae is common in deep burial samples. Further
criteria are bent twinning lamellae and diminution
within echinoderm single crystal.
• Wacke- and packstones often display no thin-sec-
tion criteria, pointing to alteration of skeletal grains.
Bryozoan and ostracod fragments may have been pro-
tected by the surrounding micrite. The size of micrite
matrix crystals is enlarged up to 30 m. The texture
resembles that of fine-grained marbles and is charac-
terized by patchy extinction.
Equant spar calcite cements (ESC) are character-
ized by equigranular limpid crystals exhibiting a dis-
tinct extinction. Crystal boundaries are irregular, in
places serrated. Crystals are large, up to several milli-
meters. The cements are common in voids, often oc-
cluding the last remaining pore spaces. Interpreting
equant spar calcite cements is difficult, because they
are formed under deep burial conditions as well as in
near-surface environments. A high ferrous iron content
is common in burial ESC, but can also exist in early
cements formed in near-surface anoxic conditions
(Scholle and Halley 1985). A deep-marine origin of
equant spar cement has been reported by Freeman-
Lynde et al. (1986) in Bahama escarpment limestones.
The effect of burial diagenesis on platform and basinal
carbonates:
The burial diagenesis of platform and basinal car-
bonates exhibits similar effects (increase in tempera-
ture and pressure with depth, porosity reduction, squeez-
ing of pore waters, dehydration of minerals, increase
in the influence of saline pore waters, and chemical
compaction following mechanical compaction), but is
different with regard to other criteria:
Platform carbonates are characterized by (1) rapid
sedimentation; (2) variable pore waters ranging in sa-
linity from freshwater and brackish to marine and hy-
persaline; (3) fine- to coarse grained sediments often
associated with evaporites; (4) metastable and stable
carbonate minerals; (5) variable diagenetic and com-
paction potential; (6) possibility of the influx of mete-
oric water; (7) carbonate source for cement provided
initially provided by meteoric dissolution, later by pres-
sure solution; (8) decreasing porosity with depth and
overburden, but strongly varying with pore water flux
rates and different compaction intensities depending
from different primary sedimentary fabrics.
Basinal carbonates are characterized by (1) slow
sedimentation; (2) initially marine pore waters; (3) fine-
grained sediments often associated with clay and or-
7.4.4 Facies-Controlled Diagenesis
Cementation is strongly facies-related. Marine ce-
mentation can be traced from beaches to deeper water
and depends on the porosity and fabric of the sediment.
Beaches and tidal settings are characterized by alter-
nating meteoric-vadose, marine-vadose, and marine
conditions that contribute to the formation of beach-
rocks, hard carbonate crusts and diverse vadose and
marine cement types. Common cements are micro-
crystalline, bladed or fibrous Mg-calcite, and arago-
nite cements.
7.4.4.1 Carbonate Platforms and Ramps
The spatial distribution of interparticle cement in
carbonate sediments is primarily a function of the en-
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