Geology Reference
In-Depth Information
Plate 119 Standard Microfacies Types: SMF 12 and SMF 13
SMF 12. Limestones with shell concentrations
Criteria:
Bioclastic rudstones or densely packed floatstones characterized by accumulations of commonly
one-type shells or echinoderm fragments. Shells may be bivalves, brachiopods, gastropods, or crinoids. The
prevailing fossil group should be noted by additional letters: SMF 12-S (coquina composed of shells without
specific assignment), SMF 12-B
S
(coquina composed of bivalve shells), SMF 12-B
RACH
(brachiopods),
SMF 12-G
ASTRO
(gastropods). Most echinoderm accumulations occur from crinoids (SMF 12-C
RIN
).
Occur-
rence:
Rock-building
concentrations of shells
originate in various environments, from the coast to the deep sea.
Bivalve shell beds are formed in platform interior settings including restricted platforms and tidal flats (FZ 8)
and open platforms (FZ 7), reefs and slopes (FZ 5 and FZ 4), and toe-of-slope and deep-marine settings (FZ 3,
FZ 2 and FZ 1). Shell concentrations are very common in mid-ramp settings. Accumulations of bivalve shells
are caused by various processes including current concentrations, storm wave and tempestite concentrations,
transgressive lag and condensation concentrations, or prolonged continuous accumulation of shells, because
sedimentation fails to keep up with hardpart accumulation (see Fig. 3.6 and Fig. 3.7).
Crinoid concentrations:
Limestones consisting of abundant and densely packed echinoderm fragments (e.g.
'encrinites') represent a specific facies type formed in various settings including slopes, protected platforms,
reefs and mounds. This type has been included by Wilson in SMF 12, but should be specifically differentiated
because of its paleoenvironmental significance. Be aware that crinoid packstones also may be the result of
chemical compaction (indicated by clay seams, stylolites, and corroded bioclasts). Crinoid accumulations occur
both in micritic limestones or they form packstones in which the crinoid columnals are cemented by syntaxial
calcite. The accumulations may be autochthonous (open sea shelf - FZ 2; foreslope - FZ 4; mounds - FZ 5) or
allochthonous (deep shelf margin - FZ 3; foreslope - FZ 4). Allochthonous accumulations often are debris flows
or turbidites. Crinoid concentrations are common in mid-ramp settings.
Additional pictures:
Pl. 94/7 (SMF 12-
T
RILO
), Pl. 95/2 (SMF 12-C
RIN
).
The original definition of SMF 12 was restricted to bioclastic rudstones or grainstones characterized by abun-
dant shell hash, and deposited in an environment of constant wave or current action that removed carbonate mud
by winnowing. Actually, mud may be removed by winnowing (bioclastic rudstones), but can be still preserved
within and between the shells (bioclastic floatstone, -> 1). Winnowing, therefore, should not be regarded as a
critical definition criterion.
SMF 13. Oncoid rudstones and grainstones
Criteria:
Millimeter- to centimeter-sized oncoids, predominantly cyanoids and porostromate oncoids, form-
ing a grain-supported fabric, sometimes in association with ooids and fine-grained bioclasts. Bimodal grain-size
distributions are common. Note that this SMF does not categorize oncoid floatstones (see SMF 22!).
Occur-
rence
: Ancient oncoids were formed in different settings (see Sect. 4.2.4.1), both in high-energy and low-energy
environments. Oncoid rudstones and grainstones were common in open-marine winnowed edge sand areas (FZ 6),
open shelf lagoons (FZ 7), the environs of platform patch reefs, and in backreef areas behind larger reef com-
plexes. On ramps grain-supported oncoids originated predominantly in shallow inner ramp settings.
Additional
picture:
Pl. 12/1.
1
SMF 12B
S
.
Bivalve floatstone.
Concentration of bivalve shells resulting from high population density on a slope (FZ 4).
The absence of bioerosion, abrasion and encrustation contradicts transport. The recrystallized bivalves are double-valved
(center) or occur as single valves. Matrix is a lime mudstone. Late Triassic: Steinplatte, Tyrol, Northern Alps, Austria.
2
SMF 12C
RIN
.
Crinoid floatstone.
Crinoid stem fragments and arm plates (arrows) are embedded within a bioclastic
wackestone matrix containing fine-grained echinoderm debris. Note the mixture of complete and broken elements. This
crinoid limestones originated at the flanks of a mud mound (FZ 5; see Pl. 144). Mississippian (Waulsortian facies): Muleshoe
Mound, Sacramento Mountains, New Mexico, U.S.A.
3
SMF 13
.
Oncoid-ooid rudstone.
Large oncoids formed by skeletal and non-skeletal cyanobacteria and encrusting foramin-
ifera (white arrows) in association with smaller bioclasts and radially-structured ooids. The ellipsoidal shape of the on-
coids is controlled by the shape of the cores (bivalve shells). Note the irregular knobby surface (black arrows), indicating
the absence of reworking of the oncoids. The oncoids shown at the bottom right are compacted and dissolved at their
boundary. The oncoids formed in moderately high-energy, shallow waters near a platform edge (FZ 6). The radially-
structured ooids were swept into the environment of oncoid formation from protected parts of the platform. Late Jurassic:
Switzerland.
