Geology Reference
In-Depth Information
As opposed to marine oncoids, lacustrine and flu-
vial oncoids often exhibit distinct sets composed of mor-
phologically identical laminae, e.g. couplets of dark and
light laminae. Alternating thin micritic laminae and
thicker laminae with vertical structures reflect the dif-
ferent compositions of the original algal associations
(Monty and Mas 1981; Freytet and Plaziat 1982; Moik
1990; see Pl. 8/3). Laminae thickness and micromor-
phology depend on the interplay between carbonate pre-
cipitation and sediment binding on algal filaments and
the decay of the benthic microbial communities
(Golubic 1976; Burne and Moore 1987). Binding and
inorganic carbonate precipitation appears to be quanti-
tatively more effective than biologically induced pre-
cipitation (Pentecost 1978).
As opposed to marine oncoids, studies of non-ma-
rine oncoids allow a higher resolution of the develop-
ment and time involved in environmental changes.
Growth types of freshwater and brackish oncoids and
stromatolitic structures are powerful aids in reconstruct-
ing the depositional history of lacustrine, fluvial and
deltaic sediments (e.g. Leinfelder 1985). The form, size,
and microstructures are used to distinguish depositional
zones of fluvial systems, e.g. channel base, abandoned
channels, or the mouthes of rivers (Anadon and Zamar-
reno 1981; Nickel 1983; Mäcker 1997). Distributional
patterns of oncoids assist in differenting specific lacus-
trine and fluvial depositional settings in coastal playa
lakes and hypersaline lakes (Herbig 1994). Common
microfacies criteria of non-marine oncoid-bearing lime-
stones are micrite, peloids, black intraclasts, small ra-
dial ooids, stromatolites, gastropod and other mollusk
shells, and ostracods (Link and Awramik 1978). Diag-
nostic criteria for ancient freshwater oncoids are nu-
clei formed by freshwater mollusks, lamination patterns
analogous to modern freshwater oncoids, well-pre-
served microstructures, occurrence together with ter-
restrial sediments, and often large sizes.
Box 4.9.
Selected case studies of ancient lacustrine-flu-
vial spongiostromate and porostromate oncoids and
oncolites.
Pleistocene:
Awramik et al. 2001; Brochier et al. 1986;
Casanova 1985; Dixit 1984.
Tertiary:
Anadon and Zamarreno 1981; Bertrand-Sarfati
et al. 1994; Bignot 1981; Bradley 1929; Colom 1993;
Crouzel et al. 1972; Dixit 1984; Freytet and Plaziat
1982; Herbig 1991, 1994; Klähn 1926; Leinfelder and
Hartkopf-Fröder 1990; Link et al. 1978; Llombart and
Krauss 1982; Mäcker 1997; Moik 1990; Nickel 1983;
Obrhel 1984; Ordonez et al. 1983; Rutte 1953; Speck
1949; Surdam and Stanley 1979; Truc 1978; Weiss
1969.
Cretaceous:
Carozzi 1989; Monty and Mas 1981.
Jurassic:
Leinfelder 1985.
Triassic:
Clemmensen 1978.
Permian:
Clausing 1990; Schäfer and Stapf 1978; Stapf
1989.
Carboniferous
: Bertrand-Sarfati and Fabre 1972; Obrhel
1977.
Devonian:
Donovan 1975; Fannin 1969; Obrhel 1976.
ferent types of bioclastic nuclei (e.g. unionid bivalves,
gastropods, decapod crabs). Mollusk shells are more
commonly used as nuclei for lithoclasts or gravels.
'Hollow' central parts of non-marine and marine on-
coids or nuclei consisting of silt or sandy material can
be caused by the disintegration and subsequent infill-
ing of plant stems acting as nuclei (Jones and Goodbody
1985); those oncoids might be indicators of the exist-
ence of former macrophyte meadows. A common fea-
ture, particularly of larger lacustrine oncoids, are fine
radial structures (Pl. 12/7) sometimes emphasized by
vertically arranged micro-cracks. Some of these radial
structures are inaugurated by vertical growth patterns
of algae.
The composition of non-marine or extremely mar-
ginal marine oncoids depends on the growth patterns
of algae, cyanobacteria and sometimes also of foramin-
ifera and other organisms. The abundance and shape of
oncoids is only sometimes hydrodynamically con-
trolled. Many lacustrine oncoids originate from in-situ
growth. Lacustrine oncoids have ellipsoidal and sphe-
roidal, sometimes cylindrical shapes. Common sizes
vary between 15 and about 200 mm. Sphaeroidal on-
coids are generally attributed to fluvial channels and
bottom lags of ponds; more irregular forms occur
preferrentially in shallow parts of lakes (e.g. Nickel
1983, Casanova 1985), but these relations should not
be transferred to all ancient examples (Leinfelder and
Hartkopf-Fröder 1990).
Ancient marine oncoids:
Phanerozoic marine on-
coids were formed in at least eight different settings
including (1) tidally influenced marginal-marine envi-
ronments, (2) open-marine shelf lagoons, (3) the envi-
rons of platform patch reefs, (4) back-reef areas, (5)
associated with platform margin reefs, (6) on the upper
slope, (7) basins, and (8) on pelagic platforms. These
settings were best studied for Late Jurassic carbonates,
particularly in Switzerland and France. Some of these
settings can be differentiated by the types and compo-
sition of the oncoids, and in the associated sedimen-
tary facies. Fig. 4.17 is a simplified summary of the
main criteria of Late Jurassic oncoites and oncoid-bear-
ing limestones. Oncoids are frequent constituents of
