Geology Reference
In-Depth Information
Reworked and redeposited mud peloids are common
in protected shallow-marine environments. Larger lime
mud intraclasts and smaller mud peloids can originate
if lime mud dries out and small chips become commi-
nuted and rounded, or if storms and currents disturb
the bottom sediments.
micritized and recrystallized ooids or bioclasts. Reid
et al. (1991, 1992) and Reid and MacIntyre (1998) em-
phasize the widespread occurrence of micritization/re-
crystallization processes affecting foraminiferal and
algal grains, and leading to the formation of micro- and
cryptocrystalline grains. Peloids originating from the
micritization of ooids have been called
pseudopeloids
(Lakschewitz et al. 1991; Pl. 10/3) aiming to distin-
guish these peloids from peloids that represent micrit-
ized bioclasts.
A never ending story - A comment on the term
'pseudo-ooid':
The original definition of this term
(Bornemann 1886) referred to transported and worn
fragments of fine-grained limestone clasts observed in
thin sections of the Triassic Muschelkalk. Later uses of
the term connoted well-rounded, structureless carbon-
ate grains of very different origin (Flügel and Kirch-
mayer 1962). But this connotation coincides with the
definition of pseudopellets as well as with that of small
well-rounded intra- and lithoclasts. Some authors still
use pseudo-ooid to describe ooids with poorly preserved
structures or as a name for strongly micritized grains.
However, the term pseudo-ooid should no longer be
used now.
(7) Pelletoids
(Fig. 4.12; Pl. 38/6): The term was intro-
duced by Blatt et al. (1972) for microcrystalline grains
that have been generated by the recrystallization of vari-
ous carbonate particles, such as skeletal grains (e.g. fora-
minifera) and ooids. Pelletoids are often amalgamated
and have diffuse outlines due to compaction and ag-
grading crystal growth. Some pelletoids may show
vague residual internal structures of the original grain.
Distinguishing between pelletoids and bahamite peloids
can be difficult.
-> Note that some authors use the term 'pelletoid' for
textures exhibiting pellet-like grains which are believed
to have been formed by incipient or incomplete recrys-
tallization of micrite. Other authors use the adjective
'pelletoid' to describe limestones with (non-altered) pe-
loids.
(5) Peloids resulting from the internal molds of fos-
sils
(Fig. 4.12; Pl. 132/8): Internal molds of thin-shelled
ostracods, mollusks and foraminifera can be preserved
as small peloids when the shells are dissolved.
Peloids resulting from syn- or postdepositional alter-
ations of grains:
In-situ formation:
(6) Bahamite peloids: Micritized grains
(Pl. 10/3, Pl.
43/1, Pl. 121/4): This category comprises isolated and
composite peloids whose formation is in some way
comparable to that of morphologically similar grains
in the modern Bahama Banks. Intensive micritization
of bioclasts and of ooids, leading to the complete loss
of the microstructure of the grains, can produce round
micrite grains. These grains occur in association with
aggregate grains, ooids and micritic intraclasts. The pe-
loids are generally larger than algal peloids.
Originally the name
bahamite
was proposed for an-
cient shallow-marine limestones with grains that closely
resemble grain assemblages found in the interior of the
Bahama Banks (Beales 1958), but also known from
other modern shallow-marine environments (Kendall
and Skipwith 1969; Logan 1974; Pusey 1975). The term
refers to carbonates composed of accretionary and com-
posite grains including aggregate grains and peloids.
Gygi (1969), studying Jurassic platform carbonates,
introduced the name
bahamite grain
for structureless
microcrystalline peloids and aggregates of these peloids.
Genetically these peloids are altered fecal pellets and
(8) Microbial peloids
(Pl. 8/6, Pl. 10/1): There are in-
creasingly more reports about the in-situ formation of
carbonate particles in modern microbial mats, on an-
cient micrite crusts, and within the peloidal textures of
mud mounds (Box 4.7; Friedman 1978; Javor and
Castenholz 1981; Pickard 1992; see Facies, volume 29,
1993). These particles correspond morphologically to
peloids, ooids, oncoids and aggregate grains. In-situ
growth of modern oncoids has been reported from lakes,
lake margins and sabkhas (Jones and Wilkinson 1978),
and rivers or streams (Freytet and Plaziat 1965; Hart-
kopf-Fröder et al. 1989). Ancient
in situ
growth has
been claimed for Tertiary lacustrine oncoids (Leinfelder
and Hartkopf-Fröder 1990).
Algal
and
cyanobacterial pellets
formed in situ
(
benthic peloids
) occur in microbial mats of salinas,
saline lakes and intertidal settings (Gerdes et al. 1994;
Kazmierczak et al. 1996). The rounded micritic grains
as well as autochthonous micrite (Sect. 4.1.2) are the
result of biochemical precipitation triggered by micro-
bial activity leading to the decomposition of organic
