Geology Reference
In-Depth Information
nae, Dahanayake 1978) most commonly represent
molds of calcimicrobes or algae, but in some cases may
be caused by diagenetic alteration of peloidal fabrics.
Type 2 oncoids are common in low-energy environ-
ments. Type 3 is of particular interest because it re-
flects the response of microbiota to short-term envi-
ronmental changes. Type 4 is often characterized by
sessile foraminifera or porostromate calcimicrobes (e.g.
Girvanella
).
Girvanella
tubes are usually visible in the
exterior parts of the oncoids, and rapidly loose their
shape to the inner oncoid parts.The encrusting foramin-
ifera depend on hard substrates, and therefore testify to
the existence of sporadic lithification during oncoid
growth. The diversity of type 4 can be both low and
also very high, thus reflecting the appropriate condi-
tions for the growth of epiphytes and epizoans in shal-
low subtidal environments with low sedimentation
rates. The fabric of Type 5 has been called fenestral.
Both Type 4 and Type 5 may lack a well-defined lami-
nation or the meshwork characterizing Type 5 may form
non-laminated oncoids.
Non-laminated oncoids are rounded micritic grains
commonly containing abundant relicts of calcified
biota. Shapes are often irregular, lobate and not strictly
Plate 11 Oncoids: Calcareous Nodules Formed by Microbes, Algae and Other Encrusting Organisms
Oncoids are mm- to cm-sized, rounded or irregularly formed grains consisting of a layered micritic cortex and a
bio- or lithoclastic nucleus. The layers originate from crusts of sessile organisms (predominantly calcimicrobes
and algae, but also foraminifera, bryozoans, serpulids and others). These organisms enhance and initiate the
precipitation of calcium carbonate, act as encrusters and binders, trap fine-grained sediment or enhance synsedi-
mentary lithification. Oncoids are common in marine and non-marine environments. Basic oncoid types are
distinguished by the existence of skeletal or non-skeletal microfabrics (porostromate and spongiostromate on-
coids) and by the prevailing biotic composition (e.g. microbial oncoids, foraminiferal oncoids). The shape,
internal structures and biotic composition denote paleoenvironmental factors (e.g. water energy, sedimentation
rates) and specific settings (see Box 4.11). The main settings of Phanerozoic oncoids have changed over time:
Paleozoic spongiostromate oncoids grew predominantly in lacustrine and transitional marine environments;
Cambrian to Jurassic porostromate oncoids flourished in marine subtidal environments. Starting in the mid-
Jurassic, porostromate oncoids were increasingly substituted for by spongiostromate oncoids, which became the
only type in marginal-marine and particularly in lacustrine settings. In normal marine settings oncoids were
replaced by rhodoids starting with the Late Cretaceous.
Oncoids are commonly larger than ooids and vadoids (pisoids). They exhibit more irregular and often overlap-
ping layers which are considerably thicker than the coatings of cortoid grains.
1
Modern oncoid
formed by snowball-like sediment-trapping on the slimy surface of microbial films. A poorly developed
laminar fabric is indicated by micritic areas (arrows). The oncoids with a maximum size of about 5 cm occur in quartz-
bearing calcareous sands on the coast of the Red Sea. Ras Muhammed, Sinai, Egypt.
2
Detail of -> 1. The oncoid consists of strongly micritized foraminifera (F), cortoids, rounded intraclasts and small angular
quartz grains (Q). Note the open interparticle porosity (P).
3
Spheroidal spongiostromate/porostromate oncoids
. The micritic layers of the cortex are accentuated by differences in
color and the incorporation of spherical microfossils. Intergranular pores are occluded by sparitized calcitic rhombs
indicating dedolomitization processes similar to those described from Middle Jurassic reservoir rocks in the Paris Basin
(Purser 1985). The complex diagenetic history is characterized by partial dolomitization of oncoid floatstones associated
with a replacment of matrix and peripheral parts of some grains, dissolution of dolomite crystals and filling of the rhombic
molds by calcite. The limestones represent the upper part of a shallowing-upward cycle. Late Middle Jurassic: Vrhnika-
Logatec near Ljubljana, Slovenia.
4
Detail of -> 3. Vague remains of very thin, vertically arranged, branched filaments are recognizable within the gray
layers (A). In contrast, horizontally growing
Girvanella
-like tubes occur in layer B. The dark-colored zone at the top of A
point to cessations in the growth of the oncoid.
5
Foraminiferal oncoids
. Irregularly shaped and partly contorted oncoids built by the encrusting foraminifera
Nubecularia
reicheli
Rat. Bioclastic oncoid grainstone. Bioclasts used as nuclei are identical to the bioclasts occurring outside the
oncoids. Reworking of oncoids is indicated by variations in shape and size (< 2 to 5 mm). These oncoids formed on high-
energetic shoals at the boundary of the restricted and open-marine parts of a carbonate platform. Middle Jurassic (Bajocian):
Bourgogne, northwestern France.
6
Nubecularia
oncoid. The core (an echinoderm fragment, E) is surrounded by layers of densely spaced foraminiferal tests.
The chambers exhibit thick walls that touch each other (arrows). Same sample as -> 5.
