Geology Reference
In-Depth Information
cise and reproducible definitions of MFT, but the ques-
tion then arises of which rank to ascribe to MFTs within
a hierarchically structured facies concept (Aigner 1984;
Hüssner 1985)?
Microfacies types for allochthonous carbonates, de-
fined by texture and composition and based on small-
scale analyses (e.g. bed-by-bed studies), define the fun-
damental depositional unit and reveal the depositional
dynamics that were operating during transport and
deposition (e.g. storms). Facies analyses based on depo-
sitional lithofacies (e.g. wackestone, grainstone) and
the comparison of MFTs from sedimentary sequences
record both vertical and lateral changes. Vertical
changes may indicate shallowing-up or deepening-up
and reveal facies dynamics (e.g. regressive/transgres-
sive events). Lateral changes (e.g. following the dip of
a ramp) may indicate differences in water depths and
hydrodynamics. Associations of MFTs occurring within
the same lithofacies and deposited in the same general
environment (e.g. MFTs of grainstones) describe local
sedimentary subenvironments (e.g. soft and firm sub-
strates) or local processes.
MFT because its consideration does not change the en-
vironmental interpretation of the two samples. Both
samples reflect the existence of an organic reef frame-
work formed by coralline sponges within low-energy
environments. However, the depositional setting of each
of these environments is different. Both samples are
Late Triassic in age. Sample A comes from small patch
reefs in the marly-calcareous Zlambach beds of the
Northern Calcareous Alps which were formed in front
of the large platform margin reefs represented by sample
B which comes from the Dachstein limestones. The
similarity in biotic composition and texture does not
express congruent depositional sites, but does express
similar nutrient controls in habitats that were compa-
rable with regard to the ecological constraints of nutri-
ent supply, available substrate and reduced water en-
ergy.
Another sponge limestone is shown in Fig. 10.34.
This Late Jurassic sample differs distinctly from the
sponge limestones discussed above in the sponge group
(siliceous hexactinellid sponges), the association of
sponges with microbial crusts at the top of the sponges,
and the spotty matrix. The sample represents a specific
MFT characterized essentially by biotic composition.
The spotty and peloidal texture of the matrix indicates
microbial controls. Other Late Jurassic sponge lime-
stones may differ from the sample shown in growth
forms of the sponges, the absence of microbial crusts,
or matrix texture. These differences define specific
MFTs that reflect differences in ecological and deposi-
tional constraints. Some criteria are of minor or no rel-
evance for the characterization of the MFT. The exist-
ence of brachiopods (figure center) should be taken into
consideration if the group is a conspicuous constituent
of the limestone. Fracturing and differential compac-
tion are not diagnostic criteria for definition of the MFT.
Plate 88 exhibits Cretaceous limestones with rud-
ists. The term rudist limestone would be too general
for establishing a defined MFT because different rud-
ist groups prefer different habitats and environments
and occur in different depositional settings. The defi-
nition of a MFT must take into consideration the major
taxonomic groups (which can be recognized in thin
section by specific microstructural patterns), their di-
versity (e.g. mono- or multispecific) and their associa-
tion with other biota. Evaluation of diversity may be
difficult or misleading on microfacies scale for this reef
limestone, as well as for other reef carbonates, without
knowledge of outcrop and field data.
11.2 How to Differentiate Meaningful
Microfacies Types
Microfacies types for autochthonous limestones are de-
fined by different criteria from those for other lime-
stones
Autochthonous limestones (Sect. 8.2) include both
reef limestones and microbial carbonates.
Reef limestones:
MFTs are differentiated according
to the type of reef-building fossils. Generally, the most
common reef-building taxa in combination with the
dominant texture (framestone, bafflestone, bindstone
or boundstone) and matrix type are used in defining a
MFT.
The example shown in Fig. 10.36 (subsequently
called sample A) is a sponge boundstone, or more pre-
cisely a coralline-sponge boundstone. Because the lat-
ter designation does not reflect the diversity of the cor-
alline sponges contributing to the boundstone fabric,
the MFT would be better characterized as medium-
diverse coralline boundstone. The sample depicted on
Pl. 81/1 (sample B) yields the same coralline sponge
groups and corresponds to sample A in an inhomoge-
neous micrite matrix. The two samples represent the
same microfacies type even though sample B exhibits
spongiomorphids not shown in A. This additional con-
stituent is of minor importance for the definition of the
Many reef limestones require a
two-fold typifica-
tion
as demonstrated by the Figs. 11.1 (sample A) and
11.2 (sample B) which show Late Triassic coral lime-
