Geology Reference
In-Depth Information
Practical classification
Fig. 4.14 outlines a practicable classification of com-
mon oncoid and rhodoid types, based on the dominat-
ing biota involved in the formation of the grains:
(A)
Oncoids:
The main types are differentiated by
(a) the existence/absence of non-skeletal or skeletal
microstructures, the biotic composition and the occur-
rence of non-algal/cyanobacterial (zoogenic) biota, and
(b) the diversity of co-existing biota that contribute to
the cortices.
others are not.
Composite oncoids
consist of several
taxa that form alternating layers.
Common types are (1) grains built by various cyano-
bacterial and algal taxa; (2) cyanobacteria and/or algae
associated with encrusting foraminifera (e.g. 'osagid
grains': Henbest 1963; Toomey et al. 1989); and (3)
cyanobacteria and algae, associated with taxonomically
different encrusters (e.g. foraminifera, bryozoans, ser-
pulids (
Girvanella
-spirorbid oncoids: Poncet and
Lapparent 1975). Rare groups which contribute to the
formation of oncoids, particularly in non-marine and
marginal-marine environments, are spirorbid worms
and vermetid gastropods (Poncet and de Lapparent 1975;
Burchette and Riding 1977).
(B)
Rhodoids:
They are easily recognized by the
very fine micritic network of the microstructures (Pl. 54,
Pl. 56). Opposed to oncoids, the internal structure of
rhodoids is strongly controlled by the internal growth
pattern of the red algal thalli. Rhodoids, therefore, are
not necessarily laminated. Nodular thalli of very fine-
grained calcareous red algae may be confused with mi-
crite oncoids, e.g.
Marinella
Pfender, a common con-
stituent of Late Jurassic and Cretaceous platform car-
bonates (Pl.136/3).
Composite rhodoids
originate from
the growth of red algae associated with encrusting fora-
minifera and other sessile organisms. The diversity
within the cortex may be low, moderate and high, and
is a sensible measure for short-term changes of envi-
ronmental conditions.
Micrite oncoids
consist of micrite, exhibit no dis-
tinct lamination, but are comparable in shape to other
oncoid types. These oncoids result from trapping of
fine-grained sediment on the surface of vanished mi-
crobial/algal felts and/or microbially induced precipi-
tation of carbonate. The oncoids correspond genetically
to spongiostromate oncoids. Many micrite oncoids,
however, are caused by obliteration of porostromate
and other oncoids due to pervasive microboring and
recrystallization.
Spongiostromate oncoids
are poorly
or distinctly laminated grains, consisting of micritic lay-
ers around a bio- or lithoclastic nucleus. The laminae
are the result of sediment trapping and the calcification
of microbial or algal filaments. Owing to the rapid de-
composition of organic filaments, commonly only the
lamination but not the filaments are visible. Spongios-
tromate microfabrics are variable. They embrace mi-
critic, spongy, fenestral, peloidal and other fabrics.
These fabrics are produced by differences in environ-
mental factors, biotic composition and diagenetic over-
print. Comparable textures are known from recent as
well as ancient stromatolites (Logan et al. 1974; Monty
1976, 1976). Organisms which can be incorporated
within spongiostromate oncoids are foraminifera, bryo-
zoans and various microproblematica (Kutek and
Radwanski 1965; Elliott 1966; Dahanayake et al. 1976).
Porostromate oncoids
are layered or non-layered, usu-
ally micritic grains exhibiting remains of skeletal mi-
crobes and calcareous algae. Oncoids formed by rec-
ognizable cyanobacteria are called
cyanoids
. Porostro-
mate microfabrics are defined by loose or tangled, vari-
ously oriented, straight or sinuous calcified filaments.
Typical cyanoids are
Girvanella
oncoids, known from
the Cambrian to the Jurassic. A characteristic feature
of these oncoids is the alternation of dark
Girvanella
-
bearing and light barren laminae.
Girvanella
oncoids
are more rapidly lithified than spongiostromate oncoids
as evidenced by more abundant macroborings in po-
rostromate oncoids.
Foraminiferal oncoids
are formed
by encrustations of calcareous or agglutinated tests,
starting with settlement on a hard surface, e.g. a bio-
clast. Some foraminiferal oncoids are distinctly layered,
In-depth classifications:
Oncoids have been subdi-
vided according to biotic composition, internal struc-
tures (lamination), shape and growth form as well as
size (Maslov 1960; Dragastan 1964; Zhuravleva 1964;
Logan et al. 1964; Radwanski and Szulczewski 1966;
Catalov 1970; Dahanayake 1977, 1978). Similar crite-
ria have been used for categorizing rhodoids, but the
types of internal structures, systematic composition of
the red algae as well as the shapes of the oncoids are
more relevant (Bosellini and Ginsburg 1971; Bosence
1983). Catalov (1983) differentiated 'phytogenic' on-
coids (corresponding to porostromate oncoids), 'zoo-
genic' oncoids (foraminiferal oncoids), and 'zoophyto-
genic oncoids' (composite oncoids).
4.2.4.1 Oncoids
Modern oncoids
Present-day oncoids form in a wide variety of envi-
ronments, ranging from freshwater lakes, streams and
marshes to marine inter- and subtidal conditions. It is
