Geology Reference
In-Depth Information
Jurassic shelf carbonates. Most of these settings are also
documented by non-Jurassic examples (see Box 4.9).
Oncoids formed in upper slope and basinal settings are
not known prior to the Permian.
amples are known from the Liassic and Late Jurassic
of the Tethyan region (originally described as 'pelagic
oolites': Jenkyns 1972; Massari 1983). The oncoids are
characterized by polyphase coatings consisting of mi-
critic and granular laminae, the latter composed of fine-
grained skeletal debris. Encrusting organisms (e.g.
Nubecularia
) are common. Nuclei are bioclasts (e.g.
fragments of ammonites, sponges) and lithoclasts. The
sizes vary between <2mm and 100 mm. The oncoids
occur in association with microstromatolites and origi-
nated through sediment-binding and/or mineral-precipi-
tating activity of microbes in regions with low sedi-
ment input, probably at storm-wave depth.
Box 4.10 lists important case studies of Cambrian
to Cretaceous marine oncoids.
Reef oncoids:
Concentrically structured porostro-
mate and micritic oncoids are common constituents of
Paleozoic reef limestones (e.g. Devonian stromato-
poroid-coral reefs in western Canada: Machielse 1972;
late Permian coralline sponge-
Archaeolithoporella
reefs
of the Tethys region: Sano et al. 1990). Oncoids are
common constituents of Devonian back-reef sands and
fore-reef sediments. Locally porostromate oncoids grow
in place on the slope, sometimes together with stroma-
tolites. Very good examples are the capped oncoids
formed by
Rothpletzella
(Playford et al. 1976). Differ-
ences in the biotic composition of the oncoids give hints
of changes in sea level and water depths.
Pelagic oncoids:
These spongiostromate grains,
formed on the tops of seamounts and drowned plat-
forms, became more frequent during the Jurassic. Ex-
Precambrian and Phanerozoic oncoids
Oncoids occur already in Archean carbonates, to-
gether with stromatolites and ooids (Hofmann et al.
1999; Grotzinger and James 2000). The grain type is a
common constituent of Proterozoic platform carbon-
Box 4.10.
Selected case studies of ancient marine spongiostromate and porostromate oncolites.
In contrast to modern
oncoids, these oncoids were common in Paleozoic and Mesozoic subtidal and intertidal environments as well as in
slope and basinal settings. The last marine cyanoids are known from the Cretaceous. Cyanoids were replaced by coral-
line algal rhodoids during the Late Cretaceous and the Tertiary.
Cretaceous:
Carozzi 1989 (tidal setting; oncoid bars on shallow carbonate platform); Lewy 1972; Masse 1979; Weiss
1969.
Jurassic:
Bollinger and Burri 1970 (patch reefs); Colacicchi et al. 1975 (tidal setting); Dahanayake 1977, 1978, 1983
(lagoonal carbonates); Dangeard 1935; Dragastan 1969; Elliott 1966; Fezer 1988 (platform margins and open-
marine lagoonal settings, lagoonal carbonates); Gasche 1956; Gasiewicz 1983 (peritidal); Gygi 1969, 1992 (open-
marine lagoon, slope, basin); Heim 1916; Hug 1999; Jenkyns 1972 (pelagic platform); Kutek and Radwanski 1965
(peritidal and basinal settings); Kuss 1990; Leinfelder et al. 1993; Leinfelder and Schmid 2000; Massari 1983;
Massari and Dieni 1983 (pelagic platform); Misik 1966; Pümpin 1965 (reef, back-reef sands); Purser 1975 (peritidal);
Scherze 2001 (patch reef);. Schmid 1996 (lagoonal and reef settings)
Triassic:
Biddle 1983 (reef-derived material); Catalov 1970, 1983 (platforms); Fischer 1964 (tidal setting); Goldhammer
et al. 1990 (tidal setting); Hagemeister 1988; Jerz 1966 (siliciclastic-carbonate inner shelf); Kisten et al. 1990; Peryt
1977, 1980; Radwanski 1968; Schuler 1968; Tichy 1983.
Permian:
Flügel 1979 (lagoonal and reef settings); Füchtbauer 1968 (near-reef); Kerkmann 1966 (reefs, back-reef
sands); Mazzullo and Cys 1983; Peryt 1981; Peryt and Wagner 1981 (basinal setting); Rothe 1969; Termier and
Termier 1955 (reef), Toomey 1974, 1983, Toomey et al. 1988, 1989 (subtidal carbonate shelves, open-marine and
restricted shelf lagoons).
Carboniferous:
Bowman 1983 (subtidal platform); Burchette and Riding 1977; Carozzi 1989 (tidal setting; shoals in
inner ramp position); Gerhard 1985; Henbest 1963; Johnson 1946; Neal 1969; Obrhel 1977; Perret 1971; Toomey
1975; Wright 1983 (peritidal setting)
Devonian:
Kaufmann 1964; Machielse 1972 (back-reef sands and forereef setting); Pia 1932; Playford and Cockbain
1966 (forereef setting at platform margin); Poncet and Lapparent 1975 (siliciclastic-carbonate inner shelf); Wolf
1965 (reef).
Silurian:
Ratcliffe 1988 (shelf lagoon); Stel and De Coo 1977 (patch reef); Toomey and LeMone 1977.
Ordovician:
Benedict and Walker 1978; Blackwell et al. 1984 (lagoonal setting); Carozzi 1989 (tidal setting); Maslov
1960; Wood 1948.
Cambrian:
Gandin and Debrenne 1984 (associated with archaeocyathid patch reefs); Halley 1975 (peritidal setting);
Hose 1961; Youngs 1977; Zhuravleva 1964.
