Geology Reference
In-Depth Information
O r i g i n T y p e s D i a g n o s t i c c r i t e r i a
Lithified organic
1
Fecal pellets
Rounded elongated, rodshaped or ovoid darkcolored micritic
excrements
grains, rarely spherical. Commonly homogeneous or with
siltsized inclusions; rarely with defined internal structures.
Sizes <100
m to several millimeters. Sometimes associated
with bioturbation structures
. . . . . . . . . . . . . . . . . . . . Pl. 136/8
Abrasional products of
2
Algal peloids
Irregularly shaped, rounded micritic grains, exhibiting
algae and calcimicrobes
gradations from grains with relicts of algal structures to homo
geneous grains. Size <20
m to ~2 mm
. . . . . . . . . . . Pl. 136/4
Grains resulting from hard
3
Bioerosional
Scoopshaped subrounded and angular grains.
partboring and rasping
peloids
Sizes from 20
m to 100
m
activity of organisms
Synsedimentary and post
4
Mud peloids
Variously shaped micritic grains, commonly without internal
sedimentary reworking of
(Lithic peloids)
structures. Wide size ranges, poor sorting. Frequent
carbonate mud and micrite
occurrence within distinct beds or laminae . . Pl. 10/2, Pl. 121/2
Internal micritic molds
5
Mold peloids
Ovoid micritic grains, sometimes with relicts of still
of bivalved shells
undissolved shells (ostracods, small bivalves). . . . . . . Pl. 132/8
Ooids and rounded skeletal
6
Bahamite
Round micritic grains, some of which with relicts of the
grains whose microstructures
peloids
primary microstructures. Association of peloids, aggregate
have been lost through
grains and ooids. Transition of micritized bioclasts to peloids of
micritization
the same size. Larger than algal peloids . . . . . Pl. 10/3, Pl. 43/1
Ooids and skeletal grains;
7
Pelletoids
Microcrystalline grains, in places exhibiting vague residual
microstructures destroyed
internal structures. Diffuse outlines due to amalgamation
by recrystallization
and compaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pl. 38/6
Biochemical precipitation
8
Microbial
Rounded micritic grains associated with laminated and clotted
triggered by microbes and
peloids
fabrics. Sizes from <80
m to >600
m . . . . . . Pl. 8/6, Pl. 10/1
or
ga
nic
s
ubs
ta
nce
s
Chemical precipitation of
9
Precipitated
Tiny peloids within carbonate cements; consisting of a cloudy
carbonate cements with
peloids
micritic center surrounded by clear exterior rims of crystals.
or without organic controls
Occurrence in cavity fill precipitates (e.g. in reefs)
. . . . Pl. 8/5
Fig. 4.11.
Origin and diagnostic criteria of carbonate peloids.
The term 'peloid' is only descriptive until thin-section studies
reveal information providing a genetic connotation.
Criteria: Fecal pellets are commonly elongate, rod-
shaped or ovoid grains. Most fecal pellets are homoge-
neous, but some may contain silt-sized inclusions
(quartz or skeletal debris). The dark color of the grains
or the peripheral rim in transmitted light is caused by
the high content of organic matter or iron sulfides. Fe-
cal pellets occur in isolated nests and are common in
burrowed limestones. Parautochthonous occurrences
are often characterized by small, moderate to well-
sorted pellets. The sorting of fecal pellets provides a
first approximation of the hydrodynamic behavior of
these grains (Wanless et al. 1981).
Environment: Carbonate fecal pellets are produced
in tropical marine and in non-marine environments, but
are more commonly preserved in subtidal and lower
intertidal zones of inner platform or ramp settings, with
low water energy and reduced sedimentation rates. The
fossilization of the originally soft particles requires bac-
terial decomposition of the organic mucus and intra-
granular cementation by Mg-calcite or aragonite. Lithi-
fication takes place preferentially in warm shallow wa-
ters that are supersaturated with respect to calcium car-
bonate, i.e. the interior part of the Bahama platform
(Kornicker and Purdy 1957; Land and Moore 1980).
Non-marine fecal pellets occur in salt lakes and
freshwater lakes, and are known from pedogenic car-
bonates. Fecal pellets of filter-feeding zooplanktonic
crustaceans (copepods), reported from ancient lacus-
trine limestones, are elongated grains with long diam-
eters of 250 to 500
m. Pedogenic and caliche peloids
vary greatly in shape and sizes (10
m to more than
1 mm). Ovoid grains are fecal pellets produced by
worms, ants or gastropods (Klappa 1978; Jones and
Squair 1988). Sometimes vegetal microstructures are
preserved within the grains. Other caliche peloids origi-
nate from carbonate precipitation around roots and root
hairs or in association with the decomposition of veg-
etal and fungal matter (types 8 and 9 in Fig. 4.11) or by
micritization of skeletal grains (type 6 in Fig. 4.11;
Calvet and Julia 1983).
