Geology Reference
In-Depth Information
Plate 6 Micrite and Microspar: What do they Look Like in Thin Sections?
The former carbonate mud in limestones occurs as fine-grained microcrystalline calcite matrix (
micrite
). The
originally very strict definition (crystals < 4
m) was broadened for practical reasons and because of the difficulties
in genetically interpretating micrite. Micrite represents an accumulation of allochthonous sedimentary particles
forming allomicrites (e.g. calcareous nannoplankton, foraminifera or parts of algal skeletons; -> 1, 6 and Pl. 7)
or a product formed in place on the sea floor (-> 5) or within the sediment (-> 3, 4) by physicochemical,
microbial and biochemical processes (automicrite). Regardless of an autochthonous or allochthonous origin, the
occurrence of micrite is commonly regarded as an indicator of low-energy depositional environments in protected
shallow areas or in deep water below the fair-weather wave base. Many problems are involved with this paradigma.
The term
microspar
(microsparite) refers to a calcite matrix with rather uniform crystal size and equant
crystal shape. The origin is explained by the recrystallization of micrite-sized calcite crystals during recrystallization
or as one-step process of cementation and calcitization of aragonite precursors (see Fig. 4.5).
There exist six main criteria which should be considered in a thin-section analysis of fine-grained limestones
and which are answered in the description of the figures of this plate:
(A) - Sea-floor micrite or internal micrite?
(B) - Overall fabric?
(C) - Textural differences in the sample?
(D) - Crystal size and crystal fabric
(E) - Fossils and biogenic textures?
(F) - Non-carbonate material?
1
Micrite types:
Note the textural difference between the rounded micrite clast, composed of homogeneous micrite, and the
inhomogenous finely bioclastic 'allomicrite' matrix (questions B and C). Bioclasts are shell debris (question E). The
sample represents an intraclast wackestone (see Pl. 16/3) formed by episodic storms on a carbonate ramp (question A).
See Pl. 102 for further criteria for storm deposits. The crystal sizes of the micrite clasts are about 10
m and exceed the
4
m limit (question D). Middle Triassic (Hauptmuschelkalk): Künzelsau, southwestern Germany.
2
Microsparite:
Equant fine-grained texture interpreted to have originated by recrystallization in the vadose zone of former
micrite (see Pl. 47/2). Late Jurassic carbonate ramp (see Pl. 136): Subsurface, Kinsau, southern Germany.
3
Automicrite:
Cauliflower structure formed within cavities of a carbonate mud mound. The inhomogenous structure
(question B) represents a succession of 'peloidal container organomicrites' (black, crystal size about 10
m, question D)
and accretionary thrombolites (irregular peloidal fabric formed within relics of soft sponges; see questions A and E).
Thrombolite structures are also shown on Pl. 50/5 and Pl. 131/5. Early Cretaceous (Albian): Soba reef area, Mazo Grande,
northern Spain.
4
Internal micrite
(question A) within synsedimentary cavities in a micritic peloidal bindstone. Note gradation, geopetal
filling and coarse sparite at the top. The cavities occur within dark micrite formed by sediment entrapment by tiny donezellid
algae (arrows; see Pl. 60/1; Question E) which contribute to the framework of decameter-sized mud mounds. Micrite
infillings of synsedimentary cavities often exhibit textures which are distinctly different from the enclosing matrix (Pl.
17/3, Pl. 20/1, Pl. 50/4). Significance of the sample: Evidence of the timing of sedimentary and diagenetic processes. The
internally precipitated micrite alters the mud/grain ratio of the pre-existing sedimentary deposits and questions the use of
micrite content an indicator of low water energy. Late Carboniferous (San Emiliano Formation, Westfalian): Cantabrian
Mountains, northern Spain.
5
Sea-floor automicrite
(question A). Hemispheroids of dense micritic/fenestral microbialites (question B) contributing to
the formation of decameter-sized platform-margin reefs (Neuweiler 1993). The finely laminated structures disappear with
increasing sediment supply. Autochthonous formation of microbial micrite (originally Mg-calcite) by in-situ calcification
is indicated by the purity of the micrite (no non-carbonate material; question F) and the protrusional outer margins (arrows).
Slight changes in sediment input are shown by the alternation of black micrite and micrite rich in fine bioclastic debris
(predominantly sponge spicula; question E). Note circular cross section of burrows pointing to a rapid alternation of soft
and hard bottom conditions (question E). Early Cretaceous (mid-Albian): Soba reef area, Vasco-Cantabrian basin, northern
Spain.
6
Pelagic sea floor allomicrite
with abundant planktonic foraminifera (questions A and E). Note differences in size and
packing of the calcitic foraminiferal tests which may point to burrowing in the sediment. The sediment is dominated by
foraminiferal tests most are sand-sized. The fine-grained micrite component often results from the deposition of nannofossils
and only to a minor degree from the degradation of foraminiferal tests. SMF 3-F
OR
. Open-marine deep-water environment.
Early Tertiary (Eocene): Apennines, Italy.
-> 3: Reitner and Neuweiler 1995; 4: Hensen et al. 1995; 5: Neuweiler 1993
