Geology Reference
In-Depth Information
Plate 118 Standard Microfacies Types: SMF 10 and SMF 11
SMF 10. Bioclastic packstones and grainstones with coated and abraded skeletal grains
Criteria:
Worn and coated bioclasts deposited within a fine-grained matrix. Forming packstones and wacke-
stones, sometimes also fine-grained grainstones. This type indicates textural inversion. The dominant particles
have been transported from high-energy to low-energy environments (e.g. from shoals to swales in the proxim-
ity).
Occurrence
: Shelf lagoon with open circulation (FZ 2) and open sea shelf (FZ 7). Common in inner and
mid-ramp settings.
Additional picture:
Pl. 74/2.
SMF 11. Coated bioclastic grainstone with sparry cement
Criteria
: Most grains are coated bioclasts, exhibit micrite envelopes or are completely micritized. Additional
grain types may be rounded intraclasts, peloids and some ooids.
Occurrence:
FZ 6 (winnowed platform edge
sands) and in reefs (FZ 5). Rare in inner ramp and mid-ramp settings. The sediment formed in areas with normal
marine salinity, constant wave action at or above the fair-weather wave base or between the wave base and the
storm wave base. Coating due to micritization by microborers occurs preferentially in very shallow environ-
ments (see Sect. 4.2.3 and Sect. 9.3.4).
1
SMF 10.
Worn foraminiferal (fusulinid) packstone.
The foraminiferal shells (
Triticites
) are worn and micritized indicat-
ing transport before deposition. Note lithoclasts (black arrows). The matrix consists of silt- to fine-sand sized peloids,
micrite and sparite. The sample consists of 81% skeletal grains and 19% matrix according to point-counter analysis. Only
very few fusulinids show axial sections (white arrow) necessary for species determination. The mass occurrence of fora-
minifera is a diagnostic criteria of SMF 18-F
OR
. The attribution of the sample to SMF 10 regards the textural inversion of
the biofabric. Fusulinids living under high-energy conditions were transported downslope to local protected low-energy
areas. Textural inversion and transport are indicated by infilling of outer whorls with sediment and by the loss of outer
whorls in many tests. Open platform (FZ 7). Early Permian (Leavenworth limestone): Osage County, Oklahoma, U.S.A.
The Leavenworth limestone is a uniform sheet, 500 km long and at least many tens of kilometers broad. The mid-shelf
limestone unit represents the transgressive part of a megacyclothem, and comprises skeletal wackestones, sometimes
associated with mudstones, oncoid wackestones, and the fusulinid packstones shown in this figure.
2
SMF 11
.
Coated bioclastic grainstone
. Most grains are strongly micritized coated skeletal grains associated with some
small-sized benthic foraminifera. The sample comes from an isolated Bahamian-type carbonate platform affected by
incipient drowning. Middle Jurassic: Monte Kumeta, western Sicily, Italy.
3
SMF 11.
Coated bioclastic grainstone.
Most skeletal grains have micrite envelopes or biogenic encrustations and corre-
spond to cortoids and small oncoids (black grains). Lime mud with peloids occurs as infilling in the grain-supported
fabric (arrows), but most intergranular pores are occluded by calcite cement. Note the conspicuous wavy boundary be-
tween areas with micrite and small peloids and areas characterized by calcite cement. The boundary reflects differences in
the lithification within the sandy sediment. Bioclastic grains are mollusk shells, dasyclad green algae (DA) and foramin-
ifera (F). In the Late Triassic the association of involutinid foraminifera and dasyclads characterizes open-marine parts of
platform interior settings (see Sect. 14.2.2). In addition to the small coated grains, larger 'fenestral oncoids' (ON) occur,
formed by microbes (
Bacinella
). The sediment originated in the high-energy area of backreef sands deposited on an outer
platform. Late Triassic (Dachstein limestone, Norian): Gosaukamm, Austria.
-> 1: Toomey 1983; 2: Di Stefano 2002; 3: Wurm 1982
Fig. 14.20.
SMF 11.
Coated grainstone.
Cortoids are
associated with ooids and strongly micritized bioclasts.
The pore space between the grains is filled with stubby
fibrous rim cement (arrow) and crystal silt, indicating
a post-compactional vadose overprint of the platform
sediment. The sample comes from platform-marginal
sand shoals adjacent to a grapestone facies. Middle
Jurassic (Esfandiar limestone): Shotori Mountains,
Tabas area, Iran. Crossed nicols. After Fürsich et al.
(2003).
