Geology Reference
In-Depth Information
•
termination (pointed, acute, chisel-shaped, pyramid-
like, flattened),
erally explained by stabilization due to the action and
influence of meteoric waters.
•
appearance (clear, cloudy, with inclusions),
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twinning,
Botryoidal cements:
Pore-filling cements consist-
ing of individual and coalescing millimeter-sized
mamelons of compact, fibrous aragonite were first de-
scribed by Ginsburg and James (1976) from Holocene
reef cavities in Belize. Many botryoids contain thin dis-
continuity horizons marked by dust lines of dark, ir-
regularly shaped inclusions and/or slight irregularities
in crystal growth, or small patches or thin layers of Mg-
calcite micrite, or fragments of bioclasts (e.g. plank-
tonic foraminifera) concentrated on discontinuity sur-
faces.
Modern botryoidal aragonite cements contribute sig-
nificantly to a very rapid cementation of steep marginal
slopes with declivities between 25° and 45° (Grammer
et al. 1993) and to the formation of seismic reflector
horizons. Ancient botryoidal cements are known from
Precambrian, Devonian, Carboniferous, Permian, Tri-
assic and Tertiary reefs and slope deposits. Modern red
algae belonging to the family Peyssoneliaceae have a
hypobasal layer below the thallus, which may be
strongly calcified although the thallus is non-calcified.
Calcification consists of aragonite botryoids. This in-
dicates that some ancient reef cements (especially those
•
crystal form (scalenohedral, rhombohedral, anhedral,
subhedral; syntaxial overgrowth),
•
growth relation of crystals or subcrystals with re-
spect to the substrate (normal, subnormal, oblique,
arranged in fans, without preferred direction),
•
extinction type (straight, undulose, sweeping),
•
size (equidimensional, non-equidimensional, changes
in size toward the pore center) and width/length ra-
tio of crystals.
The cements shown in Fig. 7.8 comprise five groups:
•
Cements consisting of crystals growing from a free
substrate into the pore space
(acicular, fibrous, bot-
ryoidal, radiaxial fibrous, dogtooth, bladed),
•
pendant cements
(dripstone) underneath grains and
grain-connecting cements (meniscus),
•
pore-filling cements forming calcite mosaics
and
characterized by crystals increasing in size toward
the pore center (drusy), equidimensional crystals
without substrate control (granular), or crystals with-
out a preferred orientation (blocky),
•
overgrowth cements in optical continuum
on a single
crystal (syntaxial calcite overgrowth),
•
micritic cements
exhibiting tiny peloids within a
microcrystalline matrix (peloidal microcrystalline
cement) or consisting of micron-sized calcite crys-
tals forming fine-grained mosaics, rims or menis-
cus structures.
Box 7.6.
Common carbonate cements associations
formed in different diagenetic environments. For the
definitions see Fig. 7.8 and Fig. 7.12.
Meteoric vadose:
Types: Dripstone, meniscus, microc-
rystalline, bladed, fibrous, drusy, blocky. Cement crys-
tals small and equant. Cements irregularly distributed,
concentrated at grain contacts or beneath grains. Fab-
rics: Gravitational, meniscus, drusy and granular mosa-
ics, syntaxial overgrowth.
Discussion of specific cement types:
Acicular and fibrous cements:
Some authors con-
sider acicular and fibrous cements to be identical. The
cements are similar in regard to the shape, appearance
and growth relation of cement crystals, but differ in the
termination and width of the crystals. Isopachous fi-
brous and radiaxial cements are common in ancient reef
rocks (Fig. 7.7). They are dark colored in hand speci-
mens and honey-colored in thin sections and form mul-
tiple bands consisting of densely packed fibrous crys-
tals radiating from the substrate into growth cavities or
solution cavities.
In modern shallow-marine settings isopachous fi-
brous fringing cements formed by Mg-calcite (Schroe-
der 1973; Schroeder and Purser 1986; Aissaoui et al.
1986) or aragonite are common in marine-phreatic en-
vironments. In ancient reefs fibrous cements are com-
posed of Low-Mg calcite (Walls and Burrows 1985).
The difference in the mineralogical composition of
modern and ancient isopachous fibrous cements is gen-
Marine vadose:
Types: Dripstone, meniscus, fibrous.
Fabrics: Isopachous, gravitational, meniscus, granular
mosaics, syntaxial overgrowth.
Meteoric phreatic:
Types: Dogtooth, blocky, dripstone,
meniscus, fibrous, microcrystalline, peloidal microcrys-
talline. Cement crystals are a bit larger than vadose crys-
tals. Cements more homogeneously distributed than va-
dose cements. Fabrics: Circumgranular; isopachous,
gravitational, equant, granular and blocky mosaics;
syntaxial overgrowth.
Marine phreatic:
Types: Fibrous, acicular, bladed,
botryoidal, radiaxial, dogtooth, microcrystalline. Fabrics:
Circumgranular, isopachous, crusts, splays, botryoidal.
Burial:
Types: Blocky, poikilotopic, dogtooth, radiaxial.
Fabrics: Drusy, equant and granular mosaics; syntaxial
overgrowth.
