Geology Reference
In-Depth Information
precipitated in marine and nonmarine diagenetic envi-
ronments (see Milliman et al. 1985 for discussion).
In general, micrite is understood to be the fine-
grained matrix of carbonate rocks and the fine-grained
constituent of carbonate grains. The crystal size of mi-
crites ranges from cryptocrystalline (not resolvable with
the petrographic microscope) to microcrystalline (in-
dividual crystals recognizable with a petrographic mi-
croscope but not visible with a binocular microscope
and hand-lens). The original micrite definition relies
strongly on crystal size limits which can only be rec-
ognized in petrographical thin sections (Pl. 6) or the
use of SEM (Pl. 7). An example of a 'typical' micrite
corresponding to Folk's definition are the famous Ju-
rassic lithographic limestones of Solnhofen in south-
ern Germany (Fig. 4.4; Pl. 46/1).
Terms often used more or less synonymously with
micrite are
lime mud, lime ooze
as well as
lime mud-
stone
(Pl. 46/1),
calcimudstone
and
calcilutite
. The last
three terms are useful field terms.
Microfacies thin sections are approximately 30
m
thick, and are commonly thicker than petrographical
thin sections. This may cause difficulty in measuring
true crystal sizes. Therefore, upper crystal size limits
for micrite have come into use: 10
m (Elf-Aquitaine
1975), 20
m (Dunham 1962), and 30
m (Leighton
and Pendexter 1962). Bosellini (1964) differentiated
'micrite I' (crystal size <4
m) and 'micrite II' (4-
30
m). The broadening of the definitions has resulted
in being subsumed micrite and microspar into one cat-
egory.
A conspicuous micrite type is
minimicrite
(Folk
1974), consisting of Mg-calcite and calcite crystals with
a size range mostly below 1
m (Pl. 6/6). Minimicrite
appears black in transmitted light and white in direct
light. Minimicrite occurs in lime mudstones, wacke-
stones, and microbialites, and constitutes the tests of
porcelaneous foraminifera. It is found in calcimicrobes
(Pl. 7/6) and in the skeletons of coralline red algae (Sect.
10.2.1.2). Bernier (1994) stressed the fact that litho-
graphic limestones composed of very fine micrite (crys-
tals frequently as small as 2
m) and characterized by
specific controls on their origin should be clearly sepa-
rated from other micrites by the designation lithographic
micrite.
(e.g. tidal channels, algal mats, lagoons, platforms),
reefs, and deep-marine ocean floors.
Main hypotheses starting with Sorby (1879) explain
the origin of microcrystalline calcite in limestones as
(1) in-place formation of fine-grained carbonate trig-
gered by biochemical and physicochemical factors,
(2) post mortem disintegration of calcareous algae,
(3) physical or biological abrasion of skeletal material,
(4) accumulation of pelagic calcareous plankton, and
(5) a result of diagenetic processes including cementa-
tion and recrystallization (Fig. 4.1). Honjo (1969) dif-
ferentiated micrites formed by the growth of carbonate
in place or resulting from the destruction, accumula-
tion and deposition of pre-existing carbonate particles
characterized by subhedral polygonal calcite grains
meeting at interfaces (
orthomicrites
), and micrites com-
posed of calcareous pelagic biota (
nannoagorite
). These
primary micrites
are often contrasted with the
second-
ary micrites
and
pseudomicrites
resulting from diage-
netic processes (Wolf 1965).
Several
genetically
defined terms
are in use to des-
ignate the major modes of micrite genesis:
Automicrite
(Wolf 1965) refers to 'autochthonous' micrite inter-
preted as the result of in-place formation of fine-grained
calcite or aragonite on the sea floor or within the sedi-
ment as an authigenic product triggered by physico-
chemical, microbial, photosynthetic and biochemical
processes (Pl. 6/3, 5; Pl. 50/2, 4). The term was revived
in the context of studies of mud mounds and biocalci-
fication (Reitner and Neuweiler 1995, Neuweiler 1995).
Automicrite exhibits specific fabrics and has a uniform
mineralogical and chemical composition. Automicrite
is fundamentally different from
allomicrite
(Wolf 1965),
meaning 'allochthonous' micrite and interpreted as the
sedimentary microcrystalline matrix derived from the
breakdown of various carbonate grains and deposition
of fine-grained material (Pl. 6/1, 6; Pl. 7). Allomicrites
may contain mineralogically and chemically different
constituents. The term
'orthomicrite'
(Honjo 1969) des-
ignates micrites made of subhedral polygonal clay-sized
calcite grains meeting at interfaces.
Modern carbonate muds and ancient fine-grained
carbonates:
Modern carbonate muds originate in ma-
rine and non-marine sites, in warm water and cold wa-
ter environments and in shallow and deep settings. Mud-
producing processes which can be found in modern car-
bonate environments and which were also operating in
the formation of ancient carbonates are:
•
Classification of micrites:
Few topics in carbonate
sedimentology have been debated as controversially as
the origin and classification of fine-grained carbonate
rocks. Today, fine-grained carbonate muds originate in
non-marine environments (e.g. pedogenic and lacus-
trine settings; Sect. 2.4.1), and marine environments
including shallow marine inter- and subtidal settings
the effects of metabolic and other processes of bac-
teria, cyanobacteria and algae on the precipitation
of carbonates,
Text continued on p. 80
