Geology Reference
In-Depth Information
controlled by microbes which induce mineralization and
act in trapping and binding sediment (Monty 1973
;
Champetier and Joussemet 1979). Different sources
contribute to mineralization. Manganese nodules result
from enrichment of deep oceanic bottom waters in man-
ganese from submarine volcanisms, basalts and rift-
zone hydrothermal fluids. Manganese and ferroman-
ganese oncoids, crusts and pavements are widely known
throughout the Mesozoic of the Tethyan region in con-
densed pelagic limestones (Jenkyns 1970). Nodular
phosphorites containing phosphatic oncoids originate
under conditions of marine upwelling, but input of phos-
phorus from river drainage must also be taken into ac-
count (see Pl. 110). Common features of non-carbon-
ate oncoids are variations in the contribution of encrust-
ing organisms (particularly foraminifera: Greenslate
1974), the geometry and arrangement of oncoid lami-
nae and the shape of the nodules. The size ranges from
<1 mm to 2-10 cm (manganese micro- and macro-
nodules: Pattan 1993).
Examples of ancient ferruginous oncoids are limo-
nitic Middle Jurassic nodules known as 'snuff-boxes'
and believed to have been formed in an oxic environ-
ment with periodical current influence (Gatrall et al.
1972
;
Palmer and Wilson 1990). These oncoids are as-
sociated with episodes of reduced sedimentation. The
oncoids exhibit asymmetric cortices.
The original ferric composition of the laminae was
probably under the control of non-photosynthetic mi-
croorganisms. Phosphatic oncoids exhibit dense lami-
nated and microstromatolitic fabrics (Krajewski 1983
;
Martin-Algarra and Vera 1994). A genetic interpreta-
tion of phosphatic oncoids and stromatolites formed at
discontinuity surfaces of drowned carbonate platforms
is shown in Fig. 4.16.
Fig. 4.16.
Formation of phosphatic oncoids
on a Mesozoic
pelagic swell. The ellipsoidal and subspherical oncoids are
several centimeter in size. The microstromatolitic laminae
were formed by microbes, phosphate minerals, trapped pe-
lagic micrite, and encrusting foraminifera.
The nuclei of the microstromatolitic oncoids are mud
clasts as shown in Fig.
A
:
1
Ancient non-marine oncoids:
Oncoids formed in
ancient lake systems have been known since the Pre-
cambrian (Buck 1980), but have been studied more thor-
oughly only from the Tertiary (Box 4.9). The oncoids
provide information about lake depositional systems
and provide insights into the regional climate and lake
level fluctuations (Awramik et al. 2001). Lacustrine
oncoids are commonly associated with high-energy,
often mixed siliciclastic-carbonate near-shore environ-
ments. They can mark the initiation of climatically in-
duced transgressive phases. Conditions favorable for
oncoid formation are high carbonate input associated
with the increase in precipitation and the contribution
of supersaturated groundwaters. Reworked oncoids
indicate falls in lake level. Oncoids formed in soda lakes
are known from Tertiary lake deposits (Dixit 1984).
Freshwater and saline oncoids can be separated by dif-
growth of microbes (m) on a hardground (hg);
2
deposition of pelagic sediment and burial of microbial
mats;
3
formation of clasts by burrowing and/or currents;
4
encrustation of isolated clasts and and rolling under mod-
erate currents;
5
disymmetric upwards development of microbial lamina-
tion;
6
renewed growth of microbes (m) on the oncoid.
Fig.
B
demonstrates the origin of endostromatolites in
voids within the oncoids:
1 oncoid grown on a micritic clast with a glauconitic rim;
2 burrowing and boring (b) affecting the oncoid and the
nucleus, growth of microbes in the cavity (m);
3 infilling by pelagic sediment (stippled).
Modified from Martin-Algarra and Vera (1994).
