Geology Reference
In-Depth Information
formed in the upper part of the vadose zone can be trans-
ported in suspension and deposited in voids as internal
silt-sized sediment that may show grading and small-
scale cross-lamination. Crystal silt may filter down 1-
2 m below submerged paleosurfaces (Aissaoui and
Purser 1983) and is often used as an indication of va-
dose diagenetic conditions (Fig. 7.11). This interpreta-
tion should be applied with some caution, because simi-
lar or identical silt-sized internal fillings are also known
from burial settings.
7.4.2.2 Cement Fabrics
By examining the spatial relations between cements,
grains, substrate and pores, cement fabrics can be sub-
divided into five groups (Fig. 7.12):
1 Symmetrical cements around grains represented by
isopachous and circumgranular cement rims, distin-
guished by the cement type and thickness of the ce-
ment rim.
2 Asymmetrical cements concentrated on the under-
side of grains or developed at grain contacts.
3 Cement crusts, fans and hemispheres forming milli-
meter- to centimeter thick structures.
4 Calcite mosaics, differentiated by size, orientation
and the type of crystal boundaries.
5 Syntaxial overgrowth cement, usually associated
with echinoderm grains.
Fig. 7.11.
Crystal silt within a solution void.
The void is lined
with scalenohedral dogtooth cement (arrows point to the vari-
ously shaped terminations of crystals) and was subsequently
filled with very small calcite crystals. Note the conspicuous
textural difference between the silt and the dark microcrys-
talline matrix of the limestone. The void was formed in car-
bonates representing the slope between a shallow-marine plat-
form and a basin. Dogtooth cement and crystal silt record the
influx of meteoric water from exposed parts of the nearby
platform (Flügel and Koch 1995). Late Triassic: Steinplatte,
Tyrol, Austria. Scale is 1 mm.
structures. Raggioni are typically 20-30 mm long, may
reach 250 mm. The width of the largest ray crystals is
about 4 mm. Terminations are square-ended or feath-
ery. Raggioni are interpreted as pseudomorphs of cal-
cite after aragonite cements formed in meteoric and ma-
rine-vadose, hypersaline as well as marine-phreatic
waters.
7.4.3 Cement Types and Diagenetic
Environments
Cement types can be diagnostic of particular diagenetic
environments, but caution is required, because identi-
cal cement types may be formed in different diagenetic
environments, as documented, e.g. by equant drusy spar
cement precipitated in near-surface meteoric environ-
ments and under deep burial conditions. Careful atten-
tion must be paid to isotopes, trace elements, and catho-
doluminescence fabrics to correctly deduce the precipi-
tation environment.
Crystal silt - internal sediment or cement ?
The term
crystal silt
(Fig. 7.11; Pl. 34/8 Pl. 39/8)
describes a poorly sorted gray internal matrix consist-
ing of tiny, often rhombic, sometimes angular and very
angular calcite crystals of 5-40
m size. It occurs in
molds, solution-enlarged molds, fenestral voids, inter-
particle pores, and small reef cavities. Crystal silts have
various origins. They are known from continental (e.g.
caliche), transitional (e.g. tidal) and marine environ-
ments, and have been interpreted as vadose crystal silt
of supratidal origin (Dunham 1969), a product of the
destruction of limestones and dolomites (Upper Juras-
sic; Lang 1964), marine internal sediment (because of
the association of silt and marine microfossils), or re-
worked microcrystalline cements. Calcite crystals,
Diagnostic cement criteria:
Meteoric cements
exhibit
• commonly Low-Mg calcite, because meteoric water
has a very low Mg/Ca ratio, Mg is leached and High-
Mg calcite is converted to Low-Mg calcite,
• aragonite dissolution and precipitation of drusy cal-
cite spars,
• blocky, granular or mosaic fabrics that fill the pores
completely,
