Geology Reference
In-Depth Information
Box 4.11.
Significance of oncoids and oncoid limestones
Paleoenvironmental proxies
Biotic composition and lamination types reflect environmental factors that control microbial and algal growth as
well as encrusting communities (light, salinity, water energy, sediment input). The availability of light is not necessarily
a limiting factor because cyanobacteria and various green algae also grow in areas of low light intensities (e.g. on the
undersides of oncoids).
Salinity:
Oncoids originate in freshwater, brackish waters and marine waters. Ancient freshwater and brackish-water
oncoids are differentiated from marine oncoids by nuclei formed by fresh water mollusks, specific lamination patterns
analogous to modern freshwater oncoids, better preserved microstructures and the association with terrestrial sedi-
ments.
Water-energy levels:
Oncoids are often used as indicators of high energy, low intertidal to shallow subtidal environ-
ments, because it is thought that oncoids require frequent overturning and rolling to form, and that the degree of turbu-
lence determines oncoid shapes. These generalizations do not hold in many freshwater and subtidal environments,
where spongiostromate and porostromate oncoids originate without significant rolling but nonetheless exhibit concen-
tric laminae. Stationary growth is shown by asymmetrical shapes and asymmetrical widths of laminations. Rolling
appears to be indicated by subspherical shapes, well-laminated cortices, concentric symmetrical growth patterns, and
abrasion of algal or cyanobacterial structures. Cessations in rolling are manifested by strongly encrusted laminae sur-
faces. Lack of rolling is revealed by branched and lobate oncoid shapes. Multistage oncoids exhibiting a change from
laminar to lobate growth forms record a change from common rolling to less frequent rolling. Low-energy and high-
energy oncoids are better differentiated by the combined approach of oncoid criteria and texture types: Low-energy,
quiet-water conditions are indicated by large micritic single and multiple oncoids that exhibit discontinuous growth
layers (oncoid type R, Fig. 4.15) and form floatstones and wackestones. Quiet-water conditions are also indicated
nonlaminated or only weakly laminated meshwork oncoids (type 5, Fig. 4.15). Higher-energy environments are re-
flected by small oncoids, that exhibit regular laminations and form packstones and grainstones. Redeposition of oncoids
is indicated by smoothing and breaking, co-occurrence of different oncoid types, and oncoids found in a matrix different
than the micrite forming the oncoids.
Paleo-water depths
are reflected in biotic composition, nuclei and lamination types. Deeper-water oncoids are often
un-laminated or have densely spaced micritic laminae (e.g. in the Permian Zechstein). This is a favorable setting for
continuous oncoid growth. The slowly growing
Girvanella
oncoids are considered indicators of reduced sedimentation,
preferrably in deeper settings at depths of some tens of meters. Spongiostromate oncoids develop over shorter periods in
unstable ecological conditions. Oncolite beds, intercalated in platform sequences, can reflect breaks or hiatuses in sedi-
mentation. Short-term alternations of stationary phases and oncoid-building phases are shown by changes in lamination
and encrustation types, micro-unconformities of laminae surfaces, and the incorporation of sediment within the oncoid.
Sealevel fluctuations
Shallowing-upward sequences and cyclic depositional
patterns:
Shallowing is indicated by stromatolitic crusts over-
growing oncoids, marked changes in size, shape and composition of oncoids, and differences in depositional texture
types. A good example was described by Ratcliffe (1988). Oncoids are characteristic constituents of shallow subtidal
members of muddy and grainy platform cycles (e.g. Fischer 1964; Goldhammer et al. 1990; Purser 1975).
Transgressive and regressive phases:
Oncoids often occur at the base of transgressive sequences formed in shallow
low-energy environments (e.g. Wright 1983) and mark maximum flooding surfaces. An oncolitic wackestone facies
with whole fossils may indicate regressive conditions.
Stratigraphic gaps and condensed sequences:
Breaks in sedimentation are manifested by non-carbonate oncoids and
by oncoids with ferruginous or glauconitic coatings, associated with microstromatolites, hardgrounds and lithoclastic
beds.
Regional correlations
In ramp and platform settings, oncolite limestones can be excellent marker beds that can be followed laterally for
tens and hundreds of kilometers. Examples are known from the Late Triassic (Fig. 4.18B) and the Jurassic.
Depositional settings of marine oncolites
Compositional and morphological features of oncoids, and their association with other grains and fossils are valuable
criteria in recognizing depositional settings on platforms, slopes and basins (Fig. 4.17).
Reservoir rocks
Micritic oncolitic limestones are regarded as poor reservoir rocks, but exceptions are known, e.g. oncoid limestones
associated with microbial reef carbonates (Late Jurassic Smackover Formation in the eastern Gulf Region: Becher and
Moore 1979), and productive Cretaceous platform carbonates of Texas (Bebout et al. 1979) and offshore Brazil (Carozzi
and Falkenhein 1983). The critical differentiation of oncoids and other coated grains is important; misleading environ-
mental interpretations and exploration targets may result if oncolites are confused with subaerial fabrics (Gerhard 1985).
