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ates together with 'katagraphs' (non-laminated carbon-
ate structures of different size, and a structure consist-
ing of micritic particles and peloids: Zhuravleva 1964,
1979). Conspicuous criteria of many Precambrian on-
coids are their large size (up to tens of centimeters) and
radialfibrous microfabrics interpreted as bacterially in-
duced structures. Until the Vendian-Cambrian transi-
tion, oncoids are represented by spongiostromate and
micritic growth types. The first calcimicrobes forming
porostromate oncoids appear in the Late Vendian.
Peryt (1981) suggested a distinct change in the ma-
jor oncoid types and their environments during the
Phanerozoic. He assumes that Paleozoic spongio-
stromate oncoids were predominantly formed in lacus-
trine and transitional marine environments, whereas po-
rostromate Girvanella oncoids flourished in marine
subtidal environments from the Cambrian to the Juras-
sic. Starting in the mid-Jurassic, porostromate oncoids
were replaced by spongiostromate oncoids, which be-
came the only type in marine settings and continued to
flourish in lacustrine settings. Monty (1979) made the
point, that spongiostromate oncoids were substituted
in marine settings by rhodoids starting in the Early Ter-
tiary. This concept is supported by the predominance
of cyanoids in Paleozoic shelf carbonates. However,
porostromate and spongiostromate oncoids may occur
in close association in the same beds, pointing to a much
stronger biological control on oncoid formation than
previously thought.
Fig. 4.18. Weathered surfaces of oncoid limestones.
A : Oncolite (oncoid packstone) composed of spheroidal and
ellipsoidal meso- and macro-sized oncoids embedded within
argillaceous bioclastic micrite. Most oncoids are Girvanella
oncoids (cyanoids) or composite oncoids made by cyanobac-
teria and encrusting foraminifera. Note the high amount of
small broken oncoid debris between the larger well-preserved
oncoids and aligned oncoids, indicating reworking by cur-
rents. Early Permian: Carnic Alps, Carinthia, Austria.
B : Oncoid wackestone with variously sized and poorly sorted
ellipsoidal oncoids. The argillaceous carbonate matrix con-
tains fine-bioclastic debris. Note the large size of the nuclei
and the thin micritic coatings. Late Triassic (Carnian): Obir,
Karawanken Mountains, Carinthia, Austria.
Significance of oncoids in microfacies analysis
Oncoids provide important microfacies data. Box
4.11 summarizes some significant points.
Two case studies
Early Permian oncolites and rhodoliths of the Car-
nic Alps, Austria - different composition and different
setting (Fig. 4.18A, Pl. 12/1): Oncolite beds consisting
of porostromate meso-sized oncoids with ranges be-
tween 10 and >50 mm are intercalated in shallow-ma-
rine inner and outer platform carbonates. Four types
can be recognized according to their biotic composi-
tion: (1) Girvanella -foraminiferal oncoids of near-coast
settings with strong siliciclastic input (Fig. 4.18A);
(2) large composite oncoids (size up to 70 mm) com-
posed of Girvanella and Claracrusta of open-marine
inner platforms with minor siliciclastic input; (3) large
red algal-foraminiferal rhodoids made of Archaeolitho-
phyllum and calcitornellid foraminifera, and associated
with fusulinids, dasyclad algae and mollusks, occur-
ring in open platform settings; and (4) centimeter-sized
spherical Archaeolithoporella -oncoids, associated with
cement-rich platform-margin reefs. Similar types oc-
cur in comparable settings in Late Carboniferous and
Early Permian platform carbonates of the Tethys and
in the United States (e.g. Toomey 1983; Toomey et al.
1988, 1989).
Late Triassic oncoid-bearing beds of the Calcare-
ous Alps, Austria - regional marker beds (Fig. 4.18B):
The huge middle and upper Late Triassic carbonate plat-
forms of the Alps are separated by Carnian siliciclastic-
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