Geology Reference
In-Depth Information
Granular:
Calcite cement consisting of relatively equidimensional pore-filling small crystals.
Common in interparticle pores, generally without distinct substrate control. Formed in meteoric-
vadose, meteoric-phreatic and burial environments. Can also originate from recrystallization of
pre-existing cements. Pl. 10/2.
Blocky:
Calcite cement consisting of medium to coarse-grained crystals without a preferred
orientation. Characterized by variously sized crystals (tens of microns to several millimeters),
often showing distinct crystal boundaries. Xenotopic and hypidiotopic crystal fabrics common.
High-Mg calcite or Low-Mg calcite. Typically in meteoric (meteoric phreatic and vadose) and
burial environments; rare in marine hardgrounds and reefs. Precipitated after the dissolution of
aragonite cements or grains or as late diagenetic cement filling remaining pore space. Blocky
textures can also originate from recrystallization of pre-existing cements. Pl. 20/1, Pl. 28/2, Pl.
34/1.
Syntaxial calcite overgrowth cement:
Substrate-controlled overgrowth around a host grain
made by a single crystal (usually High-Mg calcitic echinoderm fragments). Overgrowth often in
crystallographic lattice continuity with the host grain. Echinoderm overgrowth is often zoned.
Color differences between the skeletal grain and the overgrowth cement can be conspicuous.
Overgrowth cements from near-surface marine, vadose-marine and meteoric-phreatic environ-
ments are inclusion-rich and cloudy, in contrast to clear overgrowth from deep burial environ-
ments Syn.: Grain overgrowth cement, syntaxial echinoderm cement, syntaxial cement rim,
syntaxial overgrowth rim cement. Pl. 31/3-4, Pl. 34/3-4, Pl.144/5; Fig. 7.10.
Peloidal microcrystalline cement:
Characterized by a peloidal (or pelleted) fabric composed
of tiny peloids (size <100
m) within a microcrystalline calcite matrix. The peloids consist of
micrite-sized crystals bearing a radiating halo. Shallow-marine. Common in modern and ancient
reefs. Possible interpretations: Chemical and/or microbially induced precipitation (Sect. 4.2.2).
Pl. 8/5.
Microcrystalline or micrite cement:
Micron-sized curved rhombic crystals.
Forms thin coat-
ings around grains, lines intraskeletal pores, fills pores completely or constructs bridges be-
tween grains (contributing to meniscus cement). Mg-calcite. Micritic cement fringes should be
distinguished from micrite envelopes (Sect. 4.2.3). Often associated with peloidal cements. Pl. 31/
3-4, Pl. 32/1-4, Pl. 33/2.
Fig. 7.8.
Cement types. Part 2.
from which precipitation occurs (Folk 1974; Mucci
and Morse, 1984), but is also determined by the sup-
ply of carbonate ions as shown by the relationships
between the degree of supersaturation, crystal min-
eralogy and morphology (Given and Wilkinson
1985),
•
nucleation, controlled by the mineralogy of the sub-
strates (aragonite or calcite). Nucleation and growth
rates of cement crystals in turn control cement crys-
tal morphology,
•
crystal growth dependent on the presence of suffi-
cient Ca ions, and therefore, on porosity/permeabil-
ity conditions.
•
the mineralogy of the substrates (e.g. aragonite ce-
ment overgrowth on aragonite grains and Mg-cal-
cite on Mg-calcite grains). Substrate control of ce-
ment substrate is an important factor in reefs and
subtidal carbonate sands, where the mineralogy of
the carbonate cement tends to reflect the mineral-
ogical composition of the reef skeletons or the bio-
clastic grains (Friedman et al. 1974),
7.4.2 Morphology and Fabrics of Cement
Types
Description and differentiation of cement types are
based on the morphology and arrangement of cement
crystals. Common cement types formed in meteoric,
marine or burial environments are shown in Fig. 7.8,
some of their fabrics in Fig. 7.12.
•
the rate of precipitation,
•
the composition of organic matter within grains and
on grains. Organic-rich pore waters hinder carbon-
ate precipitation even from supersaturated waters,
•
the activity of microorganisms mediating carbonate
precipitation (Sect. 9.1.1).
7.4.2.1 Cement Types
Precipitation of carbonate cements may be described
as a succession of three stages:
•
Criteria used for describing cement types are
•
shape (needle-like, columnar, microcrystalline, tiny
crystals around a micritic center),
oversaturation of a pore fluid within a pore space,
