Geology Reference
In-Depth Information
cements of Mg-calcitic coralline algae. Reef cements
are bladed or microcrystalline Mg-calcite or aragonite.
Microcrystalline cement is commonly pelleted. The fab-
ric of the reef cement is controlled by its access to cir-
culating water. Voids with greater permeability may be
filled by isopachous or equant cements, more confined
voids with larger and irregular crystals. Thick botryoidal
crusts of aragonite occur in larger reef cavities. Com-
mon reef cements are radiaxial cements (Sect. 7.4.2.1),
botryoidal cements(Fig. 7.14), isopachous fibrous ara-
gonite cements, and isopachous fibrous High-Mg cal-
cite cements forming blades. Syngenetic aragonite ce-
ments forming fans and composite fans of acicular crys-
tals are common constituents of Permian and Triassic
reef limestones (Fig. 7.14; Mazzullo and Cys 1977; Flü-
gel et al. 1984; Davies and Nassichuk 1990; Liu and
Rigby 1992). Mg-calcite is the dominant cement in most
modern and many ancient reefs, and occurs mostly as
rim cements, microcrystalline crusts, or as peloidal ce-
ments that geopetally fills cavities in the reef frame-
work. The syngenetic origin of reef cements is indi-
cated by (a) multiple cement generations alternating
with cement exhibiting biogenic encrustations, mi-
croborings, and peloidal sediment; (b) reworking of
cement crusts and subsequent cementation of the clasts.
Dust lines around and between the cement fans are
interpreted as being of microbial origin (initial growth
of aragonite crystals within biofilms).
Plate 34 Marine, Meteoric, and Burial Carbonate Cements
This plate displays marine (-> 1, 7), meteoric (-> 5,6) and burial cements (-> 1, 3, 4). Thin sections allow a first
interpretation of crystal morphology and fabric with regard to the diagenetic environment. A critical assessment
of the environment requires cathodoluminescence fluid inclusions analyses as well as and stable isotope studies.
1
Bioclastic grainstone. Interparticle pores between fossils enclosed within oncoids (T: trilobite) are filled with
submarine
calcite cement
(thin rim consisting of acicular crystals, black arrow) and
blocky burial cement
(white arrow). The trilobite
fragments show the typical shepherds crook shape (see Pl. 94/3). Late Silurian (Ludlow): Burgwik, Hobbugen, Gotland
Island, Sweden.
2
Radiaxial calcite cement
growing on trilobite fragments. The large radiaxial cement crystals have curved boundaries and
show a cloudy appearance due to inclusions. Note the distinct pores typical of some trilobite carapaces. The shell consists
of prismatic calcite crystals growing with their c-axes perpendicular to the skeleton surface. Early Devonian (Emsian):
Erfoud, Anti-Atlas, southern Morocco.
3
Syntaxial overgrowth
(arrow) on echinoderm fragments. Because echinoderm fragments act as single crystals, they are
commonly nucleation sites of calcite cements. Clear calcite is added in optical continuity to the echinoderm fragment.
Sediment adjacent to the monocrystal crinoid may have been dissolved as the syntaxial overgrowth developed. The
crinoid fragment at the left side of the photograph shows distinct cleavage lamellae. Syntaxial overgrowth by clear crys-
tals points to
burial
diagenesis. In contrast, syntaxial overgrowth in near-surface marine, meteoric or mixing-zone envi-
ronments is generally inclusion-rich and cloudy. Cambrian: Handler Range, Antarctica.
4
Syntaxial overgrowth
(arrows) on echinoderms. When viewed under crossed nicols, the cement crystals show the same
interference colors as the enclosed single crystal echinoderms and are part of the same crystals typical of syntaxial over-
growth. Prolongation of twin lamellae into the syntaxial cement as well as the inclusion-free clear crystals indicate the
continuing growth of the echinoderm calcite under
burial
conditions. Jurassic: Switzerland.
5
Leached corals, filled with internal sediment and calcite. The walls of the corals are almost completely replaced by
calcite. Some septal structures are preserved as relicts at the periphery (arrow). Dissolution occurred during the subaerial
exposure of a reef and karstification (Bernecker et al. 1999). Infilling of (red) micrite sediment took place subsequent to
a renewed change to submarine conditions. Late Triassic: Tropfbruch Adnet, Salzburg, Austria.
6
Gravitational cement
represented by pendant cement (arrows) beneath variously shaped vadoids formed in a
marine-
vadose
shelf interior environment. Pendant cements reflect growth outward to capillary water-air interfaces of partly
filled interparticle pores. Pisoid grainstone. SMF 26. Late Permian: Entrance Carlsbad Cavern, New Mexico, U.S.A.
7
Marine phreatic
isopachous bladed calcite cement (arrows) around dasyclad algae and aggregate grains. Remaining
interparticle pores were filled with crystal silt.
Crystal silt
composed of tiny calcite crystals, sometimes associated with
microfossils. This internal sediment is often used as an indication of vadose diagenetic conditions (as in this figure), but
is also known from burial environments. Middle Permian: Velebit, Croatia.
8
Solution void, occluded by
scalenohedral calcite cement
(black arrow) and silt-sized sediment (white arrow) consisting
of tiny mechanically deposited calcite crystals and broken-off tips of cement crystals (
crystal silt
). See -> 7. Lowermost
Cretaceous: Subsurface, Kinsau, Bavaria, Germany.
