Geology Reference
In-Depth Information
Box 4.1.
SEM criteria
used in the distinction of fine-grained microcrystalline limestones of different origin and diagenetic
history (based on Fischer et al. 1967; Flügel 1968; Wise et al. 1970; Minoura 1974; Moshier 1989; Peszat 1991;
Lasemi et al. 1993; Munnecke 1997).
Microfabric
Grain size
: Is measured by counting the longest diameter of 100 to 200 grains. Allows attribution to grain size-based
classifications. 'Sorting' in micrites, as the result of competition between space and grain growth, can be used as
measure of the textural maturity of fine-crystalline carbonates. Grain size ranges may reflect differences in the
depositional environment (e.g. reef micrites and micrites in limestone/shale sequences). Differences in predominant
crystal sizes provide indications of the potential aragonite-dominated or calcite-dominated precursors of calcite mi-
crites.
Grain contacts:
Grain contacts seen under the SEM are straight, curvilinear or serrate. The percentage of specific grain
contacts reflects the degree of diagenetic changes (e.g. recrystallization).
Grain shape:
Differentiation of anhedral, subhedral and euhedral micrite grains. Anhedral and subhedral grains forming
a hypidiotopic fabric appear to be common in micritic rocks composed of grains > 4
m. Irregular, interlocked grains
form an 'amoeboid mosaic', in contrast to block-like grains forming a 'pavement mosaic' common in limestones
affected by thermal stress. Most micrite grains are angular to subangular. Subrounded and rounded grains result from
intergranular solution and crystal overgrowth.
Inclusions in crystals:
Liquid and gas inclusion pits are common in microspar crystals as well as in larger spar crystals.
The composition of the inclusions indicates the conditions under which recrystallization took place.
Microfenestrae:
Some micrites contain round voids, 40 to 80
m in size, which are filled with blocky calcite. The
preservation of this microfenestral fabric is used as evidence against a strong mechanical compaction of the sediment
prior to lithification.
Boundary between calcite matrix and fossils:
Diagenetic stages are distinguished by differences in the boundaries be-
tween matrix and fossils: (a) Sharp boundaries between matrix and Low-Mg calcite fossils (e.g. brachiopods, bryo-
zoans, ostracods, trilobites) with well-preserved ultrastructures; (b) moderately sharp boundaries between matrix
and fossils (recrystallized tabulate and rugose corals, stromatoporoids); and (c) boundaries between matrix and fos-
sils (e.g. mollusks) which are difficult to recognize.
Microporosity
: SEM studies reveal numerous types of micropores (5-10
m wide) in micritic limestones, including both
primary and solution-enhanced intercrystalline pores, as well as micromolds, microvugs and microchannels (Pl. 7/6;
see Chap. 5). Microporosity is developed in many subsurface micritic limestones, including significant petroleum
reservoirs in Mesozoic and Tertiary deposits of the Middle East, North Africa, and the North American Gulf Coast.
Skeletal grains
A main advantage of SEM studies of micrites is the possibility of recognizing the occurrence and abundance of
planktonic nannofossils (Pl. 7/3-5, Pl. 77/23) in mudstones and wackestones in both deep-water and shallow-water
settings. SEM differentiation of silt-sized and smaller bioclastic grains derived from the breakdown of shells helps to
define micrites formed by the accumulation of disintegrated invertebrate skeletons (group 7: Fig. 4.1).
Three categories can be distinguished according to
differences in grain size and composition:
4.1.1 Micrite
Terminology:
The term 'micrite' is the abbrevation of
'microcystalline calcite'. Originally, 'micrite' was pro-
posed as a genetic term referring to lithified mechani-
cally deposited lime mud (Folk 1959). Most authors,
however, now use the term in a non-genetic descrip-
tive sense for a rock composed of fine-grained calcite
crystals and particles formed in place or by the accu-
mulation of fine-grained pre-existing carbonate mate-
rial. This broad definition even includes 'microcrystal-
line cement' and 'micrite cement', precipitated as mi-
cron-sized Mg-calcite in reefs, beachrocks, hardgrounds
and caliche, and occurring in primary mud-free, grain-
supported sediments of carbonate slopes (Wilber and
Neumann 1993). Friedman (1985) has strongly opposed
the use of the term micrite for microcrystalline calcite
1
Micrite
, characterized by a crypto- to microcrystal-
line crystal texture. The original definition by Folk
(1959) sets a grain-size limit of <4
m.
2
Microspar
, characterized by a fine-grained calcite
matrix consisting of crystals ranging from 5 to about
30
m in diameter, most commonly about 5 to 7
m,
and often exhibiting a rather uniform size distribu-
tion.
3
Calcisiltite
(calcite silt), characterized by a fine-
grained matrix composed of detrital silt-sized cal-
cite particles. Silt comprises a size grade between 2
and 62
m (Assally et al. 1998). The upper size limit
is approximately the smallest size which can be dif-
ferentiated with the unaided eye.
