Geology Reference
In-Depth Information
tissue (e.g. sponges) and organic substance. Small-
scaled peloids occurring in microcavities off Lizard Is-
land, Great Barrier Reef, grow in organic mucilages
enriched by Ca-binding proteins (Reitner and Neuweiler
1995).
Tuberoids are considered a special kind of peloid.
The term was suggested for rounded and nodular, dark-
colored and variously sized micritic particles (Fritz
1958) occurring in abundance in bedded limestones
adjacent to sponge reefs. These grains are common in
Jurassic and Cretaceous limestones with remains of sili-
ceous sponges and micritic crusts (Pl. 116/1; Kott 1989).
The grains are interpreted as precipitates of carbonate
induced by decaying organic matter.
Diagenesis of peloids
Modern marine peloids consist of High-Mg calcite
and/or aragonite, freshwater peloids are composed of
Low-Mg calcite (Brand and Veizer 1983; Rao et al.
1994). Peloid limestones appear to have suffered re-
markably little compaction (Beales 1965). The preser-
vation of carbonate peloids of fecal origin is only pos-
sible if lithification takes place rapidly.
The precipitation of Mg-calcite peloid cement plays
an important role in the submarine cementation of up-
per slope sediments. Gradations from amorphous mi-
crocrystalline calcite to clotted peloidal micrite and pe-
loidal micrite, followed by the in-situ precipitation of
polypeloidal aggregates have been observed on Baha-
mian slopes (Wilber and Neumann 1993).Selective
dolomitization of peloids is common in fecal pellets as
well as algal peloids and mud grain peloids.
(9) Precipitated peloids (Pl. 8/5): Inorganic and organi-
cally induced chemical in-situ precipitation has been
proved for the origin of very small (20-60 m, usually
40 m) peloids within calcite cements and occurring
within inter- and intraskeletal cavities of reefs as well
as in non-reef cavities (Macintyre 1985; Chafetz 1991;
Reitner and Neuweiler 1995). These peloids are a com-
mon characteristic of submarine Mg-calcite cements
(Macintyre et al. 1968; Shinn 1969; Land and Goreau
1970; Alexandersson 1972).
The peloids consist of well-developed dentate rims
of microcrystalline rhombic calcite forming small ro-
settes (5-10 m in length) around a nondescript cloudy
anhedral nucleus.
Significance of peloids
Ancient peloid limestones account for thousands of
cubic kilometers of lagoonal, shelf, bank, reef-core and
mud mound deposits, but they are relatively rarer than
other limestone types.
Paleoenvironmental proxies: Many shallow-marine
peloidal grainstones have a scarcity of biotic constitu-
ents or contain only specific biota (ostracods, benthic
foraminifera, green algae) indicating conditions were
inadequate for many animals with regard to salinity and
substrate. By contrast, peloidal packstones and wacke-
stones with abundant fecal pellets and burrows indi-
cate adequate nutrients and oxygen.
Depositional proxies: Many ancient peloid lime-
stones appear to have formed in shallow warm-water
environments (banks, shelf- and reef lagoons). Fine-
grained peloidal limestones are generally regarded as
typical of shallow, low-energy, restricted marine envi-
ronments. This generalization is supported by many
case studies, but the examples should be critically evalu-
ated for the following reasons:
1
This bimodal texture has been explained as the re-
sult of
(a) initial rapid precipitation of anhedral magnes-
ium calcite, followed by a second stage of slower pre-
cipitation of larger euhedral Mg-calcite, forming the
dentate rims (Macintyre 1977, 1978; Lighty 1985);
(b) intermediate stages of the development of ad-
vancing crystal growth from microcrystalline Mg-cal-
cite crystals (Alexandersson 1972);
(c) crystal growth around a nucleus of suspended
silt-sized clastic particles that were flushed into cavi-
ties by wave pumping (James et al. 1976); and
(d) growth of cement crystals induced by bacteria
(Buczynski and Chafetz 1991; Sect. 4.1.2). Peloidal car-
bonate cements occur as peloidal fill (e.g. in small reef
cavities), in the form of rim cement and coatings on
grains, and as thin dense crusts on the surface of coral
skeletons and reef rubble.
Accumulation of peloids may record specific pres-
ervation sites rather than specific depositional envi-
ronments.
2
Mass occurrences of fecal pellets can be allochtho-
nous or autochthonous, and the size and sorting of
these peloids, therefore, may not be a measure of
current energy.
3
Peloids may be deposited near the place of their ori-
gin but can also be transported to shallower or deeper
environments. Understanding of the hydrodynamic
behavior of carbonate fecal pellets is important for
interpretating of the origin of peloidal carbonates
(Wanless et al. 1981).
Common peloid microfabrics
Carbonate peloids contribute to the formation of four
widely distributed microfabrics (Box 4.7) important in
the context of facies interpretations.
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