Geology Reference
In-Depth Information
quency of redeposited reef- or platform-derived mate-
rial. Basinal limestones are commonly described using
only a few microfacies types (maximum 8 MFTs). The
difference in the number of MFTs for shallow-marine
and deep-marine limestones reflects the greater num-
ber of microfacies studies dealing with shelf and reef
carbonates and also the greater diversity of shallow-
marine benthic biota used in defining microfacies types.
These differences have to be taken into consider-
ation in the definition of MFT. Samples A and B com-
prise one microfacies type (MFT 1), whereas sample C
represents another microfacies type (MFT 2). The
microfacies types are discriminated by environmentally
sensitive criteria which allow the recognition of spe-
cific habitats. MFT 1 is dominated by
Neoanchicodium
grains, MFT 2 is dominated by dasyclad grains. The
dasyclads comprise at least three taxa. Echinoderm
grains occur in MFT 1 but not in MFT 2. MFT 2 differs
from MFT 1 by a greater frequency of fusulinids, al-
though this feature may not be relevant for the discrimi-
nation of the microfacies types, but may simply indi-
cate very localized concentrations of tests by sporadic
currents.
Does the detailed differentiation of MFTs make
sense? If you are satisfied with the statement that the
samples indicate shallow-marine deposition in a high-
energy environment, further studies will not be war-
ranted. If, however, you would like to interpret this shal-
low-marine high-energy depositional environment in
terms of subenvironments and habitats, you will need
MFTs whose diagnostic criteria reflect specific settings
and environmental conditions. Such additonal criteria
can be derived from taxa and major groups of Early
Permian calcareous green algae which exhibit specific
distribution patterns. Facies analyses of several Early
Permian platforms have shown that udoteacean algae
(
Neoanchicodium
) dominate in the distal parts of open-
marine platforms, whereas the
Gyroporella-Mizzia-
Connexia
dasyclad association of sample C is a com-
mon feature of Permian proximal platform areas.
11.3 Making Microfacies Types
Plate 100/1 (subsequently called sample A), Pl. 100/
2 (sample B) and Pl. 101 (sample C) show three thin
sections of
Early Permian shelf carbonates
from the
Southern Alps. The samples A and B are from the same
section, sample C is from a different but coeval local-
ity.
The three samples are similar in depositional tex-
ture (grainstone), sorting and gross composition (more
skeletal grains than composite grains). They are also
similar in the occurrence of calcareous algae, fusulin-
ids, smaller foraminifera and microbial and encrusted
aggregate grains, as well as in the generic attribution
of the fusulinids. These coincidences indicate deposi-
tion in a shallow-marine, high-energy environment. The
shared criteria would unite the three samples in a single
facies type defined by grain composition and deposi-
tional texture.
A closer look, however, reveals differences: several
biota occur only in some samples - gastropods only in
sample A, echinoderms only in samples A and B, and
thin shells, corals and tetrataxiid foraminifera only in
sample C. These differences might appear negligible
but there are also additional differences concerning the
quantitative ranking of the constituents and the system-
atic attribution of algae. Ranking in samples A and B is
similar with regard to
Neoanchicodium
and aggregate
grains, both of which are the most abundant grains. In
sample C, however, the first two positions are occu-
pied by dasyclad algae and aggregate grains, followed
by fusulinids.
Another
example of making microfacies types
is
shown in Fig. 11.4 which shows three thin sections of
Mesozoic
sediments with serpulids.
Using the occur-
rence of serpulids only, the three samples would be at-
tributed to a single MFT. This, however, leads to a loss
in information. The scarcity of serpulid tubes in sample
A separates this sample from samples B and C which
would be placed in an other MFT. The attribution of
sample A to a distinct MFT is also supported by the
Plate 101 Defining Microfacies Types
This plate displays an example of how microfacies types can be defined. The sample yields some microfacies
criteria in common with samples A and B on Pl. 100, but differs in environmentally-sensitive characteristics.
Skeletal grainstone. Sample
C
displays calcareous algae (dasyclads - D), foraminifera (fusulinids - F, palaeotextulariid - P)
and tetrataxiid -T, and smaller foraminifera), and thin shells (S). Composite grains are represented by microbial and en-
crusted aggregate grains (AG; see Pl. 15/4, 5). Interparticle pores are filled by carbonate cements. The diagnostic microfacies
criteria that attribute the sample to a different MFT different from that of samples A and B (on Pl. 100) are the abundance of
specific dasyclads comprising an assemblage of
Gyroporella
(GY),
Mizzia
(MI) and
Connexia
(CO) that typify proximal
platform habitats. Early Permian (Asselian) platform carbonates: Seikofel, Sexten Dolomites, Italy.
