Geology Reference
In-Depth Information
might not take part in reefbuilding, and they are also
members of the dweller guild of level-bottom commu-
nities. Despite these difficulties, the SMF Types based
on ecologic groups have been retained, since they al-
low a first approximate assignment of samples.
Other criteria used in defining SMF Types are
spe-
cific fabrics,
e.g. lamination or fenestral structures (Pl.
123), or
combinations of diagenetic and depositional
features
(laminated evaporitic carbonates, Pl.125/4;
pisolitic carbonates, Pl. 126/1, 2).
14.3.1 Revised Standard Microfacies Types
The following pages summarize the diagnostic criteria
of the Standard Microfacies Types and indicate their
distribution in the Facies Zones of the Wilson model
and in the different part of carbonate ramps. Let us start
with a short list (Box 14.4).
Box 14.4.
List of Standard Microfacies Types.
The order
of the SMF numbers follows approximately the order of
the Standard Facies Zones in the Wilson Model going
from the basinal SMF Type 1 to SMF 26 that character-
izes subaerial exposed areas.
Can the SMF Types be used for other carbonate depo-
sitional models?
The SMF Types were defined for a model describ-
ing the sedimentation on a rimmed carbonate shelf and
warm-water platform-reef environments in tropical lati-
tudes. The occurrence and distribution of the SMF
Types are strongly dependent on water depth, and that
in turn depends on the configuration and topography
of the shelf. Some microfacies types of inner and shal-
low parts of carbonate ramps correspond to SMF Types
of platform interior sediments. Back-ramp tidal and
lagoonal areas are similar in microfacies associations
to tidal flat and lagoonal environments of platforms.
Other microfacies types on ramps and platforms, how-
ever, differ in their location. SMF 15, characterized by
concentric ooids, occurs in the platform model in a plat-
form-edge position, in the ramp model in an inner and
mid-ramp position. Outer ramp microfacies types in-
clude SMF Types known from the Wilson model, but
also types not specifically distinguished in the platform
model. Similar situations exist in comparing micro-
facies types of non-rimmed platforms and epeiric plat-
forms and ramps with SMF Types of rimmed platforms.
Major discrepancies exist between the Standard
Microfacies Types and microfacies types of cold-wa-
ter carbonates. The latter differ in composition (lack of
ooids, aggregate grains), skeletal grain associations and
skeletal mineralogy (see Sect. 12.2), texture and weak
cementation. Many temperate and cool-water micro-
facies types hardly correlate with SMF Types at all.
Look at the pictures of cool-water limestones on
Pl. 106/1-3 and try to find comparable microfacies types
in the SMF classification that were set up for warm-
water platform carbonates! The only SMF Type with
an approximately similar biotic composition is SMF 9.
This SMF is defined by strongly burrowed skeletal
wackestones and a wide spectrum of fossil groups also
comprising the fossils of these cool-water samples. Be-
cause of the too generalized definition of SMF 9, this
assignment provides little help in interpreting the envi-
ronment of the samples. A comparison with microfacies
types of ramps is much more advisable.
SMF 1
: Spiculitic wackestone or packstone, often with
calcisiltite matrix. Subtype emphasizes burrowing.
S
MF 2
: Microbioclastic peloidal calcisiltite with fine
grainstone and packstone fabrics.
SMF 3
: Pelagic lime mudstone and wackestones with
abundant pelagic microfossils. Subtypes differenti-
ate the groups of planktonic organisms.
SMF 4
: Microbreccia, bio- and lithoclastic packstone or
rudstone.
SMF 5
: Allochthonous bioclastic grainstone, rudstone,
packstone, floatstone, breccia with reef-derived biota.
SMF 6
: Densely packed reef rudstone.
SMF 7
: Organic boundstone. Subtypes try to differenti-
ate the kind of contribution by potential reefbuilders
to the formation of reefs and other buildups.
SMF 8
: Wackestones and floatstones with whole fossils
and well-preserved endo- and epibiota.
SMF 9
: Strongly burrowed bioclastic wackestone.
SMF 10
: Bioclastic packstone and wackestone with
abraded and worn skeletal grains.
SMF 11
: Coated bioclastic grainstone.
SMF 12
: Limestone with shell concentrations. Subtypes
characterize shell-providing fossils.
SMF 13
: Oncoid rudstone and grainstone.
S
MF 14
: Lag deposit.
SMF 15
: Oolite, commonly grainstone but also wacke-
stone. Subtypes highlight the structure of ooids.
SMF 16
: Peloid grainstone and packstone. Subtypes dif-
ferentiate non-laminated and laminated rocks.
SMF 17
: Grainstone with aggregate grains (grapestones).
SMF 18
: Bioclastic grainstone and packstone with abun-
dant and rock-building benthic foraminifera or cal-
careous green algae. Subtypes describe the system-
atic assignment of the various groups.
SMF 19
: Densely laminated bindstone.
SMF 20
: Laminated stromatolitic bindstone/boundstone.
SMF 21
: Fenestral packstone and bindstone. Subtypes
characterize fenestral voids and the contribution of
calcimicrobes.
SMF 22
: Oncoid floatstone and wackestone.
SMF 23
: Non-laminated homogenous micrite or micro-
sparite without fossils.
SMF 24
: Lithoclastic floatstone, rudstone or breccia.
SMF 25
: Laminated evaporite-carbonate mudstone.
SMF 26
: Pisoid cementstone, rudstone or packstone.
Text continued on p. 712
