Geology Reference
In-Depth Information
4
Peloid pack- and grainstones are not restricted to
shallow-marine environments but are also important
constituents of slope and basin sequences formed
by turbidites and debris flows.
hibits a circumgranular non-laminated micritic rim com-
monly called a 'micrite envelope'. Because of the sig-
nificance of these grains as facies indicators, a sepa-
rate grain type category called 'cortoid', was proposed
(from 'cortex', Latin for bark; Flügel 1982).
4.2.3 Cortoids - Carbonate Grains Charac-
terized by Micrite Envelope
Micritization and micrite envelope
Micritization is a process whereby the margins of
carbonate grains or the total volume of grains are re-
placed by crypto- or microcrystalline carbonate crys-
A specific category of carbonate grains are bioclasts,
ooids or lithoclasts and peloids, whose periphery ex-
Box 4.7. Common fabrics in peloidal limestones usually interpreted as products of microbially or biologically induced
carbonate production.
Micritic clotted fabric (Pl. 10/1)
Texture: Densely spaced, variably sized globular and irregular peloids forming amalgamated clots; commonly within a
microspar, micrite or spar matrix.
Origin : (a) Recrystallization of carbonate mud and peloidal micrite (Cayeux 1935; Bathurst 1970; Leeder 1982),
(b) diagenetic alteration of soft-pellet grainstones to wackestones (Bathurst 1970) (c) diagenetic amalgamation of
precipitated peloids (Reid 1987), (c) diagenetic alteration of soft-bodied organisms (Bourque 1984), (d) diagenetically
modified algal debris (Coniglio and James 1985), (e) products of in situ calcified mats of benthic coccoid cyanobacteria
(Kazmierczak et al. 1996), and (f) grazing and decay of algal mats (Pratt 1982).
Occurrence: Common in tidal laminated carbonates, in mud mounds, and reefs (Sun and Wright 1983; Neuweiler
1993) as exemplified by Triassic reefs, where about 75% of the reef core framework can consist of precipitated
peloids occurring in cavities, intraskeletal voids and organic crusts (Reid 1987).
Sparry peloidal fabric (Pl. 8/5)
Texture: Abundant, very small peloids formed in situ and consisting of a microcrystalline core surrounded by a dentate
crystal rim (type 9: Fig. 4/11). The peloids are part of submarine carbonate cements.
Origin: Chemical or biochemical precipitation (type 9, Fig. 4/11).
Occurrence: Common in intra- and interskeletal voids of reef rocks similar to clotted fabrics, associated with internal
sediments and carbonate cements and within crusts associated with corals and sponges.
Thrombolititic peloidal fabrics (Pl. 50/5)
Texture: Non-laminated structure. Abundant peloids of irregular shape and size, some retaining filamentous structures.
Common calcite 'spherulites' (peloids) consisting of radially-oriented, non-ferroan calcite, typically 12-20 m in
diameter with a cloudy micrite core, 1-10 m in diameter.
Origin: (a) In-situ precipitation. The formation of in-situ spherulites within the sediment is controlled by supersaturation
of CaCO 3 in solution and a site for nucleation. Supersaturation of CaCO 3 is caused by (a) the release of CO 2 and
NH 4 on the decay of organic matter and (b) bacterial sulphate reduction. Bacteria can form spherulites in aerobic,
anaerobic, agitated and non-agitated laboratory conditions, but experimental precipitation of spherulites without the
presence of bacteria points to the possibility that spherulitic textures also can form as a by-product of decayed
organic matter. Spherulites in stromatolites have been interpreted as calcified cyanobacteria. (b) Allochthonous
grains. Deposition of small allochthonous fecal pellets and algal peloids within cavities (James et al. 1976) or
formation of pellets by cryptic filter feeders (e.g. clionids: Land and Moore 1980). These interpretations are supported
by the fact that peloids can settle at the base of cavities.
Occurrence: Thrombolitic fabrics occur in lacustrine limestones (Pl. 131/5) and beachrocks, and are common in laminated
tidal and subtidal carbonates (stromatolites) as well as in subtidal shelf carbonates, mud mounds and non-skeletal
microbialite reef frameworks (particularly in the Carboniferous and Permian, Jurassic and Cretaceous).
Biocementstones (Tsien 1981) - Sect. 8.2
Texture: Fine-grained micritic limestones consisting of peloids, delicate small and/or poorly calcified organisms contained
within abundant, localized carbonate cement.
Origin: Fabric, formed by free-living microorganisms (cyanobacteria, algae, sulfur-reducing and sulfate-reducing
bacteria) and small encrusting benthic invertebrates, which trap and build carbonate mud from bottom waters,
resulting in the formation of small micritic 'protopeloids' (Tsien 1985). Amalgamation of the peloids leads to the
formation of peloidal automicrite. A large part of the limestones is formed by biologically induced carbonate cements.
Occurrence: Reef frameworks, specifically in the Permian and Triassic (Webb 1996).
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