Geology Reference
In-Depth Information
tals. Incomplete micritization leads to the formation of
cortoids, whereas complete micritization can bring
about a gradual to total alteration of the original grain
(Pl. 134/7) and the formation of bahamite peloids
(Fig. 4.11). The term micritization was worked out in
the context of studies of modern lagoonal carbonates
(Bathurst 1966). Later, the term was expanded to in-
clude all processes resulting in obliteration of original
carbonate microstructures through gradual alteration to
cryptocrystalline textures. Micritization processes are
controlled by biological and chemical factors and take
place in shallow- and deep-marine as well as in terres-
trial and lacustrine environments.
Micritization of aragonitic skeletal grains has been
described from the Bahamas and the Persian Gulf, mi-
critization of Mg-calcite skeletal grains from Florida
and Belize (Reid et al. 1992). Micritization processes
are fundamental in loosening the surface, abrading and
rounding carbonate grains, completely destroying the
original structures of skeletal and other grains. Persis-
tent micritization results in the formation of carbonate
muds (Sect. 4.1.1).
The term
micrite envelope
was first used by Bathurst
(1964) in the study of syngenetic and early diagenetic
changes affecting modern skeletal grains. The term re-
fers to a thin, non-laminated coating of very fine micri-
te around carbonate grains, particularly skeletal grains
or ooids. Modern envelopes consist of aragonite or
High-Mg calcite. In limestones, the calcitic envelopes
of cortoids appear white in reflected light and dark in
transmitted light. The thickness of the envelopes var-
ies between a few
m and about 500
m.
Origin of cortoids
Micrite envelopes originate from destructive and
constructive processes that take place at or near the sedi-
ment-water surface. Both photosynthetic and non-pho-
tosynthetic organisms play a major role in the forma-
tion of cortoids. Common modes are:
Fig. 4.13.
Origin of cortoids.
A
- Three-step destructive mi-
critization related to microborings (adapted from Bathurst
1966). A1: Endolithic cyanobacteria, algae or fungi produce
(1) tube-like microborings (black) on the surface of bioclasts.
(2) Vacant tubes (white) originating after the death of the
endoliths are (3) infilled with microcrystalline carbonate ce-
ment (stippled in A2). Multiple boring and filling result in
the formation of micrite envelopes.
B
- Destructive micritization by concurrent cyanobacterial
microboring and filling of boreholes (modified after Reid and
Macintyre 2000). Microboring (black) just beneath the grain
surface produces tunnels which are filled by radial-fibrous
cement as the organism advances.
C
- Constructive micritization caused by precipitation of mi-
crocrystalline calcite around exposed filaments of endo- and
epilithic algae and cyanobacteria (adapted from Kobluk and
Risk 1977a). C1: Initial microborings; C2: Colonization of
the grain surface by endo-epilithic filamentous algae; C3 and
C4: Precipitation of calcite cement on the surface of filaments;
C5: Resulting micrite envelope. Most micritic rims are thin-
ner than 500
m.
(1)
Destructive micritization related to microboring
organisms:
The classical model is a three-step pro-
cess (Bathurst 1966; Fig. 4.13: A1 and A2).
(a) Microbolic products of microendoliths lead to
biochemical dissolution of skeletons (Ehrlich 1999)
leaving microborings (Golubic et al. 2000). Endo-
lithic microborers attacking the surface of bioclasts
and ooids, particularly in shallow-marine environ-
ments, produce tiny little voids (size < 1
m to about
50
m) which are colonized by filamentous cyano-
bacteria, green and red algae as well as fungi (Sect.
9.1). Boring organisms can colonize a grain within
