Geology Reference
In-Depth Information
14.3.2 Discussion of Standard Microfacies
Types
shoal material may be transported by storms onshore
as well. The interpretation of SMF 4 sediments requires
information on the geometry and thickness of the al-
lochthonous limestone beds and their position within
the sequence.
One of the difficulties of the Standard Microfacies
concept is the highly generalized definition of SMF
Types of fossiliferous limestones. These limestones,
represented by SMF 8, SMF 9, SMF 10 and SMF 11,
form the bulk of bedded shelf carbonates. SMF 8 to
SMF 11 are common both in the Facies Zone FZ 7 of
the open shelf lagoon in euphotic depths, and in the
Facies Zone FZ 2 of the deep shelf within depths of
tens to hundreds of meters. Environmental interpreta-
tions derived from these SMF Types must be checked
by palecological and biofacies criteria (see Chap. 15
for examples). The assignment of samples to SMF 8
and SMF 9 can be difficult, because both microfacies
types may show strong variations in the frequency of
skeletal grains and other grain types.
SMF 12 summarizes accumulations of various shells
formed in different ways. A sound interpretation of
samples attributed to this SMF depends on informa-
tion on the stratigraphic and geometric characteristics
of the shell bed, thickness and lateral extension, posi-
tion within the sedimentary sequence, and the kind and
taphonomy of fossils (Kidwell et al. 1991; Fürsich and
Oschmann 1993; Fürsich 1995).
SMF 13 and SMF 22 are characterized by oncoids.
These types are well-defined. Most of the environmental
information is included in the composition of the on-
coid cortices. Take a look at the organisms contribut-
ing to the growth of the oncoids, the type of the lami-
nation, and consider breaks in growth indicated by en-
crusting and boring organisms.
How many samples fit into the SMF Type categories?
A common experience is that not all thin sections
can be precisely assigned to a specific SMF Type. A
review of fifty papers describing and illustrating the
microfacies of Paleozoic and Mesozoic carbonate rocks
shows that about 70% of the local microfacies distin-
guished by the authors correspond to established SMF
Types. Difficulties occur with local microfacies types
represented by wackestones and grainstones contain-
ing various types of grains and of fossils. Many of these
samples are sediments of the platform interior (FZ 6-
FZ 8) and of the deep shelf (FZ 2). In these environ-
ments mixing of grains by burrowing has a strong im-
pact on the composition of the sediment. In addition,
storms frequently contribute to sediment mixing in in-
terior platform environments. 'Mixed grain composi-
tions' are also common in ramp carbonates (Sect.
14.3.4.2). These samples should be studied using the
'compositional maturity' concept proposed by Smosna
(1987), see Sect. 6.2.1.4.
SMF Types assist in facies interpretations but must be
refined
More than two third of the SMF Types distinguished
by Wilson (1975) describe sediments deposited above
the fair-weather wave base and between the wave base
and the storm wave base. Microfacies types of deep-
water sediments are clearly underrepresented.
SMF types comprise autochthonous and parauto-
chthonous sediments deposited more or less within the
place of their origin, and allochthonous sediments,
transported from shallow to deeper depositional envi-
ronments.
The autochthonous background sedimentation in
basinal and deep shelf settings is represented by SMF 1
and SMF 3. SMF 2 indicates mixing of autochthonous
sediment and fine-grained eroded and redeposited ma-
terial. The muddy and often burrowed sediments of
SMF 1 and SMF 3 may present indications for bottom
currents. The microfacies of these bioclastic wacke-
stones is common in limestones characterized by nodu-
lar fabrics (see Sect. 5.1.6).
SMF 4, SMF 5 and SMF 6 are allochthonous sedi-
ments, whereby the composition of SMF 5 and SMF 6
provide some hints to the existence of reefs. These reefs
might have developed at platform margins, in an upper
slope position, or on non-rimmed platforms. Micro-
facies and fossils in clasts of breccias or foreslope rud-
stones allow source areas to be recognized (see Sect.
16.2.5). Note that reef material as well as lithified sand
SMF 14: A very specific SMF Type is SMF 14 (Pl.
9/7) introduced by Wilson for lag deposits. Lag depos-
its are residual accumulations remaining on a surface
after the finer material has been removed. Lags repre-
sent slow accumulations of coarse material in the win-
nowing zone. The particles are chemically and physi-
cally resistent through long periods of non-deposition
and persistent winnowing of sediment on the sea floor.
The criteria
of carbonate lag deposits are coated,
rounded and worn particles, in places mixed with pe-
loids or ooids that are blackened and iron-stained. Phos-
phate grains e.g. fish bones and teeth and phosphatized
organic remains are common (Pl. 110/2, 3, 5) as well
as glauconite and quartz grains. Coarse lithoclasts may
be present, sometimes forming lag conglomerates. Fos-
sil shells are often leached; steinkern preservation is
common. Lag deposits are characteristically thin beds.
