Geology Reference
In-Depth Information
Features characterizing cements include:
raphy
. Because crystal shapes observed under the light
microscope are often a product of neomorphism (Sect.
7.6), a combination of conventional light microscopy
and cathodoluminescence microscopy is strongly rec-
ommended. The two methods permit distinctly differ-
ent levels of observations. Light microscopy exhibits
the optical properties as well as shapes and fabrics of
crystals under transmitted or polarized light. Cathodo-
luminescence microscopy offers a possibility to differ-
entiate between primary growth forms of the cement
crystals, cements which have been affected by recrys-
tallization, and other neomorphism processes (Pl. 5).
•
often clear and clean appearance, with well-defined,
often (but not always) straight crystal boundaries,
•
sharp contact between spar and particles,
•
spar between grains does not penetrate into or cut
across grains,
•
presence of two or more generations of spar,
•
straight crystal edges and frequent triple junctions
with 180° angles (enfacial junctions),
•
long axes of crystals often normal to grain surfaces,
•
increasing crystal size away from grain surface.
Cements represent an important record of the di-
agenetic history of carbonate rocks. Studies of thin sec-
tions are of paramount importance in recognizing di-
agenetic environments and determining paragenetic se-
quences in carbonate rocks. Success in reconstructing
the diagenetic history depends heavily on the ability to
recognize the primary crystal shapes of the cements
and their time succession as reflected by
cement stratig-
Cathodoluminescence facies
, defined by crystal
shape, fabric and luminescence, is increasingly being
used to interpret diagenetic environments (Lavoie and
Bourque 1993). The data derived from petrographical
analysis must be also combined with geochemical data
(stable isotopes, and trace and minor elements). Oxy-
gen and carbon isotopes have proven extremely valu-
Plate 31 Modern Marine and Meteoric (Freshwater) Cementation: SEM and ThinSection Criteria
Marine pore-filling carbonate cements are precipitated from environments ranging from marine to meteoric
to burial settings. Shallow-marine cements consist of aragonite (-> 1, 2) and High-Mg calcite cements (-> 3,
4). Calcite cements ( -> 5, 6) are common in the subaerial and near-surface meteoric as well as in burial environ-
ments. Examples are from Fuerteventura, Canary Islands (Spain). Modern beach sands along the arid coasts of
the Atlantic island consist of volcanic and carbonate grains. The sands are cemented by High-Mg calcite or
aragonite into beachrocks (-> 3-4) forming sheets up to 30 cm in thickness. Cementation takes place in the
intertidal zone and along the shores of wind-protected bays. Most skeletal grains are fragments of red algae,
mollusks, foraminifers and echinoderms. Precipitation of the cements is caused by the evaporation of sea water
in interparticle pores at low tide. Subaerially exposed Pleistocene dune sands exhibit minor meteoric cemen-
tation (-> 5-6).
1
Aragonite
fibrous cement
. The crystals form a meshwork of needles. This type of crystal is not commonly recognized in
ancient carbonate rocks. Marine-phreatic zone. SEM.
2
Thin section of -> 1.
Fibrous cement
rim covers rounded litho- and bioclasts. Note the longer crystals along grain contacts
and within narrow intergranular voids (appearing black).
3
Microcrystalline High-Mg calcite cement
. Closely spaced rhombohedral crystals, sometimes forming rosette-like aggre-
gates. Note the conspicuous open intercrystalline porosity. Beachrock, formed at the interface of land and sea within the
marine-vadose
environment. SEM.
4
Thin section of -> 3. Equant crusts of
microcrystalline High-Mg calcite
cement (black arrows) around carbonate and non-
carbonate grains. A part of the open interparticle voids (stippled white) is occluded by automicrite connecting micro-
grained sedimentary particles (black). Beachrock. Microbial automicrite is common in intertidal solution cavities of
recent beachrocks (Neumeier 1999).
5
Dogtooth calcite cement.
Scalenohedral and elongated rhombohedral crystals with blunted terminations. Length/width
ratios are approximately 3:1. Note that dogtooth cement can no longer be taken as unequivocal evidence of meteoric
environments, because it also occurs under marine-phreatic, shallow-burial and hydrothermal conditions (Reinhold 1999).
Meteoric horizons in Pleistocene eolian calcarenites. SEM.
6
Thin section of -> 5.
Dogtooth cement
developed on coralline algal bioclasts (CA) and non-carbonate grains. In the
meteoric phreatic zone below the water table, pores are filled with water and large cement crystals can grow on all
surfaces. Note different spacing and different length of the crystals. Arrows point to broken crystals indicating dissolution
or preparation effects. Open interparticle pores appear black.
