Geology Reference
In-Depth Information
The name pellet should be restricted to grains of fe-
cal origin; for all the other micrite grains the term 'pe-
loid' is recommended. Many peloids are subrounded
or rounded, but ovoid and rodlike shapes may also oc-
cur. The dimensions of the silt- to fine-sand-sized par-
ticles vary within a range of a few m to a few mm, but
most calcareous peloids are rarely larger than 500 m,
and commonly exhibit a diameter of < 200 m, often
30 to 100 m.
sites with abundant peloids are the Persian Gulf and
the sea off Egypt (Stoffers et al. 1980). Peloids are also
generated within algal/cyanobacterial mats; examples
are known from sea-marginal hypersaline pools along
the shores of the Gulf of Aqaba, the Red Sea, and Ca-
nary Island (Friedman et al. 1972; Gerdes and Krumbein
1987).
The 'peloidal question': What are the genetic subtypes
of ancient peloids?
The origin of ancient peloids is often in doubt and
the attempt to distinguish them is limited to recogniz-
ing of peloids of fecal, detrital or in situ precipitated
origin (MacIntyre 1985). There are, however, some
clues for better genetic specification of peloids. Sev-
eral terms have been suggested for characterizing ge-
netic subcategories. Shape, size and sorting as well as
the composition of the grain associations are diagnos-
tic criteria for interpreting the origin of peloids. The
following peloid subcategories are used in microfacies
studies (Fig. 4.11).
Comparison with other grain types
Peloids differ from ooids and oncoids by the ab-
sence of concentric or radial internal structures, and
from small rounded intraclasts by their uniform shape,
good sorting and small size (<200 m). Peloids are gen-
erally smaller than ooids, pisoids and oncoids.
Occurrence
Peloids occur in rock-building abundance forming
peloid limestones (Pl. 121/1-3) or as only minor con-
stituents of carbonate rocks together with other grain
types (Pl. 122/1). Peloids constitute grain- or mud-sup-
ported fabrics (contributing to wackestone, packstone,
grainstone and bindstone textures), are found as isolated
or amalgamated grains (Pl. 50/2), and occur within lay-
ered (Pl. 50/2, 5) and clotted textures (Pl. 10/1). Pe-
loids are common in shallow-marine tidal and subtidal
shelf carbonates and in reef and mud mounds, but are
also abundant in deep-water carbonates. By contrast to
the abundance of peloids in tropical shallow-marine car-
bonate, peloids are rare or absent in non-tropical cool-
water carbonates.
Peloids of biotic origin:
(1) Fecal pellets (Fig. 4.12; Pl. 136/8): Rounded, usu-
ally fine-grained particles caused by organisms that eat
mud, digest organic matter from the mud and excrete
the non-digested lime-mud. It should be kept in mind
that many authors still use the term 'pellet' and not 'fe-
cal pellets' for peloids produced by carbonate mud-in-
gesting organisms.
Producers of fecal pellets in shelf environments are
deposit- and detritus-feeders, herbivorous animals and
plankton-feeders, including arthropods, mollusks (in-
tertidal gastropods and polyplacophorid chitons),
worms, echinoderms, and fishes. Deposit-feeding crus-
taceans, gastropods, holothurians, and worms are quan-
titatively important. Callianassa , a shallow-marine
thalassinid decapod crab, deposits as much as 4.5 mm/
year of fecal pellet mud. The assignment of fossil fecal
pellets to specific pellet-producing animals is difficult
because fecal pellets of most invertebrates lack spe-
cific morphological criteria. Exceptions are gastropod
and crustacean fecal pellets, which are characterized
by specific dimensions and morphological criteria. De-
capod fecal pellets are pierced by longitudinal canals
whose number, size and outline in transverse sections
are used for taxonomic classification ( Favreina , Pl. 93/
5, Sect.10.2.4.7). Fecal pellets accumulate not only in
modern tropical and temperate intertidal and subtidal
settings, but also in deep-marine environments where
planktonic shells (e.g. coccolithophorids) are trans-
ported to the sea bottom via fecal pellets of zooplank-
tonic and nektonic animals (see Fig. 4.4).
Modern peloids
Marine peloids are conspicuous constituents of low-
energy shallow tidal and subtidal platform carbonates
(Pl. 3/1) but are also found in sheltered reef cavities,
and slope and basinal settings. The Great Bahama Bank
is a good example of the importance of peloids for the
makeup of shallow-marine mud-rich carbonates (Imbrie
and Purdy 1962; Purdy 1963; Neumann and Land 1975;
Harris 1979). The 'pellet-mud facies' and 'mud facies'
of the restricted interior platform, originating in very
shallow waters with increased and fluctuating salini-
ties and covering an area of about 10 000 km 2 , com-
prises abundant peloids forming up to 75% of the total
sand fraction. Most pellets are lithified organic excre-
ments (fecal pellets) produced by carbonate ingesting
organisms, particularly by polychaete worms, gastro-
pods and some crustaceans. Open interior parts of the
platforms are floored by peloid and aggregate grain
sands. These peloids result from the micritization of
various carbonate grains ('bahamite peloids'). Other
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