Geology Reference
In-Depth Information
Fig. 4.15.
Descriptive subdivision of laminated oncoids
in regard to growth patterns and the composition of the laminae.
A
- Basic geometric types of oncoids found in recent shallow-marine settings and related to varying degrees of agitation and
movement of the grains (modified from Logan et al. 1964): Type C (characterized by
C
oncentrically stacked spheroid layers,
SS-C); Type R (oncoids consisting of
R
andomly arranged hemispheroidal layers); and Type I (biconvex lenticular oncoids
exhibiting
I
nverted hemispheres, SS-I). Irregular random growth also can produce
L
obate growth forms (Type L). Type I
describes a particular case characterized by the interruption of oncoid growth, partial destruction and renewed growth after
reworking and transport.
B
- Five types of laminae are commonly composed of: 1 - Laminated fabric consisting predominantly of micritic laminae
(Pl. 12/7), 2 - Micritic laminae exhibiting tiny sparry spots whose distribution accentuates the laminar structure of the oncoid
(Pl. 11/3), 3 - Couplets of micritic and sparry laminae (Pl. 12/3), 4 - Micritic laminae including or alternating with layers of
recognizable encrusting microfossils (Pl. 119/3), 5 - Laminae exhibiting an open meshwork with spar-filled voids (e.g.
Pl. 12/4, Pl. 118/3). Nucleus (black), micrite (stippled), sparry calcite (white).
spheroidal. The biota are represented by calcimicrobes,
foraminifera and various skeletal metazoans with spe-
cific growth forms and succession patterns. Organisms
contributing to Late Jurassic non-laminated oncoids in-
clude foraminifera, serpulids, bryozoans, chaetetid cor-
alline sponges,
Bacinella
,
Marinella
, and porostromate
calcimicrobes. Oncoids devoid of a continuous lami-
nation have been called
pseudoncoids
(Dahanayake et
al. 1976).
to the depositional environment from other areas or
whether the oncoids were formed within the deposi-
tional environment. Carozzi et al. (1983) uses the 'in-
dex of biotic diversity of benthics'; the IBDB relates
the number of groups or taxa forming the nuclei to the
bioclasts occurring together with the oncoids.
Ancient oncoids
Most ancient oncoids consist of calcite, sometimes
with minor admixtures of clay minerals, quartz and or-
ganic matter. The composition of these admixtures usu-
ally corresponds to that of the minerals occurring in
the vicinity of the oncoids.
Biotic composition of the cortex:
The diversity of
the microfossils in oncoids reflects small-scale envi-
ronmental conditions that assist in interpretating long-
term environmental trends (Scherze 2001). Autochtho-
nous 'foraminiferal-algal' oncoids composed of inti-
mately associated calcimicrobes and encrusting tubu-
lar foraminifera may indicate deeper-water settings
(Peryt and Peryt 1975).
Non-carbonate oncoids
include phosphatic, ferrugi-
nous and manganese nodules as well as, less commonly,
oncoids consisting of pyrite (Schieber 1999) or chlo-
rite (Misik and Sucha 1997). The first three types are
often associated with pelagic carbonates. The oncoids
are found together with stromatolitic crusts on discon-
tinuity surfaces, often corresponding to major strati-
graphic gaps and related to eustatic highstand phases.
The formation of these oncoids appears to be strongly
Nucleus:
The shape of the nuclei often determines
the shape of the oncoid grain. A comparison of bio-
clasts acting as nuclei with the skeletal grains of the
sample indicates whether the oncoids have been brought
