Microfacies Data: Matrix and Grains - Microfacies of Carbonate Rocks - page 123

Compositional Types Examples Dominating Biota

Micrite oncoid Pl. 12/2 Microbes Note: Micrite oncoids and

spongiostromate oncoids

Spongiostromate oncoids Pl. 11/34; Pl. 12/7; Microbes and algae also originate from micritization

(nonskeletal) Pl. 89/3; Pl. 131/5 of porostromate oncoids

Porostromate oncoids Pl. 12/1,3,4; Calcified cyanobacteria, e.g. Cyanoids (formed by calci

(skeletal) Pl. 142/11; Pl. 136/4 algae microbes); Girvanella oncoids

Foraminiferal oncoids Pl. 11/56; Pl. 51/2 Encrusting foraminifera e.g. Nubecularia oncoids

Pl. 111/18

Composite oncoids Pl. 51/9; Pl. 119/3 Association of cyano e.g. Girvanellaforaminiferal oncoids;

bacteria, algae, GirvanellaSpirorbis oncoids;

foraminifera or other foraminiferalalgal oncoids

encrusting organisms

Rhodoids Pl. 56/8 Calcareous red algae e.g. Coralline algal rhodoids;

Archaeolithophyllum rhodoids

Composite rhodoids Pl. 12/6; Pl. 149/6 Associations of red e.g. Foraminiferared algal rhodoids

algae and other
encrusting organisms

Macroids Foraminifera or bryo e.g. Acervulinid foraminiferal

zoans, or other macroids; bryozoan macroids

encrusting organisms (bryoids)

Fig. 4.14. Practical classification of oncoids, rhodoids and macroids.

The name macroid was suggested by Peryt (1983)
for organically coated grains with sizes >10 mm (see
Fig. 4.14). Presently the term is used for cm-sized coated
grains formed predominantly by encrusting metazoans
or protozoans, sometimes associated with algae (e.g.
balanid macroids, serpulid macroids). Foraminiferal
macroids constructed by Acervulina or Gypsina are
common constituents of Cenozoic warm- and cool-
water carbonates (Perrin 1992). Nodular growth forms
produced by free-living corals were called coralliths
(Gill and Coates 1977).

oncoids: micro-oncoid or microids (<2 mm), piso-on-
coid (2 to 10 mm), and macro-oncoid or macroids (>10
or >20 mm). Most modern and Tertiary macroids which
are not formed predominantly by coralline algae (e.g.
foraminiferal macroids) exhibit sizes ranging from
about 10 to 100 mm. Since the abundance of very small
and of exceptionally large oncoids may indicate spe-
cific hydrodynamic controls, the differentiation of mi-
cro-oncoids and macro-oncoids is meaningful. The term
piso-oncoid should be replaced by meso-oncoid for on-
coids with sizes from about 2 mm to about 10 mm, but
usually only up to 5 mm, because the suffix piso- is too
close to 'pisoid' which refers to a grain category dis-
tinctly different from oncoids.

Composite growth types: Many oncoids and rhod-
oids consist of two or more encrusting organisms that
occur in abundance and form repetitive or randomly
distributed structures. Examples are oncoids formed by
calcimicrobes and encrusting foraminifera, or oncoids
built by foraminifera and coralline algae (Pl. 149/6).
The latter have been called 'for-algaliths' (Prager and
Ginsburg 1989), but a simple descriptive designation
as foraminiferal-coralline algal oncoids of these com-
posite oncoids is sufficient, too (Fig. 4.14). Highly di-
verse oncoid communities indicate rather stable condi-
tions during the growth of the grains over time.

Semantic problems: Oncoid designates the indi-
vidual grain, oncolite a limestone consisting of more
than 50 % oncoids. This definition parallels the strongly
recommended use of ooid for the grain and oolite for a
sedimentary rock composed of ooids (Teichert 1958).
Analogously, the terms cyanoid and rhodoid should be
used for the the individual grains, and cyanolite and
rhodolite for the carbonate rocks. But many authors
still prefer the suffix '-lith' and not '-oid' in order to
name the individual grains ( rhodoliths: Bosence 1983).
The terms 'oncoid-bearing' or 'rhodoid-bearing' char-
acterize limestones with less than 50% oncoids.

Size catagories: Kutek and Radwanski (1965) and
Peryt (1983) proposed names describing size grades of