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Department of Geology and Paleontology, Faculty of Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic


Department of Geology of Mineral Deposits, Faculty of Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic

(Manuscript received October 3, 1996; accepted in revised form December 12, 1996)


 Pelagic chlorite and chlorite-hematite oncoids occur in red nodular limestones of  the Toarcian and Oxfordian

age, thought to represent intervals of condensed sedimentation. At a single locality, they were  also found in an
intraformational limestone  breccia with fragments of a calcrete. Intraclasts, fragments of hardgrounds and belemnite
rostra form oncoid nuclei. Wrinkled laminae and pseudocolumnar structures characterize their cortices. Encrusting
foraminifers are important builders of the studied oncoids. Rare synsedimentary cracking of oncoids is indicated by
the presence of neptunic microdykes. Sets of chlorite and hematite concentric laminae responded differently during
compaction and tectonic processes. A peculiar phenomenon is the recrystallization (aggrading neomorphism) of the
micrite admixture in hematite and chlorite oncoids; new-formed larger calcite grains contain the hematite pigment
arranged in dendritic patterns. The hematite pigment also formed characteristic “teeth” in the early diagenetic calcite
veinlets. The oncoids are formed by Fe-chlorite, polytype IIb. The origin of the chlorite was connected neither with
volcanic activity nor with the initial metamorphism. The condensed sedimentation with mineralized oncoids was
associated either with local conditions or with the sea level high stand.

Key words:

 Western Carpathians, Jurassic limestones, chlorite oncoids, hematite oncoids, encrusting foraminifers.


Calcite oncoids are concentric structures formed mainly by
cyanophytes and green algae. Their concentric laminae origi-
nate by the adhesion of fine grains of sediment on the muci-
lagineous surface of the algal mats and also by the precipita-
tion of the calcium carbonate in response to the withdrawal
of the carbon dioxide by algae during photosynthesis.

Oncoids can also consist of other minerals — e.g. hema-

tite, manganese oxides, phosphatic minerals, chlorite. Such
“mineralized” oncoids are of bacterial origin (other groups
than Cyanobacteria). Meanwhile calcite oncoids are restrict-
ed to the shallow euphotic zone, bacterial (non-calcite) on-
coids lack such a dependence. For instance, manganese on-
coids (“nodules”, “concretions”) are formed on the deep
ocean bottom. Non-calcite oncoids usually contain a certain
portion of microcrystalline calcite; the replacement of micrit-
ic laminae e.g. by hematite is frequent. “Mineralized” on-
coids are typical in zones of condensed sedimentation, which
are represented mostly by red nodular limestones and associ-
ated hardgrounds.

Chlorite oncoids are a very rare variety and are thus de-

scribed here in detail. Our chlorite oncoids are unique be-
cause  encrusting foraminifers significantly contributed to
their growth. We found only two mentions about the chlorite
“concretions” in the literature, both indicated as chamosite
concretions: Athanasov (1961) from the Jurassic strata of
Bulgaria and Birkenmajer (1977, p. 48) from the Middle Ju-
rassic Flaki Formation in the Pieniny Klippen Belt of Poland.

Through the courtesy of M. Krobicki we had the opportunity
to study some samples from the last mentioned locality for
comparative purposes. These oncoids are very similar to ours
by the abundance of encrusting foraminifers, they are also
formed by Fe-chlorite, polytype IIb.

Geological setting

Chlorite and chlorite-hematite oncoids were found at two

stratigraphic levels of the red nodular limestone facies of
Late Liassic (Toarcian) and Late Jurassic (Oxfordian) age.

Localities in the Toarcian limestones of the Krížna Nappe,

Ve ká Fatra Mts.

Information about the chlorite and hematite oncoids was

first published under the term chlorite-hematite concretions
(Mišík 1964, p. 71). Five localities are known in the follow-
ing valleys: Bystrická, Gaderská, Suchá, Horná Turecká and
Terlenská dolina (Fig. 1). The horizon with the chlorite and
hematite oncoids occurs in the uppermost part of the red nod-
ular limestones (Adneth Limestone) which overlain by  grey
siliceous spotty marls. The Adneth Limestone (red nodular)
in this area is of Pliensbachian-Toarcian age. The age of its
upper part is indicated by a Toarcian ammonite (Rakús
1964). The profile of the Liassic sequence in the Bystrická
Valley (Fig. 2) may serve as a representative section. An ad-
ditional profile in the Horná Turecká Valley was published in
the previous paper (Mišík 1964, Annexe 4).

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86                                                                                          MIŠÍK and ŠUCHA

Localities in the Oxfordian limestones of the Tatric Unit,

Malá Fatra Mts.

Chlorite and hematite oncoids occur in the Zázrivá Valley

at three neighbouring localities (Fig. 1): 1 — The lowest part
of the Bralo quarry; 2 — About 200 m to the east of it; 3 —
In the core of a borehole for the planned Párnica dam.

The host rock is red indistinctly nodular limestone. In the

Bralo quarry, an intraformational breccia is also present. On
the basis of the occurrence of the first “Cadosinidae” and the
absence of Saccocoma and Tintinnidae an Oxfordian age is
indicated (Borza 1984). The overlying grey micritic lime-
stones of Kimmeridgian to Barremian age were described
from the same quarry under the name of Lučivná Limestone
by Polák & Bujnovský (1979) and  Michalík et al. (1986).
Both papers fail to mention  the red limestones with oncoids.

Microscopical study of Liassic oncoids

from the Ve ká Fatra Mts.

The Adneth Limestone from the five previously mentioned

localities contains chlorite, hematite and combined chlorite-
hematite oncoids with a  considerable admixture of micrite
(Pl. I: Fig. A). They possess ovoid shapes flattened by com-
paction, with diameters up to 5 cm and wrinkled concentric

The host rock is a biomicrite-wackestone. Spicules of silici-

sponges (monaxone and rhaxa) that have been dissolved and
filled by calcite predominate among the biodetritus. Other

Fig. 1. 

Localities with the chlorite and hematite oncoids in the Ju-

rassic red nodular limestones.

Fig. 2.

 Profile of the Liassic strata (Krížna Nappe) in the Bystrická

Valley, Ve ká Fatra Mts.

Plate I:

 Fig. A. Chlorite-hematite oncoid in the red nodular Ad-

neth Limestone. Upper Liassic (Toarcian) of the Krížna Nappe,
upper part of the Gaderská Valley. Polished slab, natural size.
Fig. B.

 Intraclast perforated by boring organisms served as the

core for the chlorite oncoid. Oxfordian, Tatric Unit, in front of the
Bralo quarry, Zázrivá Valley. Thin section. Fig. C. Chlorite oncoid
penetrated by a microdyke, with endostromatolites along the core
margin. Toarcian, Horná Turecká Valley. Fig. D. Close-up from
the previous. The voids after the boring organisms were filled by
endostromatolites. Fig. E. Syntaxial rim overgrew the belemnite
rostrum corroded by boring organisms; core of the hematite on-
coid, Toarcian, Gaderská Valley. Fig. F. Chlorite oncoid on the
corroded belemnite rostrum, Toarcian, Horná Turecká Valley.
Fig. G.

 Chlorite-hematite oncoid with wrinkled laminae, Toarcian,

Bystrická Valley. Fig. H. Pseudocolumnar structure in the oncoid;
chlorite and hematite laminae partly replaced by calcite, Toarcian,
Gaderská Valley. Scale bars = 0.1 mm, if no other indication.


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PLATE I                                                                                                 87

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88                                                                                          MIŠÍK and ŠUCHA

constituents are short “filaments” (juvenile shells of Bosi-

), rare echinoderm plates, foraminifers (mainly Lenticuli-


 sp.), juvenile ammonites and ostracods. The bioturbation

is extensive. Silt size grains of clastic quartz are very rare.
Authigenic idiomorphe plagioclases occur in one sample.

The nuclei of the oncoids are represented mostly by intrac-

lasts which may contain fragments of older oncoids. The mi-
crofacies of the intraclasts are usually not identical to those
of the host rock, consisting mostly of a wackestone depleted
in biodetritus with a predominance of other bioclasts, e.g.
echinoderm plates; spicules  predominating in the host rock.
The occurrence of little “hooks” sometimes with forked ends
(Pl. IV: Figs. C, E) are typical for them in the thin sections.
They might represent tiny juvenile chambers of encrusting
foraminifers-nubecularids. Their association with dark red
portions of limestone might suggest a symbiosis with ferric
bacteria. The nuclei usually bear traces of boring organisms
along their rims. Larger borings were used by endostromato-
lites for inward growth in an opposite direction to the oncoid
growth (Pl. I: Figs. C, D). Occasionally bored belemnite ros-
tra formed the oncoid cores (Pl. I: Figs. F, G). Their margins
are  densely bored by algae and fungi;  syntaxial overgrowths
on the rostra margins formed at the expense of micrite were
observed (Pl. I: Fig. E).

The cortices of the oncoids consist mostly of wrinkled

laminae (Pl. IV: Figs. E, F). A pseudocolumnar structure
(SH-stromatolite — Pl. I: Fig. H) is sometimes present form-
ing the second order rhythms. In some cases the pseudocol-
umns are deformed away from the radial direction by com-
paction (Pl. II: Fig. A). The pseudocolumnar structure is
never observed for  the early stage of growth. Encrusting for-
aminifers preferentially occupied saddles between the col-
umns where they were better protected. Their presence pre-
vented the growth on saddles and thus accentuated the
pseudocolumnar fabric. The enhanced growth of encrusting
foraminifers between the pseudocolumns was also observed
by Martin-Algarra & Vera (1994, Fig. 9E). The “anticlinal”
part is usually more strongly stained by Fe-oxides.

Growth conditions changed very often, which resulted in

the alternation of hematite, chlorite and calcite laminae and
variable amounts of encrusting foraminifers. Carbonate mud
was trapped in the hematite cortices. Visual recognition of
calcite is enhanced by the recrystallization of micrite within
the oncoid. Quartz grains of silt size and rare echinoderm
plates occur within the cortices.

The micrite admixture in the hematite oncoids frequently re-

crystallized under the influence of Fe-oxides (Mišík 1968,
p. 129–130, Figs. 1–3). By the aggrading neomorphism (Folk
1965) larger limpid, rarely yellowish calcite grains up to
0.5 mm long were formed. In the thin sections, they contain
sometimes rosy triangular points of crystallographically ar-
ranged hematite pigment or fan-arrays of hematite inclusions
(Pl. II: Figs. C, D). The grains of pseudosparite were some-
times corroded by Fe-oxide. A single calcite crystal may con-
tain several rosy triangular patterns. Similar recrystallization
was also observed in the hematite-calcite oncoids from the
Adneth Limestone of the Silica Nappe (Pl. II: Figs. B, E).

Encrusting foraminifers contributed substantially  to the

formation of oncoids. They are sometimes so abundant that

the term “mobile microreefs” can be applied. Nubecularids
with microcrystalline test possess the plane basement and
chambers bulging towards the oncoid periphery (centrifugal
growth — Pl. II: Figs. F, G). Occasionally larger tests are ag-
glutinated from  tiny quartz grains  (Tolypamminidae, Mini-

 sp.). Thin-walled tests are rarely replaced by chlorite,

and are almost isotropic in the polarized light. The chambers
are infilled either by limpid calcite or by opaque Fe-oxide
and rarely by chlorite, showing that they were empty at the
beginning of the diagenesis. Hayes (1970) described  infill-
ing of the encrusting foraminifers by chlorite (polytype Ib) in
a Pennsylvanian limestone. Our samples contain not only in-
filling, but also perfect replacement of tests by chlorite IIb.
The replacement of foraminifers Involutina liassica by chlo-
rite was recognized long ago (Mišík 1961).

 From other genera Planiinvoluta sp., trochospiral types

with large umbillicus (Pl. III: Fig. B), a test with planispiral
coiling of juvenile stage and other forms were found (Pl. III:
Figs. D–F). Some tiny unilocular sections occur; they are
comparable to those “hooks” (supposed juvenile non-en-
crusted nubecularids — Pl. IV: Figs. C–E) found mainly in
the cores or in immediate vicinity of oncoids. The smallest
size of the encrusted unilocular tests is only 0.038 mm, while
the dimensions of the mentioned  objects scarcely attain
0.022 mm. It should be stressed that encrusting foraminifers
were not found in the surrounding Adneth Limestone.

Neptunic microdykes occurred several times within the on-

coids.  They represent 1–2 mm thick synsedimentary cracks
filled by micrite, mostly sterile or with the tiny bioclasts of
indeterminable detritus (Pl. I: Fig. C, Pl. IV: Figs: F, G). In
only one case a microdyke filled by red micrite contained
foraminifers and sponge rhaxa filled by radial-fibrous calcite
(Pl. IV: Fig. G). Microdykes penetrating up to the core were
rarely observed. The undulating very thin microdykes ap-
proximately vertical to the layering originated during the

Concentric veinlets parallel to the lamination and filled by

fibrous to thin-prismatic calcite  occur in almost all oncoids.
They can be interpreted as retractional cracks; their thickness
was increased by recrystallization of the neighboring oncoid
matter. The calcite infilling often contains hematite pigment
arranged in aggregates displaying triangular patterns in thin

Plate II:

 Fig. A. Chlorite-hematite oncoid with the pseudocolum-

nar structure, in its lower part replaced by calcite; an early diage-
netic transversal veinlet contains teeth-like arranged hematite pig-
ment within the calcite grains. Adneth Limestone, Toarcian,
Gaderská Valley. Fig. B. Recrystallized calcite grains with zonal
arrangment of the hematite pigment in an originally hematite-mi-
crite oncoid. In the upper part of the photo the host rock-biomi-
crite with short “filaments” is visible. Adneth Limestone, Liassic,
Silica Nappe, under the Kornalip saddle near Drnava. Fig. C. Re-
crystallized calcite grain with the arborescent arrangement of the
hematite pigment in an oncoid, Toarcian, Gaderská Valley. Fig. D.
The same, Toarcian, Ve ká Turecká Valley. Fig. E. Hematite-mi-
crite oncoid with the recrystallized calcite grains disturbing fine
concentric laminae; locality as B. Fig. F. Encrusting nubecularid
foraminifers in the hematite oncoids; locality as C. Fig. G. En-
crusting foraminifers as the substancial component of the oncoid.
The same locality. All scale bars = 0.1 mm.


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PLATE II                                                                                                 89

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90                                                                                          PLATE III

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sections (Pl. V: Fig. C). Their terminations tend always to-
ward the centre of the veinlet. Several oblique veinlets cutting
the lamination also possess  an early diagenetic infilling which
grew  synchronously from both sides  (Pl. V: Fig. A). Mobility
of the Fe-oxides was possible only in the very early phase of
diagenesis, we never found them in any epigenetic (tectonic)
calcite veinlet within the red nodular limestones. The younger
extensional veinlets in the oncoids originated by  tectonic pro-
cesses due to  differences in the plasticity of the host rock and
the oncoid. They cut the oncoids approximately perpendicular
to the bedding. Their infilling is of clear asbestos-like aggre-
gate with S-bended calcite fibres. They wedge out after enter-
ing from the hematite set of laminae into the more plastic chlo-
rite set and are reestablished in the next more rigid hematite
set. The youngest epigenetic veinlets are filled by clear aggre-
gate of the isometric calcite grains.

The mineral composition determined by optical microsco-

py (thin section — Pl. V: Figs. B, E; electron micrograph of
replica - Pl. V: Fig. D) shows that the chlorite in the oncoids
forms sheaf-like and fan-like aggregates (Pl. V: Fig. E). The
occurrence of the anatas pigment in chlorite was observed
(Pl. V: Figs. F, G). The calcite and quartz admixture are vari-
able. The fine-clastic quartz is included in agglutinated
tests of encrusting foraminifers. Tiny pyrite grains are
rare. Hematite represents another variety of oncoids or oc-
curs in combined chlorite-hematite oncoids. More detailed
mineralogical characterization of the chlorite and hematite
(X-ray diffraction study) is given below. As an example of
the chemical composition a bulk analysis of the chlorite on-
coid from the Liassic limestones, Bystrická dolina Valley, is
included (data in wt. %):


= 43.31 %, TiO


 = 0.42 %, Al




 = 13.20 %, Fe





6.07 %, FeO = 9.79 %, MnO = 0.13 %, P




 = 0.16 %, CaO

= 12.19 %, MgO = 5.05 %, K


O = 0.28 %, Na


O = 0.50 %,

Loss of ignition = 9.44 %. The analysis indicates that the cal-
cite admixture in the oncoids was about 20 %.

Oxfordian chlorite oncoids

from the Malá Fatra Mts.

Oncoids from Zázrivá Valley display many similarities with

the Liassic chlorite oncoids described above. Their diameter
attains up to 7 cm (Pl. VI: Fig. A). The cores are also biomi-
critic intraclasts, often bored by lithophags (Pl. I: Fig. B) con-
taining some ostracods, “filaments” (juvenile bivalvian
shells), echinoderm plates, globochaets, sponge spicules and
juvenile ammonites.

The host limestone is a rosy biomicrite with small nodules,

and radiolarian or Globuligerina microfacies. The nodules

containing both microfacies were observed together in the
same thin section indicating the partial redeposition. Other
remains are represented by globochaets, bivalvian fragments,
rhaxa (voids after rhaxa and radiolarians are filled by cal-
cite), Cadosina parvula Nagy, Colomisphaera sp., foramini-
fers (Lagenidae), rare ostracods, echinoderm plates, aptychi,
fragments of ammonites, rhyncholites, belemnite rostra and
phosphatized fish scales. The clastic quartz is very rare or to-
tally absent in the sections. The occurrence of the first “Ca-

 namely Cadosina parvula indicates the Oxford-

ian age (Borza 1984). The Callovian age of some samples
with Globuligerina (“protoglobigerina”) microfacies without
Cadosinidae” cannot be ruled out.

The cortex is composed of finely wrinkled concentric lami-

nae of chlorite displaying leafy aggregates under  polarized
light. Macroscopically, the chlorite oncoids are always green,
in  thin section green or light-brown, sometimes with an  alter-
nation of both colours. Some chlorite laminae contain abun-
dant micritic inclusions, or are stained by  hematite pigment.
The admixture of microcrystalline calcite is well visualized in
the case of the recrystallization (aggrading neomorphism).
New formed roughly isometric calcite grains sometimes con-
tain fan-like arranged hematite pigment. The oncoids rarely
contain clastic quartz; exceptionally some rhaxa were attached
to the growing oncoid. Tiny anatas inclusions in the chlorite
are rare.

The Upper Jurassic oncoids were also substantially built

by the encrusting foraminifers, such as Miniacina with tests
agglutinated of the very fine-grained quartz (PI. III: Fig. A),
nubecularids and other types. Several tests were replaced by

Early diagenetic veinlets are filled by calcite aggregates

containing hematite inclusions arranged in triangular pat-
terns (Pl. IV: Fig. B, Pl: VI: Fig. I). Concentric retractional
veinlets were broadened by recrystallization.

The locality Bralo-quarry differs from the other as only

small chlorite oncoids (up to 0.7 cm) occur there, but thin
chlorite and hematite crusts and planar chloritic stromatolites
are frequent. The host rock — an intraformational limestone
breccia with rosy, grey, white, red and green fragments is
also different. Pale red and grey intraclasts contain marine
biodetritus, white and red fragments originating from a disin-
tegrated calcrete. Green clasts are hardground fragments, but
the majority of the chlorite aggregates are part of the scarce

Rosy and grey intraclasts belong to the biomicrite-wacke-

stone with radiolarian microfacies, rhaxa, globochaets, rare
ostracods, echinoderm plates, foraminifers (including Globu-

 sp.), “filaments”, Colomisphaera sp., belemnite ros-

tra, rhyncholites and aptychi.

Fragments of calcrete.

 White and white-red clasts (up to

5 cm across) show  patterns of a fine-grained breccia. Small
white microsparite clasts bear traces of the synsedimentary
cracking (Pl. VI: Fig. D); some cracks (microdykes) are un-
fossiliferous micrite with pyrite pigment. They contain pelle-
tal grains and irregular micritic envelopes and spots within
the microsparite (Pl. VI: Fig. E). Some of the small clasts
were silicified, replaced by microquartz (Pl. VI: Fig. B),
which initiated as usual the formation of tiny Fe-carbonate

Plate III:

 Fig. A. Chlorite oncoid full of encrusting foraminifers,

Oxfordian, Tatric Unit, borehole S-1, 12.5 m, Párnica, Malá Fatra
Mts. Fig. B. Encrusting foraminifers in the chlorite oncoid, Toar-
cian, Bystrická Valley. Fig. C. Encrusting foraminifers possessing
juvenile stage with planispiral coiling in a chlorite oncoid, Toar-
cian, Horná Turecká Valley. Fig. D. The same. Fig. E. Encrusting
foraminifers in the chlorite oncoid, Toarcian, Suchá Valley. Fig. F.
The same. All scale bars = 0.1 mm.


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92                                                                                                PLATE IV

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PLATE V                                                                                                  93

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94                                                                                                PLATE VI

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rhombs (Mišík 1991). The isotopic composition of a white
clast was 



C = +2.23 ‰ PDB, 



O = –0.7 1‰ PDB which

is characteristic for marine water. The matrix joining the
white clasts of the second order is formed by an aggregate
displaying fluid structure formed by  tiny elongated calcite
grains - imperfect miniature scalenohedra (Pl. VI: Figs. B, C);
the fluidal aggregate used to be stained by limonite. Fossil
remains are completely absent which contrasts strongly with
their richness in the described Jurassic limestones. The par-
ent rock did not originate in the marine environment; several
of the described phenomena point to eroded calcrete. The
isolated clasts of the marine Jurassic limestones in the  cal-
crete could be considered as an ancient slope debris (Pl. VI:

Fig. C). It is necessary to note that the traces of a temporary
local emersion during the Jurassic were not documented in
the Central Western Carpathians prior to the present study.
The paucity of outcrops containing the Upper Jurassic strata
of the Tatric Unit in the Malá Fatra Mts. prevented further

The scarce matrix of the Jurassic intraformational breccia

consists of red biomicrite with echinoderm plates, foramini-
fers, “filaments”, Colomisphaera etc. Rare angular clastic
quartz attains up to 0.3 mm across. Several thin milimeter
thick chlorite and hematite hardgrounds are part of the matrix.

Chlorite crusts mostly with stromatolite lamination occur

sometimes immediatly between the fragments of the breccia;
they were compacted to seams with a zigzag structure (Pl.
VI: Fig. F). Ptygmatic calcite veinlets are also of compac-
tional origin (Pl. VI: Fig. H). A part of the stromatolites con-
tain encrusting foraminifers and quartz silt. Some chlorite
crusts are homogeneous, without any internal structure and
with pyrite grains. The chlorite aggregates  sometimes pos-
sess calcite rims formed by the calcite cement growing into
the retractional interspaces  during the dehydratation of the
aggregates (Mišík & Šucha 1994). The chlorite oncoids at
this locality are small and rare. Their cores stained by limo-
nite might represent fragments of hardground.

Besides chlorite, hematite hardgrounds also occur there.

These hematite crusts without lamination originated  by the
staining and replacement of the underlying limestone. Their
mechanical properties so differing from the host limestone
caused an intensive cracking of the hematite crusts during
the tectonic pressure; they are penetrated by a dense set of
the thin vertical calcite veinlets.

Mineralogical characteristic of chlorite

and hematite oncoids

Chlorite, calcite, hematite, quartz, pyrite and anatas were

identified by their optical microscopy. Further mineralogical
analyses were carried out by the following methods.

The material of the oncoids was ground to pass a 0.16 mm

sieve and subsequently disintegrated by ultrasonic probe and
treated by sodium acetate buffer at 80 


C to remove as much

of the carbonate matter as possible. Then the clay fraction
< 2


m was separated by sedimentation. X-ray diffraction

analysis (XRD) was conducted using a Philips diffractometer
PW-1710 and a Siemens D-500, both equipped with Cu  radi-
ation. Oriented specimens prepared by sedimentation onto
glass slides were analyzed in air dried state and after satura-
tion with ethylene glycol (8 hours at 70 


C). Infrared spectra

were obtained on a FTIR spectrometer Nicolet Magna 750
equipped with a DTGS detector. Each sample was recorded
in the 4000–400 cm


 spectral range in the transmission

mode with a resolution of 4 cm


. The KBr pressed-disk

technique (0.4 mg sample and 200 mg KBr) was used.

Four samples of chlorite oncoids from the following locali-

ties were analysed: N1, N3 (both Suchá Valley), N4
(Bystrická Valley) of Toarcian age; N2 (Zázrivá Valley) of
Oxfordian age. The chlorite composition of both stratigraphi-
cal horizons is almost identical.

Plate IV:

 Fig. A. Encrusting foraminifer in the chlorite oncoid,

Toarcian, Krížna Nappe, Horná Turecká Valley. Fig. B. Encrusting
foraminifer in the chlorite oncoid, Oxfordian, Tatric Unit in front
of the Bralo quarry, Zázrivá Valley. Fig. C. Hook-like section (per-
haps juvenile stage of an encrusting foraminifer) in the host rock
close to the hematite oncoid, Toarcian, Gaderská Valley. Fig. D.
Section in the form of tiny hooks and rings (perhaps unilocular ju-
venile stage of nubecularid foraminifer) in the core of a hematite
oncoid; the same locality. Fig. E. Encrusting foraminifers and iso-
lated “hooks” in the Fe-oncoid. Toarcian, Bystrická Valley. Fig. F.
Synsedimentary crack in the hematite oncoid (neptunic micro-
dyke) with micrite infilling, Toarcian, Gaderská Valley. Fig. G.
Microdyke consisting of the biomicrite-wackestone with ostra-
cods, lagenid foraminifers and rhaxa - evidence of the synsedi-
mentary cracking of the chlorite oncoid. Firstly, the inicial calcite
cement was precipitated on the crack walls, later the micrite pene-
trated into the residual space, Toarcian, Horná Turecká Valley. All
scale bars = 0.1 mm.

Plate V:

 Fig. A. Early calcite veinlet in the Fe-oncoid containing

rosy points, oriented hematite inclusions, Toarcian, Gaderská Val-
ley. Fig. B. Chlorite oncoid, Toarcian, Suchá Valley. Fig. C. Cal-
cite veinlet with the rosy points-oriented hematite pigment, per-
haps a retractional crack concordant with the oncoid laminae;
Oxfordian, in front of the Bralo quarry, Zázrivá Valley. Fig. D.
Chlorite sheets separated from the oncoid, Toarcian, Gaderská
Valley; suspension under the electrone microscope, magnification


Fig. E. Thin section of the same oncoid, polarized light.

Fig. F.

 Anatas pigment in the chlorite oncoid, Toarcian, Horná Turecká

Valley. Fig. G. The same. All scale bars (except of D) = 0.1 mm.

Plate VI:

 Fig. A. Chlorite oncoids on the weathered surface of the

Oxfordian limestone (the diameter of the white circle is 24 mm);
in front of the Bralo quarry, Zázrivá Valley, Malá Fatra Mts. Fig.

 Fragment of a calcrete with the fluidal patterns formed by tiny

calcite crystals-imperfect scalenohedra and partial silicification
(white parts); clast from the intraformational carbonate breccia
containing chlorite oncoids. Oxfordian, Tatric Unit,  Bralo quarry,
Zázrivá Valley. Fig. C. The same. Fig. D. Another clast of the cal-
crete with desiccation cracks includes a limestone fragment with
the crinoidal plate proceeding from the ancient scree. Fig. E. Clast
of the calcrete with irregular concentric structure. Fig. F. Chlorite
hardground with the marked compaction patterns. Fig. G. Crinoi-
dal plate with recrystallized margins (white) within the chlorite
hardground; the crystallization of chlorite caused the calcite re-
crystallization. Fig. H. Ptygmatic veinlets in the chlorite hard-
ground (dehydratation accompanied by the compaction). Fig. I.
Calcite veinlet with points pigmented by the hematite in a Fe-
hardground within the intraformational breccia. Figs. B–I are from
the same locality. All scale bars = 0.1 mm.


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96                                                                                          MIŠÍK and ŠUCHA

Chlorite mineralogy

Several aspects of the chlorite chemistry and structure were

determined by XRD and IR spectroscopy on the < 2 mm frac-
tion. The XRD patterns give sharp and symmetrical basal
chlorite reflections with no changes after saturation by ethyl-
ene glycol for all samples. The intensity distribution is roughly
the same for all the studied samples (Table 1). Higher intensi-
ties of 002 and 004 than 001 and 003 reflections indicate a
high Fe content in octahedral sites of the chlorite structure.
Two plots based on the basal peaks intensity were used for
estimation of the iron content in the chlorite structure (Oinu-
ma 1973; Weiss 1992). Both indicate a chemistry close to Fe-
chlorite. This is also supported by IR spectroscopy (Fig. 5).
Stretching bands at 3542 cm


 (Si-O) together with the band

at 652 cm


 indicate high Fe content in the chlorite structure

(Farmer 1984; Shirozu 1985). IR spectra also show a rela-
tively high content of quartz in the clay fraction (bands at
1165, 1089, 799 and 780 cm


). The IR spectra of three chlo-

rites from different localities show almost no differences.

XRD analyses of randomly oriented clay powder of 4 sam-

ples give a sharp 201 reflections characteristic for IIb poly-
type. The most intensive was a doublet of 202 at 0.259 nm
and 201 at 0.255 nm.

The XRD analyses of the red oncoids gave results typical

for hematite.

Fig. 5.

 Infrared spectra of samples N2, N3, N4.

Fig. 4.

 X-ray diffraction patterns of air dried (A) and ethylene gly-

colated (B) specimens of the < 2 


m fraction of sample N1.

Fig. 3.

 X-ray diffraction patterns of air dried oriented specimens

of the < 2 mm fraction. CH = chlorite, Q = quartz (samples N2,
N3, N4; their localization is in the text).

































































Table 1:

 Intensities of chlorite basal reflections recalculated to

100 % (data are in %).

The presence of the main mineralogical components (chlo-

rite and/or hematite) was also confirmed by XRD of random-
ly oriented bulk samples. For a more detailed analysis of the
layer silicates a clay fraction < 2


m of four samples was

separated. In the clay fraction chlorite and quartz are the ma-
jor phases (Fig. 3). In sample N1 a significant amount of il-
lite was also present. Illite XRD reflections do not change
their positions after ethylene glycol saturation (Fig. 4). Only
some intensity changes of the 001 and 003 peaks were no-
ticed. An intensity ratio I(001)/I(003) air dried vs. I(001)/
I(003) glycolated representing the amount of expandable lay-
ers in the illite structure (Srodon 1984) shows a value higher
than 1 (Ir = 1.30). This means that some  expandable layers
are still present in the illite. The illite crystallinity index


 2 theta) shows that illite crystals are either very thin or

contain many  stacking defects.

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Environmental interpretation and conclusions

The chlorite and hematite oncoids were associated with the

condensed sedimentation of the red nodular limestones facies
during the Toarcian and Oxfordian. According to the associ-
ated organic remains, they are pelagic oncoids formed below
the wave erosional base, most probably at depths of 100-200
m. At a single locality Bralo chlorite oncoids occur in the in-
traformational limestone scarp breccia with fragments of a
supposed calcrete what could demonstrate a nearby local em-
ersion. Bored intraclasts from the disintegrated hardgrounds
and belemnite rostra served as the cores for the oncoid
growth. Long-distance transport of the oncoids by currents
can be ruled out since the tests of encrusting foraminifers cut
by erosion were never observed.

The hematite oncoids lithified rapidly as can be occasion-

ally evidenced by their cracking and the micritic sediment in-
filling of these synsedimentary veinlets (neptunic micro-
dykes). The lithification of the chlorite oncoids was much
slower; its consequence was a considerable compactional de-
formation and flattening.

The aggradational recrystallization of the microcrystalline

calcite included in oncoids under the influence of Fe-oxides
is a frequent phenomenon. The recrystallized aggregates fre-
quently display a fan-like arrangement of hematite pigment.
The first generation of the early diagenetic calcite veinlets is
also remarkable for the hematite pigment arranged in trian-
gular patterns visible in thin sections.

The described chlorite oncoids are an unusual phenome-

non. They belong to the authigenic iron-rich chlorite of poly-
type IIb, a product of the diagenesis. The chlorite composi-
tion is in accordance with the analysed rocks bearing not
even the slighest traces of metamorphism, which in turn was
also shown by the analysis of the illite in sample N1. They
cannot be connected  with any volcanism; no volcanic activi-
ty is known from the Toarcian or Oxfordian from the Western
Carpathians (Mišík 1992). The nearest traces of the volcanic
activity (mostly trachytic) occur in the Adneth Limestone of
the Rumanian Eastern Carpathians (Patrulius 1960), in the
Getic Nappe of the Southern Carpathians (Sandulescu et al.
1974), and in the Lessini Mts. of the Southern Alps and
Western Sicily (Bernouilli & Peters 1970), then in the very
remote areas.

The oncoids described prior to the present study from the

Jurassic red nodular limestones (Ammonitico Rosso) always
belonged to calcite, hematite or goethite oncoids (Massari
1983; Szulczewski 1963; Farinacci 1967, p. 441). Vera &
Martin-Algarra (1994, p. 34) and Ballarini et al. (1994) ob-
served Fe-Mn oncoids in the Middle and Upper Jurassic
(Ammonitico Rosso facies). They were described by Zydor-
owicz & Wierzbowski (1986) from the same facies of the
Oxfordian age (Czorsztyn Limestone). Hematite oncoids
also occur at many other localities in Slovakia (e.g. Liassic
of the Silica Unit).

Encrusting foraminifers substantially contributed to the

growth of the described oncoids. They were mentioned first-
ly from the Mn-oncoids or nodules (Greenslate 1974; Wendt
1974). It is noteworthy that the higher concentration of the

metallic compounds did nor prevent foraminiferal growth.
On the contrary, one species of the nubecularids seems to
prefer those parts of oncoids formed by the set of hematite
laminae; a symbiotic relation to the ferric bacteria is possi-
ble. The abundance of encrusted foraminifers in chlorite, he-
matite and Mn-oncoids contrasts with their paucity in calcite
oncoids. Perhaps it might be caused by some protective
chemical compounds secreted by Cyanophyceae. Perfect re-
placement of the vitrocalcareous tests by chlorite was ob-
served, but their replacement by hematite was not found.
Neither fixed encrusting foraminifers nor redeposited ones
occur in the host limestones. Similar associations of encrust-
ing foraminifers grown on  oncoids in rather deep water at re-
mote localities (“oases”) could lead to the opinion that these
encrusting foraminifers produced a huge amount of juvenile
planktonic individuals.

The episodes characterized by mineralized oncoids and

hardgrounds could be connected either with the local condi-
tions or with the eustatic movements. The formation of tur-
bidites, the redeposition phenomena occur mainly during the
periods of the lowest sea-level (minima on the eustatic
curve); the condensed sedimentation with hardgrounds and
mineralised oncoids should be typical for the eustatic high
stands. It is in accordance with the fact that the horizon con-
taining Toarcian chlorite oncoids in red nodular Adneth
Limestone lies immediately  under the grey spotty marls,
generally considered as hemipelagic sediments.


The authors thank J. Madejová for in-

frared measurements,  I. Repčok for the isotope analysis and
T. Lyons (University of Missouri) for  critical reading of the
manuscript, T. Peryt (Panstwowy Instytut Geologiczny, Warsza-
wa), J. Michalík (Geological Institute, Slovak Academy of Sci-
ences) and I. Kraus (Comenius University). V.Š. thanks to US-
Slovak Science and Technology Project N. 92029.


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