Geology Reference
In-Depth Information
fossil groups affected. This is demonstrated, for ex-
ample, by fossils in Silurian shelf carbonates showing
decreasing susceptibility to micritization from trilobites
to ostracods, brachiopods, bryozoans, corals, and tenta-
culites (Mohamad and Tucker 1992). Micrite envelopes
are relatively resistant to dissolution, protect the grains
from being destroyed, and often contribute to the pres-
ervation of casts of primary aragonitic skeletal grains
during freshwater diagenesis (Pl. 33/4). The stability
of a micrite envelope depends on its stable mineralogi-
cal composition and on a high content of organic mat-
ter within the micritic rim. The envelopes are prone to
selective dolomitization; this may imply that the min-
eralogy of the envelopes was once High-Mg calcite
(Buchbinder and Friedman 1970).
4.2.4 Oncoids and Rhodoids
Nodular coated grains formed by microbes, algae and
other encrusting organisms are common constituents
of platform, reef and slope carbonates. These grains
represent two major categories: 1) oncoids (formed pre-
dominantly by calcibionts and algae) and 2) rhodoids
(formed by free-living calcareous red algae). Oncoids
and rhodoids are excellent indicators of paleoenviron-
mental and climatic conditions, and sea-level fluctua-
tions and depositional settings. As opposed to other car-
bonate grain types, the prevailing settings of oncoid
formation changed over the Phanerozoic and rhodoids
started to dominate from Late Mesozoic to Cenozoic.
Oncoids
(
onchos
, Greek for nodule) are unattached,
rounded, mm- to cm-sized, calcareous or non-calcare-
ous nodules that commonly exhibit a micritic cortex
consisting of more or less concentric and partially over-
lapping laminae around a bio- or lithoclastic nucleus.
The term oncoid is not a genetic term and should be
regarded as purely descriptive. The name is commonly
used in Europe, but not in America where the term
'pisoid' (or pisolith) was and is still used both for coated
grains, called oncoids here, but also for grains similar
in internal structure but larger than ooids (Sect. 4.2.6).
Other not very precise names applied to oncoids are
'algal balls' and 'algally-coated grains', 'mummies' or
'osagid grains'. Oncoids and rhodoids are common in
limestones and marly sediments. The latter allow indi-
vidual grains to be investigated with regard to their com-
position, size and shape. and grain assemblages. The
combined study of thin sections and acid residues aug-
ments information on the organisms involved in the
formation of oncoids. SEM observations and geochemi-
cal studies are necessary to evaluate the contribution
of clay material and organic matter to carbonate and
non-carbonate oncoids (e.g. Carbone and Civitelli 1974;
Andrews 1986; Scudeler Bacelle and Marusso 1983;
Palmer and Wilson 1990).
Comparison with other grain types:
Oncoids differ
from cortoids by the greater thickness of the micritic
coatings and the frequency of preserved cyanobacte-
rial, algal or foraminiferal structures. Ooids and pisoids
are commonly smaller und have regular concentric
laminae. As a rule but with a few exceptions, many
oncoids and rhodoids are considerably greater than all
other carbonate grains.
Paleoenvironmental proxies:
In accordance with the
original explanation that micrite envelopes originate
from light-dependent boring organisms (Bathurst 1966),
coated grains are often regarded as indicators of 'shal-
low-marine warm-water' environments (e.g. Leonard
et al. 1981). Having observed differences in the abun-
dance of microboring frequency and intensity at differ-
ent water depths (Vogel et al. 1995), some authors use
cortoids and micritized grains as indicators of specific
water depths, thus considering the abundance of bored
grains to be reliable indicators of shallow waters down
to a few tens of meters (Swinchatt 1969; Bernard-
Dumanois and Delance 1983).
Although there are many modern and ancient ex-
amples supporting this interpretation, caution is neces-
sary because a few records of micrite envelopes being
formed via non-light dependent microborings and
infillings of calcite cements are also known from deep-
sea settings (Friedman et al. 1971; Hook et al. 1984).
In addition, strong boring and leaching is also a com-
mon feature of colder carbonate-undersaturated marine
waters (Zeff and Perkins 1979; Campbell 1983). But
because carbonate precipitation within the boreholes
is rare or absent, typical cortoids are rarely preserved
in these settings.
Depositional setting:
Bedded grainstones with a high
percentage of cortoids and other coated grains, together
with rounded bioclasts characteristically form in areas
of constant water action, at or above wave base. These
conditions occur in the 'winnowed platform edge sands'
of carbonate platforms (standard microfacies type 11,
see Sect. 14.3.1) as well as in current-washed sand
shoals of inner ramp settings (Pl. 18/1, Pl. 33/ 8). Grainy
cortoid sediments are often associated with patch reefs
scattered across the shelves or occurring in a leeward
position behind reef belts (Pl. 118/3, Pl. 127/1).
Terminology
Various names have been proposed for organically
coated rounded grains consisting of a core and a cortex
formed by encrusting by organisms.
