Geology Reference
In-Depth Information
Spongiostromate vs. porostromate oncoids:
The
name 'oncoid' was suggested by Heim (1916) for mi-
critic grains found in Jurassic shelf limestones in Swit-
zerland. The author discussed a bacterial origin. Ap-
parently unaware of Heim's definition, Pia (1927) con-
sidered oncoids to be unattached stromatolites. Classi-
fying blue-green algae (cyanobacteria) of unknown af-
finities he distinguished two informal groups, Spongio-
stromata and Porostromata.
Spongiostromata
are nodules or heads with defined
growth forms but without (or very rarely with) visible
organic microstructures. Concentric laminae originate
from fine grains of sediment adhering to the muci-
lagineous surface of cyanobacteria and algae and also
from the precipitation of calcium carbonate in response
to the withdrawal of carbon dioxide during photosyn-
thesis.
Porostromata
comprise forms exhibiting pre-
served tubular microstructures. To subdivide the Spon-
giostromata, Pia used the terms 'Stromatolithi' and
'Oncolithi'. The former include forms which grow at-
tached to the substrate; the latter term refers to unat-
tached free nodules.
Today, the descriptive term
spongiostromate oncoid
(Pl. 11/3) is used for micrite oncoids possessing a lami-
nated dense micritic or spongy fabric without visible
filaments;
porostromate oncoid
(Pl. 12/3) applies to
grains exhibiting fine micritic-walled tubes, generally
less than 100
m in size (Monty 1981). Since these
tubes are prone to obliteration by recrystallization, the
apparent lack of tubular structures is not an unequivo-
cal sign of spongiostromate oncoids.
The co-existence of oncoids with well-preserved po-
rostromate microstructures and laminated oncoids of
the same size and shape, but without tubular micro-
structures may point to diagenetically altered poros-
tromate oncoids ('paraporostromate oncoids': Catalov
1983).
Skeletal and non-skeletal - a basic subdivision:
Be-
cause the Porostromata include fossils of different sys-
tematic position, Riding (1977) introduced the term
skeletal stromatolites
for “stromatolites in which the
organisms responsible for their formation are commonly
preserved as calcified fossils”. The term
non-skeletal
is identical with 'spongiostromate oncoid'. Skeletal
stromatolites and porostromate oncoids are typically
constructed by calcified cyanobacteria, but green al-
gae and microproblematica also take part in the forma-
tion of non-attached or attached structures. Recent coun-
terparts of
Girvanella,
a common microfossil in poro-
stromate oncoids, consist of Low-Mg calcite. The same
mineralogy is inferred for ancient
Girvanella
oncoids,
but ferroan calcite in some
Girvanella
skeletons indi-
cates a High-Mg calcite mineralogy, too (Richter and
Füchtbauer 1978).
Rhodoids and cyanoids - more sensitive categories:
Since it is desirable to separate coated nodular grains
of different origin and formed in different environments,
specific names have been coined, starting with the term
'rhodolite' for “nodules and detached branch growth
with a nodular form composed principally of coralline
algae” (Bosellini and Ginsburg 1971). Because rhodo-
lite has priority for a variety of the mineral garnet, the
name was later replaced by 'rhodolith'. Peryt (1983)
suggested the name
rhodoid
for nodular grains formed
mainly by coralline but also by other red algae.
Finally, Riding (1979, 1983) proposed the name
cyanolith
or
cyanoid
for porostromate oncoids formed
predominantly by calcified filamentous and coccoid cy-
anobacteria that are commonly preserved as tubes. Cor-
responding stromatolites were called 'skeletal stroma-
tolites'. Oncoids with tiny tubular microfossils that can
not be identified as being of cyanobacterial origin
should simply be called 'porostromate oncoids'.
Oncoids and stromatolites:
A particular type of mod-
ern and ancient oncoids is associated with intertidal and
shallow subtidal algal mats and stromatolites. These
oncoids are microbialites and sometimes formed in situ
within algal mats (Dahanayake et al. 1985). A geometri-
cal classification of modern shallow-marine stromato-
lites and oncoids (including 'algal biscuits': Gebelein
1969) was proposed by Logan et al. (1964) and
promptly adapted to ancient oncoids (e.g. Radwanski
and Szulczewski 1966). However, the premise that re-
cent marginal-marine oncoids are useful paleoenviron-
mental analogues of ancient lithified calcareous oncoids
was questioned by Monty (1972), who stated that mod-
ern freshwater algal nodules are the much better ana-
logues for pre-Cenozoic marine oncoids.
Non-algal oncoids and macroids:
The original defi-
nition of the term oncoid has been broadly expanded.
It presently covers carbonate nodules formed as bio-
genic accretions not only by microbes, algae and mi-
cro-organisms, but also by bryozoans or serpulids for
example.
Bryozoan nodules, also called 'bryooncoids' or
'bryooids', are built by free-living bryozoan colonies
associated with microbes (Hillmer et al. 1996, Scholz
2000). The laminar growth of bryozoan colonies
circumcrust litho- or bioclastic nuclei. Modern cm-sized
bryooncoids occur in temperate regions (Rider and
Enrico 1979). Ancient counterparts have been described
from Neogene and Cretaceous sediments (Balson and
Taylor 1982).
