Geology Reference
In-Depth Information
Have a look at specific criteria of matrix and grains!
Matrix specifics:
Differences in matrix texture re-
flect different controls on the formation of fine-grained
matrix. The distinction between seafloor micrite and
internal micrite is particularly important for packstones,
densely packed wackestones and floatstones as well as
for reef limestones.
Consider Pl. 44/3: most of the fine-grained sediment
in the interstices between the larger grains of this coral-
algal packstone is an internal micrite that was infilled
into a grain-supported fabric. This infilling indicates
short-term events that transported fine-grained sediment
into depositional areas characterized by fluctuating hy-
drodynamic conditions. The recognition of sedimen-
tary infilling is crucial in the definition of MFT based
on baffling processes. An example can be found in Pl.
41/1.
Limestones composed of
lithoclasts, intraclasts and
extraclasts
(Sect. 4.2.8) often yield a wide spectrum of
genetically indicative MFTs. Again, a texturally ori-
ented classification would lump together genetically dif-
ferent carbonates. Pl. 16/1, Pl. 16/3 and Pl. 16/7 are
intraclast wackestones, but the first two samples repre-
sent a MFT defined by intraclasts corresponding to
black pebbles indicative of near-coast depositional sites,
whereas the intraclasts of the sample shown in Pl. 16/7
are indicative of a tidally influenced depositional site.
Microfacies types of Proterozoic limestones
lack-
ing skeletal grains are defined by microbialite types,
by differences in intraclast categories and ooid con-
structional types, by compositional patterns of wacke-
stones and packstones consisting of extraclasts, and by
depositional fabrics (Sami and James 1993).
Grain specifics:
A purely descriptive approach to
some of the grain categories discussed in Sect. 4.2 re-
sults in a loss of information. In contrast with peloids
and cortoids, which commonly exhibit only few mor-
phological criteria that would allow further differen-
tiation, the morphological features of other grain types
offer possibilities for the discrimination of genetically
defined subtypes.
Oncoids
(Sect. 4.2.4.1) formed in different environ-
ments differ in shape and growth forms, internal growth
patterns, and the biotic composition of coatings. The
latter feature is a diagnostic criteria for MFT differen-
tiation. Consider Pl. 12, which exhibits limestones with
oncoids formed by cyanobacteria (cyanoids) and by red
algae (rhodoids). Clearly limestones with cyanoids and
rhodoid limestones represent different MFTs. A com-
parison of Pl. 12/3, Pl. 12/4 and Pl. 12/7 shows marked
compositional differences between cyanoids, which at-
tribute the samples to three different MFTs. The differ-
entiation of these three MFTs does not result from a
preference for splitting but is meaningful in the con-
text of microfacies interpretation: the cyanoids of Pl.
12/3 characterize a reef environment, those of Pl. 12/4
a platform environment and the cyanoids depicted in
Pl. 12/7 are typical for brackish-water settings.
Ooids
(Sect. 4.2.5) and
pisoids
(Sect. 4.2.6) origi-
nate in different environments and are subject to dif-
ferent controls. The descriptive criteria summarized in
Box 4.15 offer the possibility of defining MFTs for
oolitic limestones that reflect particular paleoenviron-
mental conditions (water energy and transport pro-
cesses, salinity, water depth) and depositional settings
(Box 4.17). The samples in Pl. 13/4, 5, 6 represent three
well-defined microfacies types although all samples
would be rather simply classified as oolitic grainstones.
Skeletal grains are of prime importance in defining
microfacies types
Limestones are predominantly biogenic sediments
(Sect. 2.1.2) formed by biologically controlled pro-
cesses (Chap. 9). Fossils are significant proxies for pa-
leoenvironmental conditions: skeletal grains are highly
sensitive to processes characterizing specific deposi-
tional environments.
The composition of bioclastic carbonates depends
on many factors including skeletal mineralogy and the
path of taphonomic and diagenetic processes examined
in Sect. 4.2.1 and referred to in the context of the dis-
cussion of the various fossil groups (Chap. 10). The
taphonomic filter as well as the diagenetic loss of pale-
ontological data must be thoroughly considered in the
evaluation of MFTs based predominantly on the oc-
currence and frequency of skeletal grains (see Pl. 9/7).
Nevertheless the type, association patterns, and system-
atic classification of fossils, as well as their abundance,
are the basic criteria for the discrimination of signifi-
cant microfacies types.
A differentiation of MFTs based on skeletal grains
should include answers to the questions listed in Box
11.2.
Combining the criteria summarized in Box 11.2 re-
sults in a wide range of MFTs and allows for environ-
mental and depositional interpretations on different lev-
els.
Consider Fig. 11.3 and try to answer the questions
in Box 11.2: The thin section exhibits a poorly sorted
skeletal packstone. Fossil groups represented by skel-
etal grains are echinoderms, brachiopods and mollusk
shells (low thin-section diversity; question 1). Echino-
derms and shell fragments are common, brachiopods
are rare (question 2). Echinoderms are better preserved
