Geology Reference
In-Depth Information
ies, generally accomplished by wave and current ac-
tion; this allowing phototrophic growth to occur on all
sides of the oncoids. Varying degrees of agitation and
movement of the grains result in several geometric types
(Fig. 4.15): SS-I structures (biconvex lenticular on-
coids), SS-R oncoids (oncoids consisting of randomly
arranged hemispheroids that make up the structure),
and SS-C structures (characterized by concentrically
stacked spheroids).
induced by tides or storms, day to night variations or
seasonal changes in water chemistry resulting in
changes in saturation and nutrients. Seasonal long-term
environmental changes may affect water temperature,
salinity, water level and turbulence, nutrients and sedi-
ment influx. The thickness of the laminae depends on
the phototactic response of photosynthesizing cyano-
bacteria, the makeup of the microbiota, seasonal
changes which may affect lithification and carbonate
precipitation, and the modifications of initial laminae
thickness by diagenesis. The microrelief along a single
lamina, e.g. small-scale crenulations, tufts or dome-
shaped structures, is produced by particular microbes,
but may also reflect diagenetic modifications.
How to describe oncoids and oncolites?
The analysis of oncoids requires field data and care-
ful studies of thin sections. Box 4.8 summarizes the
criteria which should be examined.
Dimensions and shape:
Oncoid size is controlled
both by environmental conditions and biological fac-
tors. The former include water energy and sedimenta-
tion rate, the latter metabolic processes of the biota in-
volved in the oncoid formation, competition with graz-
ing organisms and the size of the bioclastic or litho-
clastic nuclei used. Marked variations in oncoid size
within limestone horizons are important indicators of
changes in deposition patterns, and are used to evalu-
ate sequence boundaries as well as sea-level fluctua-
tions. The size of modern freshwater oncoids ranges
from smaller 2 mm to larger 100 mm. Ancient spongio-
stromate oncoids commonly vary from smaller 2 mm
up to 50 mm. The average size of ancient porostromate
oncoids is approximately 50 mm, but the range may
extend up to more than 100 mm.
Shape modifications both of oncoids and rhodoids
can be best described by means of the Sneed and Folk
sphericity-form diagram for particle shapes. The tri-
angle diagram displays long, intermediate, and short
axes and defines the basic form types (discoidal, sphe-
roidal, and ellipsoidal). Many oncoids are elongate or
subspherical in shape and may be flattened on one side.
Descriptive subdivision of oncoids:
Several attempts
have been made to classify oncoids with laminated fab-
rics (e.g. Dahanayake 1977; Malchus and Kuss 1988;
Kuss 1990). These classifications consider the compo-
sition of the laminae and the biota incorporated within
the cortex. Because different lamina types reflect
changes in water energy levels and water depths (e.g.
Bowman 1983), a
simple descriptive subdivision
is pro-
posed here (Fig. 4.15):
The subdivision of the
geometric types
follows Lo-
gan et al. (1964), who classified oncoids (called 'sphe-
roidal stromatolites') according to the arrangement of
the laminae, reflected in turn by the shape of the nod-
ules. Type C is characterized by concentrically arranged
laminae, Type R by randomly arranged, non-continu-
ous and overlapping laminae, and Type I by inverted
laminae. In addition, a fourth Type L for randomly
growing lobate forms with dome-shaped laminae is pro-
posed here.
The five
compositional laminae types
may occur in
isolation, but many oncoids show multi-stage sequences
consisting of several laminae types. These sequences
are used to estimate hydrodynamic changes during on-
coid growth, e.g. from quiet-water conditions (reflected
by types 1 and 2 in Fig. 4.15) to moderate to high-en-
ergy conditions (type 5). Note that a given oncoid type
can be transformed to another type by diagenetic alter-
ation.
The micrite laminae of Type 1 are microbial in ori-
gin or were formed by trapping of fine-grained sedi-
mentary material (Pl. 11/2). This type is the most com-
mon microfabric in spongiostromate oncoids and is re-
garded as indicative of an environment with constant
low-energy conditions, e.g. protected shallow subtidal
or basinal oncoids. Some of these oncoids may be com-
pletely recrystallized porostromate oncoids. Type 1 on-
coids are commonly sphaeroidal or ellipsoidal. The
sparry spots of type 2 (corresponding to 'grumous' lami-
Laminated versus non-laminated micofabrics:
Mod-
ern stromatolites and oncoids offer the possibility of
understanding the controls of microstructural elements
of oncoids. The basic lamination in ancient oncoids con-
sists of couplets seen as dark, organic-rich micritic lami-
nae, alternating with microspar or sparry as well as
lighter colored laminae poor in organic matter, and of-
ten thicker than the dark laminae. The generation of
individual laminae is intimately related to life activi-
ties of microbial assemblages. The generation of cou-
plets with two distinct laminae members implies itera-
tive changes in environmental variables that control the
life activities of microbes, e.g. sedimentation rates, pe-
riodic deposition of sediment onto the growth surface -
