Geology Reference
In-Depth Information
generally believed that many oncoids and concentric
laminae are formed by adhesion of fine grains of sedi-
ment to the mucilaginous surface of algal mats (Pl. 11/
1, 2) and also by precipitation of calcium carbonate in
response to the withdrawal of carbon dioxide by algae
during photosynthesis. Nodule turning is facilitated by
waves and tidal currents, a sloping surface, or biotur-
bation of the underlying surface by grazing herbivo-
rous fish. The contribution of cyanobacteria and other
bacteria to the formation of cyanoids and spongiostro-
mate oncoids has generated a large number of publica-
tions in the last few decades that have clearly estab-
lished the fundamental role played by cyanobacteria
(Pentecost and Riding 1986; Gerdes et al. 1994). These
observations were reinforced by experiments that suc-
ceeded in creating calcite and aragonite biocrystals
under laboratory conditions approximating natural con-
ditions (Chafetz and Buczynski 1992; Knorre and
Krumbein 1992; Castanier et al. 1999). The eminent
function of microbes for the formation of freshwater
oncoids and ooids was exemplified by Davaud and
Girardclos (2001) in Lake Geneva, Switzerland: In
surficial sediments nucleation of Low-Mg calcite oc-
curs in close association with organic films formed by
the mucus of filamentous cyanobacteria and diatoms.
In subsurface sediments the intercrystalline porosity is
reduced and the micritic porous and organic-rich coat-
ings are transformed into a dense mosaic with scattered
molds of filaments, creating a microfabric similar to
ancient oncoids.
oncoids. Smaller and discoidal oncoids (e.g.
Schneggli-
steine
in Lake Constance) exhibit a distinct lamination
with alternating porous and dense carbonate layers re-
flecting seasonal growth (Pl. 12/5). Freshwater lake on-
coids are characteristically concentrated in shallow,
near-shore zones with only slight wave- or current-agi-
tation. Their distribution allows climatically controlled
lake level fluctuations to be reconstructed (Magny
1992). As shown by oncoids in the Lake Constance,
changing textural and diagenetic features of the oncoids
are related to differences in the composition and growth
pattern of the microbes and algae that participate in the
formation of the nodules. Modern lacustrine oncoids
demonstrate that the formation of spongiostromate and
porostromate oncoids does not necessarily need strong
or repetitive turning of the nodule and that in-situ
growth is a common feature. An interesting aspect re-
lated to the nutrient controls of oncoid-forming algae
are freshwater oncoids created by industrial pollution
(Dean and Eggleston 1984).
Fluvial oncoids, formed by often highly diverse al-
gae and cyanobacteria, are extremely variable in shape
(ellipsoidal, discoidal, spherical, subcylindrical, irregu-
lar), size and internal structures (Glazek 1965, Golubic
and Fischer 1975). Many oncoids exhibit excentric
growth forms due to the contacts of the grains. The
layers are often asymmetrically arranged, interrupted
or obscured. Oncoid sizes range from smaller than 5 cm
to larger than 30 cm; large oncoids are rather common.
Many oncoids exhibit distinct concentric lamination and
radial fibrous structures.
Recent non-marine oncoids:
Free-lying, rounded ir-
regularly laminated oncoids occur in fluviatile and
lacustrine habitats; they form in association with cal-
careous tufa, and as microbial coatings on tropical karst
cavities ('terrestrial oncoids': Jones 1991). Lacustrine
and fluviatile oncoids are similar with regard to their
morphology and internal criteria. Oncoids from fresh-
water lakes have been studied in Europe and North
America (e.g. Pia 1933; Jones and Wilkinson 1978;
Schäfer and Stapf 1978; Murphy and Wilkinson 1980;
Schneider et al. 1983; Obenlüneschloss 1991). The el-
lipsoidal or subspherical, centimeter to tens of centi-
meter-sized oncoids are produced by cyanobacteria in
association with green and red algae as well as dia-
toms. The oncoids typically possess a laminated, po-
rous microstructure, with layers or tufts of micrite tubes
representing the encrusted sheets of cyanobacteria or
green algal filaments. The taxonomically differentiated
filaments form a fine intertwined network or a con-
spicuous radial microfabric. Common oncoid nuclei are
mollusks, plant debris or rock pebbles. The basic types
correspond to porostromate as well as spongiostromate
Recent marine oncoids:
In marginal-marine settings
cyanoids originate in intertidal to shallow subtidal en-
vironments with normal, low or high salinity (Golubic
and Campbell 1981; Richter et al. 1979; Neuser et al.
1982; Richter and Sedat 1983). Some spongiostromate
oncoids are formed by trapping and binding of sedi-
ment on the surface of finely filamentous mats that dis-
integrate and roll on the substrate (Gebelein 1976; Jones
and Goodbody 1985). Modern open-marine oncoids
formed in intertidal to subtidal environments (e.g. Ber-
muda, Bahamas, Netherlands Antilles) often do not dis-
play distinct laminations, because algal growth patterns
are continuously destroyed. The nuclei of these oncoids
are provided by algal chips and skeletal grains from
adjacent organisms living on seagrass. Oncoids with
concentrically stacked laminae (spheroidal structures,
type-SS structures of Logan et al. 1964) originate in
the low intertidal zone exposed to waves, and in the
agitated shallow-water zone below the low-water mark.
The prime prerequisite for the growth of these oncoids
is believed to be the overturning of the laminated bod-
