Geology Reference
In-Depth Information
14.3 Standard Microfacies Types
(SMF)
• Depositional texture types. Only a few SMF Types
correspond to just one texture type; most SMF Types
occur in two or more texture types of the Dunham clas-
sification and its modifications (Box 14.7).
Facies of carbonate rocks is studied on different scales.
Stratigraphic relationships of rock bodies, sedimentary
structures, and litho- and biofacies are the main targets
of outcrop studies. In the subsurface, rock bodies and
facies units are seismcally differentiated, using log char-
acteristics and investigating cores and cuttings.
Microfacies based on thin-section studies subdivides
facies into units of similar compositional aspect that
reflect specific depositional environments and controls.
This target requires integrating small-scale microfacies
data and larger-scale litho- and biofacies criteria. This
can be done when the criteria of textures, composition
and fossils of the limestones are considered jointly with
the help of Standard Microfacies Types.
Some SMF Types are very simply defined and rely
on the
principal and quantitatively dominant kind of
grains.
For example, limestones with abundant aggre-
gate grains represent a specific SMF Type (Pl. 121/4).
Limestones with abundant ooids, or limestones with
abundant peloids, or with dominating benthic foramin-
ifera are attributed to three other SMF Types (see Pl.
120, Pl. 121/1-3, and Pl. 122/1). These types are easy
to determine but somewhat dangerous in their use as
indicators of paleoenvironmental conditions. A thor-
ough inspection of oolitic, peloidal or foraminiferal
limestones will reveal structural, textural or composi-
tional differences that must be included in the discrimi-
nation of SMF Types, because these differences con-
tain essential information on depositional settings and
environmental controls. Oolite grainstones consisting
of abundant and well-sorted concentric ooids indicate
shallow-marine, high-energy conditions. Grainstones
and wackestones with poorly sorted radial ooids indi-
cate low-energy conditions, common in transitional-
marine and also non-marine environments. In the fol-
lowing revision of the SMF Type classification these
differences are annotated by the introduction of sub-
types (e.g. SMF 15-C and SMF 15-R for oolite lime-
stones) in order to avoid an inflation of SMF types.
Subtypes were also proposed for peloid limestones,
using differences in the fabrics, and for limestones con-
sisting of foraminifera or calcareous green algae in rock-
building abundance.
Other limestones having in common the same domi-
nant grain types (e.g. oncoids) are attributed to differ-
ent SMF Types because of
unlike carbonate textures
and different grain structures,
indicating differences in
environment and depositional setting (cf. Pl. 119/3 and
Pl. 124/2).
Standard Microfacies Types are virtual categories
that summarize microfacies with identical criteria.
These criteria are simple, non- or semi-quantitative, and
easy to recognize. Most SMF Types are based on only
a few dominant characteristics comprising grain types,
biota or depositional textures.
The SMF concept arose from the recognition of the
astonishing compositional and textural similarity of
limestones of different age formed in analogous envi-
ronments (see Fig. 14.28). Originally developed for cat-
egorizing common Late Triassic platform and reef car-
bonates, and based on the combination of texture and
paleontological criteria (Flügel 1972), the classifica-
tion was expanded and more strictly defined by Wil-
son (1975) for the history of carbonate facies over time.
Wilson distinguished 24 SMF Types and used these
types as additional criteria in differentiating the major
facies belts of an idealized rimmed carbonate shelf (Fig.
14.29).
Which criteria are used to differentiate SMF Types?
The main criteria used in differentiating SMFs com-
prise:
• Grain types, grain frequency (prevailing grain types)
and grain associations.
• Matrix types (e.g. micrite, calcisiltite).
• Depositional fabrics (e.g. lamination, grading, open-
space structures, burrowing, reworking, and redepos-
ition).
• Fossils (dominant groups, assemblages, autochtho-
nous/allochthonous occurrence, skeletal concentrations
e.g. shell accumulations, specific ecologic groups e.g.
planktonic organisms, taphonomic features e.g. pres-
ervation; micrite envelopes).
Some SMF Types are defined by
characteristic eco-
logic groups,
e.g. the abundance of planktonic micro-
fossils (Pl. 113), or the occurrence of reef-building fos-
sils (Pl. 116/2, 3). These categorizations are very broad
and include many pitfalls. Pelagic limestones with abun-
dant planktonic fossils must be subdivided at least ac-
cording to the prevailing systematic groups. The use of
'reef-building fossils' as diagnostic criteria requires
interpretating the reef- and sediment-building poten-
tial of benthic organisms. Thin sections with corals are
no evidence for the existence of reefs. Corals may be
members of the constructor guild and contribute to or-
ganic frameworks, but are also bafflers that might or
