GeolCarp_Vol52_No6_375

GEOLOGICA CARPATHICA, 52, 6, BRATISLAVA, DECEMBER 2001

375 — 386

EARLY MIOCENE BRAIDED RIVER AND LACUSTRINE

SEDIMENTATION IN THE KALNIK MOUNTAIN AREA

(PANNONIAN BASIN SYSTEM, NW CROATIA)

DAVOR PAVELIĆ, RADOVAN AVANIĆ, KORALJKA BAKRAČ and DAVOR VRSALJKO

Institute of Geology, Sachsova 2, HR-10000 Zagreb, Croatia; dpavelic@yahoo.com

(Manuscript received March 9, 2001; accepted in revised form October 5, 2001)

Abstract: Early Miocene deposits of fresh-water environments are characteristic in the Kalnik Mountain area, at the SW
marginal zone of the Pannonian Basin System. Alluvial and lacustrine sediments varying from gravel to marl accumu-
lated by different depositional processes during the Ottnangian. In the early, alluvial phase pebbly braided rivers devel-
oped. Deposition was characterized mostly by bar conglomerates and flood plain siltstones. Alluvial deposition was
controlled by both autocyclic and allocyclic processes, in a semi-arid climate. During the later, lacustrine phase, sedi-
mentation was mostly represented by marls and occasional coarser material, in a humid climate. Fresh-water deposition
was terminated by marine transgression during the Karpatian. Lower Miocene fresh-water deposits of the Kalnik Moun-
tain can be correlated with similar deposits in the wider area of Northern Croatia. The Kalnik Mountain represents the
boundary area between two Early Miocene basins, the north-western one being characterized by marine deposition, and
the south-eastern by contemporaneous fresh-water deposition, both belonging to the Central Paratethys.

Key words: Croatia, Early Miocene, braided river, hydrologically open lake, synsedimentary tectonics.

Introduction

The Lower Miocene sedimentary complex of the Kalnik
Mountain (Figs. 1, 2) disconformably overlies Mesozoic-Pa-
leogene basement, but in some localities the contacts are tec-
tonic (Šimunić et al. 1981, 1982, 1994; Fig. 3). The total thick-
ness of the Lower Miocene complex is approximately 470 m
(Šimunić et al. 1982). The stratigraphic dating of these depos-
its is still uncertain. On the basis of marine, brackish and
fresh-water faunas the deposits were correlated with coal bear-
ing deposits of Oligocene age (Poljak 1942; Anić 1952). Later,
this complex was interpreted as marine to brackish-water ne-
glecting the existence of fresh-water sediments, and dated as
Egerian to Eggenburgian in age (Šimunić et al. 1981).

Recent investigations at the Kalnik Mt demonstrate the

presence of unfossiliferous red beds directly overlying Egeri-
an-Eggenburgian fossiliferous deposits. However, a part of the
Lower Miocene succession contains an assemblage of sporo-
morphs  and  fresh-water  algae  which  have  not  been  found  in
this area before. They allow correlation with similar deposits
of Ottnangian age in the Pannonian Basin System. Early Mi-
ocene sedimentation came to an end with deposition of marine
marls during the Karpatian time (Hećimović 1995). There is
thus  a  relatively  complete  Lower  Miocene  succession  in  the
Kalnik  Mt  (Fig.  5).  The  nature  of  the  transition  between  the
stages, as well as the areal distribution is unknown at present,
and the Lower Miocene complex is still not covered by geo-
logical maps in details. In order to determine the depositional
evolution of the Lower Miocene fresh-water succession of the
Kalnik  Mt  and  to  correlate  it  with  deposits  in  other  parts  of
Northern Croatia, two geological sections, located in the cen-
tral  part  of  the  mountain  have  been  investigated  in  details
(Figs. 3, 5, 6). Lower Miocene fresh-water deposits are subdi-

vided into a Lower, unfossiliferous unit, and an Upper unit
with fresh-water fossils.

Geological setting

The Miocene rock complex of the Kalnik Mountain belongs

to the south-western marginal area of the Pannonian Basin
System (Figs. 1, 2). The sediments were deposited in the Cen-
tral Paratethys bioprovince (Rögl & Steininger 1983; Rögl
1998). The pre-Miocene basement is geotectonically interpret-
ed as part of the Supradinaricum, that is the NW part of the In-
ner Dinarides (Herak et al. 1990). The formation of the Pan-
nonian Basin System commenced in the Early and the Middle
Miocene as the consequence of the continental collision of the

Fig. 1. Geotectonic position of the Pannonian Basin System, with
location of study area.

376 PAVELIĆ et al.

African (= Apulian) and European plates, and deposition in the
entire basin was influenced by important extensional tectonics
(Horváth & Royden 1981; Royden 1988; Horváth 1993;
Kováč et al. 1997).

Marine connections between the Central Paratethys, Medi-

terranean and Indopacific oceans were temporarily established
and interrupted during the Miocene (Rögl & Steininger 1983;
Rögl 1998). The isolated nature of the Central Paratethys has
led to establishment of a local system of Miocene stages (Fig.
4). During transgressions, especially in the Early Miocene, the
Central Paratethys was not flooded completely. Therefore, the
underlying deposits were disconformably covered by deposits
of different ages, ranging from the Early to the Late Miocene.
In the Early Miocene deposition took place in different environ-
ments, including marine, brackish and fresh-water, and in some
parts of the basin continental environments also existed tempo-
rarily (Rögl & Steininger 1983; Rögl 1998; Sztanó & Józsa
1996; Kováč & Hudáčková 1997; Hudáčková et al. 2000).

Lower Miocene (Ottnangian) fresh-water sediments repre-

sent a part of Rzehakia (= Oncophora) Beds, and cover large
areas of the Paratethys. They are probably of Late Ottnangian
or, maybe, Early Karpatian age (Rögl & Steininger 1983; Rögl
1998; Nagymarosy & Müller 1988). Lower Miocene fresh-wa-
ter deposits in the neighbouring Styrian Basin (Fig. 1) studied
by multi-disciplinary stratigraphical methods are of Ottnan-
gian age (Steininger 1998).

Lower unit of fresh-water deposits

Description and interpretation of facies

The lower, unfossiliferous part of the succession is 62.8 m

thick and consists of siliciclastic rocks (Fig. 6). It is character-

ized by the predominance of siltstones in the lower part, and
common alternations of conglomerates and sandstones in the
upper part.

The sediments are subdivided into eight facies, which form

fining- and coarsening-upward cycles. The lower part of the
fresh-water deposits forms two megacycles, with the lower
characterized by coarsening-upward trend (Fig. 6).

Facies Gc – massive clast-supported conglomerates

This facies occurs in the upper part of the succession of the

Lower unit (Fig. 6). The conglomerates form horizontally bed-
ded clast-supported massive beds, 40—130 cm thick. Their
lower boundaries are erosional. The clasts are mostly of coarse
pebble size, while cobbles are uncommon and mostly found in
the basal part of units. Clasts up to 26 cm in diameter are very
rare. The matrix is composed of coarse-grained sandstone to
fine-pebble conglomerate, and in some places the facies is
characterized by a bimodal composition. The matrix content is
very variable, although it generally increases towards the up-
per parts of most beds. In some cases pebbles are imbricated
(type a

(t)

(i)

The facies represents deposits of very powerful currents, as

indicated by erosional lower boundaries, clast size and imbri-
cation of the a

(t)

(i)

type. The bimodal composition and vari-

able portion of matrix indicate multi-storey accumulation of
material, suggesting pulsation of the current velocities (Steel
& Thompson 1983). The structures of the conglomerates and
their massive appearance indicate deposition on longitudinal
bars (according to Smith 1974; Rust 1978; Steel & Thompson
1983). The cobbles in the basal parts of some beds represent
basal lags. The increased matrix content in the uppermost parts
of some fining-upward beds is explained by gradually decreas-
ing current velocities.

Fig. 2. Location map of Kalnik Mt. The boundary between Slovenian Oligocene Basin and North Croatian Basin is marked in the area of
Kalnik Mt and Medvednica Mt.

MIOCENE BRAIDED RIVER AND LACUSTRINE SEDIMENTATION IN PANNONIAN BASIN 377

Facies Gp – planar cross-bedded conglomerates

This facies also occurs in the upper part of the Lower unit

(Fig. 6). Beds are 50 to 110 cm thick, and have erosional lower
boundaries. The conglomerates are mostly clast-supported,
rarely matrix-supported, and are characterized by planar cross-
bedding. Clast sizes range from coarse pebbles and rarely to
fine cobbles up to 9 cm long. The matrix is well sorted coarse-
grained sandstone.

The clast size (up to 9 cm) and erosional lower boundaries

indicate strong currents, while planar cross-bedding suggests
deposition by avalanching mechanisms. The bed thickness (up
to 110 cm) indicates relatively large bedforms which could be
transversal bars migrating in channels of gravelly braided riv-
ers (Smith 1974; Steel & Thompson 1983).

Facies Ge – conglomerate lenses

This facies occurs throughout the succession of older depos-

its, with increased abundance in the upper part (Fig. 6). The
conglomerate beds are lens-shaped, 10—40 cm thick and up to 5
m long. They are most common within siltstones of facies F2
or overlying conglomerates of facies Gc or Gp. Conglomerates
of facies Ge are clast-supported, with clasts ranging in size
from fine- to medium-grained pebble, rarely with cobbles up to

8 cm wide. The matrix is coarse-grained, well- to medium-
sorted sandstone. Some clasts show imbrication of a

(t)

(i)

type.

The facies Ge was deposited from high velocity currents, as

indicated by erosional lower boundaries and clast size (up to 8
cm). The imbrication of the a

(t)

(i)

type indicates deposition by

bed-load traction. Association with siltstones of facies F2, in-
terpreted as flood plain deposits, indicates deposition in simi-
lar alluvial environments. The conglomerates might represent
crevasse channel deposits (Steel 1974; Hughes & Lewin
1982). Outcrops where conglomerate lenses cover bar depos-
its (facies Gc and Gp) probably represent deposits in shallow
channels cutting into bars during periods of waning flow (Rust
1978; Steel & Thompson 1983).

Facies Sh – horizontally laminated sandstone

This type of sandstone occurs in the central and upper parts

of  the  measured  succession  of  the  Lower  unit  (Fig.  6).  The
sandstones  most  commonly  alternate  with  conglomerates  of
the facies Gc, while sporadically they cover sediments of the
facies Ge (Fig. 6). The sandstones are medium to well sorted
and  form  20—40  cm  thick  beds  characterized  by  horizontal
lamination and irregular lower boundaries. They are medium-
to fine-grained, and contain rare pebbles up to 2 cm in size.

Fig. 3. Geological sketch-map of the investigated area (simplified after Šimunić et al. 1982).

378 PAVELIĆ et al.

Fine- to medium-grained bioturbated sandstones in the cen-

tral and upper part of the succession of older deposits, were
also included in this facies in spite of their massive appear-
ance. They form units 100 to 210 cm thick (Fig. 6).

The grain size and horizontal lamination indicate deposition

by traction in the upper flow regime, while their overlaying
conglomerates of the facies Gc and Ge suggest deposition on
bars and in shallow channels in periods of waning flow after
the flood, and at lower water levels (Rust 1978). The presence
of relatively thick beds could be explained by long-lasting uni-
form depositional conditions. The bioturbation was probably
caused by small mammals.

Facies Sr – cross-laminated sandstone

This facies occurs only at the top of the Lower unit (Fig. 6),

and alternates with conglomerates of the facies Ge. They form

two units, 15 and 20 cm thick, respectively. The lower bound-
aries are irregular. The beds are characterized by a weakly ex-
pressed fining-upward trend.

The grain size and cross-lamination indicate deposition by

traction from currents in the lower flow regime. The position
of the cross-laminated sandstones overlying conglomerates of
the Ge facies and the fining-upward trend suggests sediment
deposition in shallow channels under lower flow regime con-
ditions.

Facies Se – sandstone lenses

Sandstones of this facies occur in the lower and central part

of  the  Lower  unit  (Fig.  6),  where  they  are  interbedded  with
sediments  of  facies  F1  and  F2  (Fig.  6).  They  form  6—20  cm
thick  lenses,  extending  laterally  up  to  6  m  in  outcrop.  Their
erosional  bases  were  originally  concave  up.  The  sandstones
are  fine-  to  coarse-grained,  in  some  places  passing  into  silt
sized  sediments,  and  are  characterized  by  a  fining-upward
trend  and  medium  sorting.  Cross-lamination  occurs  sporadi-
cally.

Occurrences of cross-lamination indicate deposition by trac-

tion in the lower flow regime. The fining-upward trends indi-
cate  decreasing  of  flow  velocities.  Depositional  structures,
lensoid geometry and position within sediments of facies F1
and  F2  interpreted  as  flooding  plain  deposits,  suggest  a  cre-
vasse splay origin (Steel 1974; Hughes  &  Levin  1982;  Guc-
cione 1993).

Facies F1 – massive siltstone

This type of siltstone occurs in the middle part of the succes-

sion of the Lower unit (Fig. 6). Siltstone is interbedded with
sandstone lenses of facies Se. The siltstone units are 10—250
cm thick, and their lower boundaries are irregular. They are
massive, very well sorted, and characterized by grey colour.

Fig.  5.  Environmental  changes  in  the  Lower  Miocene  succession
with  the  stratigraphic  position  of  the  sections  KL-I  and  KL-II  at
Kalnik  Mt.  Compiled  and  simplified  after  Šimunić  et  al.  (1981,
1982), Hećimović (1995) and Pavelić (1998).

Fig. 4. Chronostratigraphic scheme showing the correlation of the
Central Paratethys stages to the standard time scale (after Rögl 1998).

380 PAVELIĆ et al.

The massive siltstones were deposited from suspension,

characterized by very weak flow. The massive nature, thick-
ness and interbedding with sandstone of facies Se indicate
deposition on a flood plain. The environment was not charac-
terized by reworking in subaerial conditions, as indicated by
sorting, colour and lack of bioturbation. These characteristics
also suggest frequent floods in part of the flood plain relatively
close to the river channel.

Facies F2 – modified siltstone

This type of siltstone occurs in the entire succession of the

Lower  unit  except  the  uppermost  part,  and  it  represents  the
predominant lithotype in the lower and middle part of the suc-
cession (Fig. 6). The siltstone contains lenses of facies Se and
Ge. In one outcrop the siltstones are underlain by facies Gp,
and overlain by facies Gc (Fig. 6). The facies form 0.2—14 m
thick units, and their lower bedding planes are irregular. The
siltstones contain irregular sandy zones, and uncommon peb-
bles  up  to  0.7  cm  in  diameter.  The  siltstones  are  very  rarely
clayey,  the  sorting  is  poor,  and  the  structure  massive.  They
contain  carbonate  nodules  and  ferruginous  concretions  and
scattered coal clasts. The sediments are partially bioturbated,
and predominantly by dark red or sporadically grey spots.

Siltstones were deposited out of suspension from very weak

flow, and the thickness up to 14 m indicate persistance of very
similar conditions. The association with facies Se, Ge, Gp and
Gc,  which  are  interpreted  as  alluvial,  and  the  lack  of  fauna,
suggest the same depositional environment. In alluvial settings
thick  successions  of  siltstones  are  common  in  flood  plains
(Rust  1978;  Miall  1996).  Coal  fragments  could  originated
from  the  Egerian-Eggenburgian  sediments  which  commonly
contain coal beds (Šimunić et al. 1981).

The siltstones show some signs of post-depositional alter-

ation. The red pigmentation, in some places dark-red, could be
a  result  of  chemical  disintegration  of  unstable  ferruginous
minerals and diagenetic covering of detrital grains by hematite
in conditions of rapid drying, temporary moisturizing and high
temperatures  (review  in  Collinson  1996).  The  formation  of
concretions could also have been generated by drying and in-
filtration  of  minerals  into  the  soil.  The  very  rare  irregular
sandy zones and small pebbles in the siltstone probably repre-
sent  a  sedimentary  substitute  for  rotten  vegetation  roots.  Fa-
cies F2 is accordingly interpreted as a paleosoil.

Paleotransport

Measurement of 31 imbricated platy pebbles from alluvial

conglomerates of facies Gc and Ge in the Lower unit indicate
flow direction towards the NW (Figs. 6, 7). However, the new-
est paleomagnetic results from Lower Miocene sediments
from north-western Croatia show moderate counter-clockwise
rotations generated by tectonic events in the Pliocene (Márton
et al. 2001). It means that the real flow directions might be to-
wards the N.

Vertical facies relationships

Vertical alternation of facies, and changes in average grain

size in the Lower unit show small cycles and two megacycles.

Facies associations

Two facies associations are recognized in the Lower unit.

Facies association A comprises the lower and middle part of
the Lower unit (0—45 m, Fig. 6). It is composed of siltstone fa-
cies (F2 and F1) deposited in flood plain, mostly influenced by
subaerial conditions. It contains conglomerate lens facies
(Ge), and horizontally laminated sandstones facies (Sh), which
represent shallow channels, and sandstone lens facies (Se), in-
terpreted as crevasse splays.

The upper part of succession is represented by Facies asso-

ciation B. It is composed of massive clast-supported conglom-
erates  (Gp)  interpreted  as  longitudinal  bar  deposits,  planar
cross-bedded conglomerates (Gp) interpreted as deposited on
transversal  bars,  conglomerate  lenses  (Ge),  cross-laminated
sandstones  (Sr)  deposited  in  shallow  channels,  horizontally
laminated sandstones (Sh) representing bar covers, and altered
siltstones (F2) interpreted as paleosoil facies.

Interpretation

The association of sediments deposited predominantly by

traction currents on a flood plain indicates an alluvial deposi-
tional environment for the Lower unit (Fig. 8). The conglom-
erate bodies interpreted as longitudinal and transversal bar de-
posits, and the lack of sediments deposited by gravity flows
suggest a pebbly braided river (Williams & Rust 1969; Rust
1978; Smith 1974; Steel & Thompson 1983).

A predominance of flood plain sediments (F2 and F1) in Fa-

cies association A is probably a consequence of distal position
with respect to the active channel, while thick deposits indi-
cate a frequent supply of fine-grained material by floods. Ac-
tive flow probably occurred from time to time, resulting in for-
mation  of  crevasse  channels  and  splays.  The  succession  is
characterized  by  dominant  flood  plain  over  channel  deposits
indicate  deposition  in  the  lower  alluvial  plain  of  the  braided
river (Fig. 8).

A characteristic of a pebbly braided river is the small preser-

vation potential of flood plain deposits, as braided flows show
a tendency to occupy the entire river valley, resulting in ero-

Fig. 7. Paleocurrent rose-diagrams from alluvial conglomerates
show flow direction generaly towards NW. However, the block
might be moderate CCW rotated in the Pliocene.

MIOCENE BRAIDED RIVER AND LACUSTRINE SEDIMENTATION IN PANNONIAN BASIN 381

Fig. 8. Block-diagram shows facies model for an early Ottnangian
pebbly braided river at Kalnik Mt. A relatively abrupt transition
from the flood plain to the channel belt is marked. Facies code ex-
planation see in the text.

sion of fine-grained deposits. The flood plain deposits have a
high preservation potential in conditions of free movement of
the  river  channel  belt  because  of  lack  of  valley  walls  which
would restrict this process (Friend 1978). Shifting of the chan-
nel belt could also be restricted by vegetation. However, traces
of  vegetation  are  very  uncommon,  indicating  unfavourable,
semi-arid conditions. The same conditions are indicated by the
appearance of carbonate nodules in the siltstones of facies F2
and lack of coal beds, which are common in alluvial succes-
sions in regions with a humid climate. The flood plain deposits
of Facies association A is unusually thick. Therefore, the suc-
cession  and  preservation  of  the  flood  plain  deposits  in  the
braided  river  system,  following  an  unrestricted  valley,  could
be attributed to the rapid subsidence of the basin by tectonic
influence, when the possibility of erosion of fine-grained de-
posits is minimal (see Miall 1996).

Facies association B was mainly deposited in a high-energy

environment. Pebbly facies, especially river bars (Gc and Gp),
was deposited by active flow (Fig. 8). Variable grain sizes,
from silt-sized particles to cobbles 26 cm in diameter, indicate
changes in flow velocities, while the frequency of oscillations
suggest pulsating character of the currents, which might be at-
tributed to seasonal events.

Vertical tendency – cycles and megacycles

In the lower part of the succession, both fining-upward or

coarsening-upward cycles are found. Fining-upward cycles are
dominant in the middle and upper part of the succession,
where coarsening-upward cycles may occur seldom (Fig. 6).

The fining-upward cycles have an erosional base and show a

gradual decrease in average grain-size, associated with a thin-
ning of the conglomerate beds. In the middle part of the Lower
unit succession (Fig. 6), in Facies association A, the erosional
surface is overlain by conglomerates (Ge) and sandstones
(Sh), which are interpreted as crevasse channel deposits. They
are followed by flood plain siltstones (F2 and F1), including
thin crevasse splay sandstones (Se). In the upper part of the se-
quence, in Facies association B, the basal parts of the fining-
upward cycles are composed of channel lag and bar conglom-
erates (Gc and Gp) overlying erosional surfaces (Fig. 6). They

are covered by shallow channel conglomerates (Ge) and bar
blanket conglomerates (Sh) or modified siltstones of the flood-
ing plain (F2).

Coarsening-upward cycles are characterized by gradual in-

crease of clast lengths in bar conglomerates (Fig. 6, 56—59 m
interval) or in a sharp transition from bar blanket sandstones to
bar conglomerates (Fig. 6, from 59 m to the top).

In the lower and middle part of the Lower unit (Facies asso-

ciation A), a gradual coarsening-upward trend in clast size is
recognized (Fig. 6, from the base of the section to 45 m). This
trend is indicated by increasingly common occurrences of con-
glomerate and sandstone facies with respect to siltstone facies.
Therefore, deposits of the Facies association A may be regard-
ed as a single coarsening-upward megacycle.

The conglomerates with erosional bases of fining-upward

cycles in Facies association A indicate a sudden beginning of a
deposition under high-energy conditions, while the fining-up-
ward trend indicates gradual decrease of the flow velocity. The
interpretation  of  the  conglomerates  as  crevasse  channel  fills,
and their vertical transition into flood plain siltstones suggests
that deposition might have been generated by floods. The pre-
dominance of flood plain deposits in Facies association A in-
dicates  intense  flood  plain  development  by  input  of  fine-
grained material by weak floods. The coarse-grained material
was deposited only occasionally, during strong floods, when
crevasse  channels  were  formed,  in  which  pebble-sized  sedi-
ments were deposited, overlain by sandstones. Isolated layers
of  sandstone  within  the  siltstone  suggest  sporadic,  relatively
weak floods, resulting in the formation of crevasse splays. The
interpretation  of  these  cycles  as  the  consequence  of  flood
events  suggests  independence  of  external  influences,  that  is
the influence of autocyclic processes in the flood plain.

The erosional base and composition of the fining-upward

cycles  in  Facies  association  B  also  indicate  abrupt  onset  of
depositional  events  followed  by  gradual  weakening  of  flow
velocities. The vertical stacking pattern of the cycles suggests
repetition  of  events.  The  facies  sequences  indicate  vertical
aggradation  of  bars  and  formation  of  macroforms.  As  bars
grow vertically, flow velocity decreases, resulting in deposi-
tion of fine-grained sediments, mostly sands forming bar blan-
kets.  Some  sandstone  units  are  intensely  burrowed  by  small
mammals at the top, suggesting subaerial emergence. Only in
one  example  was  bar  aggradation  concluded  by  flood  plain
siltstones, which may be explained by lateral channel migra-
tion. The maximal depths of braided channels were mostly 0.5
to 1 m, but in some cases more than 2.5 m. The fining-upward
cycles of Facies association B indicate dominance of autocy-
clic processes in formation of the upper part of the alluvial de-
posits.

The coarsening-upward cycles in Facies association B indi-

cate increasing water energy. The composition of the lower cy-
cle (Fig. 6, 56—59 m) indicate vertical aggradation of bars
comprising progressively longer clasts, probably due to the in-
creasing flow velocities. The uppermost cycle is composed of
deposits of a different facies, as the result of a specific se-
quence of depositional events (Fig. 6). The sandstones in the
cycle base, interpreted as sediments of bar blankets, were de-
posited during water lowstands, and their thickness (1.8 m) in-
dicate long periods of deposition by relatively weak flows, as

382 PAVELIĆ et al.

a consequence of the lateral migration of the channel. Biotur-
bation recorded in the uppermost part of this unit might indi-
cate long-lasting dry conditions, perhaps associated with aban-
donment of the channel. Shallow channel conglomerates in the
middle part of the cycle reflect a new flow and deposition of
coarse-grained material under higher energy conditions. The
formation of a transverse pebble bar in the upper part of the
cycle indicates very high energy and deposition in the active
channel. The topmost coarsening-upward unit may have
formed by gradual filling of a temporarily abandoned channel,
as reactivation of abandoned channels is a process typical for
braided river systems (cf. Costello & Walker 1972). Such cy-
cles thus most likely represent autocyclic processes in the allu-
vial environments.

The coarsening-upward megacycle in Facies association A

(Fig. 6, from bottom to 45 m) reflects increasing occurrences
of  sediments  deposited  in  progressively  higher  energy  envi-
ronments. This trend represents a consequence of increasingly
common  formation  of  crevasse  channels  and  splays  in  the
flood  plain,  due  to  stronger  floods.  The  upwards  increase  in
grain  size  could  be  attributed  to  the  approach  of  the  active
channel belt, due to progradation of the alluvial system.

The upper boundary of the megacycle (Fig. 6, 45 m level) is

characterized by an abrupt transition from the flood plain (Fa-
cies association A) to the channel belt (Facies association B)
(Fig. 8). The Upper unit deposits are generally interpreted as
deposits of pebbly braided river, and the migration of the chan-
nel belt may be explained in two ways. Relatively rapid tec-
tonic subsidence probably caused preservation of a thick flood
plain succession, and tectonic activity may have forced chan-
nels into more rapidly subsiding areas of the flood plain (cf.
Bridge & Leeder 1979).

Upper unit of fresh-water deposits

Description and interpretation of facies

The upper unit deposits were investigated in an 88 m thick

section (Fig. 6). They are dominated by marls (F3), while
sandstones (Sn) are subordinated. The succession shows a ver-
tical coarsening-upward trend, recognizable in the upward in-
crease in occurrence and gradual thickening of sandstone beds.

Facies Sn – normally graded sandstones

The sandstones are most common in the upper part of the

Upper unit, while they are rare in the lower part (Fig. 6). They
alternate with marls of facies F3 (Fig. 6). The sandstones ap-
pear as interbeds 1—20 cm thick, occasionally up to 60 cm
thick, with obvious trend of bed thickening towards the upper
part of the succession (Fig. 6). The bases of the sandstone beds
are erosional or flat. The sandstones are mostly fine-grained,
rarely medium-grained, with good to very good sorting. Thin-
ner beds (<20 cm) show millimetre scale horizontal lamination
in the lower part and normal grading. In some beds marl intra-
clasts are found. Thicker beds (>20 cm) exhibit no lamination,
and appear massive. The sandstones contain tiny fragments of
carbonized plant remains.

The weakly erosional bed bases indicate deposition from

low velocity flows, while the grain-size and horizontal lamina-
tion in the thicker beds suggest deposition from sandy high-
concentrated  turbidity  currents  (Lowe  1982).  Parts  of  beds
without lamination were also probably deposited from turbidi-
ty currents. Normal grading indicates gradual waning of flow.
The beds may have been deposited directly out of suspension
from  sandy  high-concentrated  turbidity  currents,  where  trac-
tion  mechanisms  are  suppressed  due  to  the  fast  deposition
(Lowe  1982).  Massive  and  relatively  thick  sandstones  may
also have been deposited from “quasi-steady currents” which
do not deposit sediment en masse but continue to flow, while
sediment  aggrades  (Kneller  &  Branney  1995).  The  massive
structure of the thicker beds suggests deposition from under-
flows (hyperpycnal flows), formed when river water contain-
ing large quantities of material due to the difference in density
and water temperature descend below the lake water-level and
deposit material at the bottom (Talbot & Allen 1996). Sand-
stones  containing  marl  intraclasts  indicate  an  erosive  system
and/or relatively close source. They could represent sediments
also deposited from such “quasi-steady currents”.

Facies F3 – horizontally laminated marls

Marl is the prevailing facies in the Upper unit. Marls show a

gradual tendency of decreasing occurrence towards the upper
parts of the succession and are interlayered with facies Sn
(Fig. 6). The marl units are from 3 cm to 8 m thick, generally
thinning upward in the succession. The marl beds have flat
bases. Marls are rarely very silty, show millimeter-scale hori-
zontal lamination and are grey to dark-grey.

Marls commonly contain fragments of partially carbonized

continental plants, but also completely preserved leaves, most-
ly concentrated in some laminae. The marls contain sporomor-
phs and fresh-water algae throughout all the succession (Fig.
9). The sporomorphs include those of subtropical ferns. Leiot-
riletes  cf.  wolffi  W.  Krutzsch  1962  is  typical  of  Ottnangian
coal-bearing  sediments.  Polypodiaceoisporites  cyclocingula-
tus W. Krutzsch 1967, Polypodiaceoisporites schoenewalden-
sis W. Krutzsch 1967 and Polypodiaceoisporites corrutoratus
Nagy 1985 (Planderová 1990) occur in Lower and Middle Mi-
ocene deposits. The pollen Pterocaryapollenites stellatus (R.
Potonie 1931) Thiergart 1937, forms minor and media also oc-
curs, and is characteristic of the Early Miocene, while individ-
ual specimens of Pterocaryapollenites stellatus f. media indi-
cate  the  Ottnangian  (Planderová  1990).  Tricolpate  pollen
(probably  Quercuspollenites)  has  also  been  determined.  The
coniferae are represented by the somewhat more common pol-
len  Pinuspollenites  type  Haploxylon.  This  type  of  pollen  is
most common in Lower Miocene deposits (Planderová 1978).
The green alga Botryococcus braunii Kützing 1849 is typical
of temperate to tropical fresh-water environments throughout
the  Tertiary,  but  is  more  common  in  deposits  of  Ottnangian
and  Upper  Badenian—Sarmatian  (Planderová  1990).  The  pa-
lynofacies composition is characterized by a dominance of vit-
rinite, accompanied by more or less amorphous liptinite. The
palynological  assemblage  clearly  indicates  fresh-water  envi-
ronments,  and  suggests  the  Ottnangian  MF-4  microfloristic
zone (Planderová 1990).

384 PAVELIĆ et al.

The facies commonly contains dispersed, monotypic assem-

blages of fresh-water molluscs Pisidium sp. with both shells
preserved. Analysis of calcareous nannoplankton indicated
complete lack of the Miocene species.

The marls were deposited in very quiet conditions, as indi-

cated by grain-size and small and thin shells of molluscs. The
horizontal lamination and molluscs paleoecology indicate
fresh-water lacustrine environment, while the dark colour, re-
mains of terrestrial plants and lack of bioturbation indicate fre-
quent anoxic conditions and deposition below thermocline, in
a relatively deep lake. This interpretation is supported by inter-
layering with sandstones deposited by gravity flows (facies
Sn).

The palynological assemblage indicate deposition in a fresh-

water basin fed by rivers, which transported fragments of ter-
restrial plants, sporomorphs of ferns and higher plants inhabit-
ing the river banks, but also with eolian input of coniferae
pollen from somewhat higher environments. The prevalence
of vitrinite over amorphous liptinite suggests dominating an-
oxic conditions at the lake floor.

Facies association

The facies association is composed of marls (facies F3) with

sandstones beds (facies Sn) (Fig. 6). The upper part of the Up-
per unit exhibits a general thickening and coarsening-upward
trend, indicated by increasing number and thickness of sand-
stone beds, accompanied by thinning of the marl beds (Fig. 6).

The marls (facies F3) of the Upper unit were deposited in a

fresh-water  lake,  mainly  in  the  basinal  part  (Murphy  &
Wilkinson 1980) (Fig. 10). The lake was relatively deep, as in-
dicated by lack of bioturbation and probably of carbonized re-
mains  of  continental  plants  caused  by  anoxic  conditions.
Abundant continental plants in lacustrine deposits indicate hu-
mid climate during deposition. The transition from alluvial to
deep-water  lacustrine  deposition  may  reflect  onset  of  a  new
tectonic phase in the extensional basin.

Gravity flows which transported terrigenous material into

lake, could have been generated by resedimentation of former-
ly deposited unconsolidated material in the delta front, or the
material was deposited from underflows directly from a river
during floods on the land. Sandstones (facies Sn) and marls
(facies F3) indicate alternation of short-lived periods charac-
terized by input of terrigenous material by gravitational flows
with long-lived periods of quiet basinal deposition typical for
a prodelta (Fig. 10). The coarsening and thickening up nature
indicate constant progradation of the prodelta.

The further Miocene depositional sequence is unclear in

Kalnik Mt. Lower Miocene fresh-water deposits are transgres-
sively overlain by Badenian deposits (Fig. 5) (Šimunić et al.
1981, 1994). Karpatian marine deposits may, however, also be
present, implying that the lacustrine phase was succeeded by
reestablishment of marine deposition already by the end of the
Early Miocene (Hećimović 1995). This event is the conse-
quence of short-lasting marine transgression, which in the
Karpatian affected wide areas of the Pannonian Basin System
due to the opening of a Paratethyan seaway to the Mediterra-
nean along the middle Slovenian corridor (Rögl & Steininger
1983). In the area of neighbouring Medvednica Mt this marine

transgression is explained by subsidence due to tectonism
(Pavelić et al. 2000).

Correlation of Ottnangian fresh-water deposits

between Sava and Drava rivers

The succession of Ottnangian fresh-water deposits at the

Kalnik Mt can be lithostratigraphically correlated with similar
rocks described from hills between the Sava and Drava rivers
in the Early Miocene North Croatian Basin (Fig. 2) (Pavelić
1998, 2001). The presence of alluvial deposits and their transi-
tion into lacustrine deposits at the Medvednica Mt was recog-
nized  by  Basch  (1983).  The  alluvial  part  of  these  sediments
was  deposited  in  a  pebbly  braided  river  flowing  towards  the
N-NE  and  E,  in  a  semi-arid  climate  (Pavelić  et  al.  1995).
Lacustrine deposits at the Medvednica Mt were laid down in a
hydrologically  open  lake,  formed  during  a  humid  period
(Pavelić 1998). Deposition was regressive with development
of  coarse-grained  fan  delta  close  to  the  end  of  the  lacustrine
phase, under the influence of synsedimentary tectonics. At the
Moslavačka  Mt  marsh  and  lacustrine  deposits  contain  rem-
nants  of  Dinotherium  bavaricum  Kaup  (Fig.  2)  (Krizmanić
1995). At the Psunj and Papuk Mountains a transition from al-
luvial  to  lacustrine  environments  was  also  documented,  and
deposition was connected with tectonic activity (Jamičić et al.
1987). The deposits at Papuk Mt were laid down in a hydro-
logically open lake in humid climate (Pavelić et al. 1998). Al-
luvial and lacustrine deposition at the Požeška Mt were influ-
enced by synsedimentary tectonics (Šparica & Buzaljko 1984;
Pavelić 1988). Alluvial deposition took place in braided allu-
vial fans in a semi-arid climate, and transport was generally to-
wards the north (Pavelić & Kovačić 1999). The lacustrine en-
vironments at Požeška Mt developed during a period of humid
climate (Pavelić 1998, 2001). Deposition at all these localities

Fig. 10. Block-diagram shows facies model for a lake at Kalnik Mt
which evolved over pebbly braided river sediments. Prodelta pro-
gradation was a characteristic for the late Ottnangian. Facies code
explanation see in the text.

MIOCENE BRAIDED RIVER AND LACUSTRINE SEDIMENTATION IN PANNONIAN BASIN 385

was terminated in the Karpatian by establishment of marine
environments (Basch 1983; Jamičić et al. 1987; Šparica &
Buzaljko 1984; Pavelić 1998, 2001).

The Ottnangian succession was thus characterized by a tran-

sition from alluvial to lacustrine environments in the wide area
between the Sava and Drava rivers. The climatic conditions
were similar at all localities. During the alluvial phase the cli-
mate was semi-arid, while humid conditions characterized the
period of lacustrine deposition. The alluvial deposits generally
show overall transport towards the north, but sometimes vary-
ing from NW to E. Terrestrial deposition was terminated at all
localities by marine transgression in the Karpatian. Sedimenta-
tion was strongly influenced by tectonic activity throughout
the Early Miocene (Pavelić 2001).

The succession from alluvial to lacustrine environments in

the  whole  region  suggests  a  regional  character  of  this  event,
indicated by similar climatic changes and relatively similar al-
luvial transport directions. The hydrologically open nature of
the lakes suggests that they were connected (Kochansky-Dev-
idé  &  Slišković  1978),  or  perhaps  there  was  one,  very  large
lake (Pavelić 1998, 2001). During the Ottnangian fresh-water
environments existed in the area between the Sava and Drava
rivers, while penecontemporaneous deposition in the area west
of the Kalnik Mt took place in marine environments (Fig. 2)
(review in Šimunić 1992). Evolution of the depositional area
west of the Kalnik Mt started already in the Egerian, and was a
part of the seaway between the Mediterranean and the Central
Paratethys. This succession can be correlated with the young-
est part of the Slovenian Oligocene Basin fill (Fig. 2) (sensu
Jelen et al. 1992). The succession of Kalnik Mt comprises the
oldest  Miocene  deposits,  which  have  not  been  found  east  of
this mountain. The Ottnangian fresh-water deposits lacking in
the Miocene basin west of Kalnik Mt are characteristic of the
SE situated area. It might be concluded that this mountain rep-
resented a boundary zone between two basins in the Early Mi-
ocene: the Slovenian Oligocene Basin with marine sedimenta-
tion  untill  the  Early  Miocene  on  the  NW  and  the  Early
Miocene  North  Croatian  Basin,  characterized  by  fresh  water
sedimentation during the Ottnangian on the SE (Fig. 2).

Conclusion

Ottnangian fresh-water deposition in the area of Kalnik Mt

took place in alluvial and lacustrine environments following
each other.

1) In the early (alluvial) phase pebbly braided rivers devel-

oped, represented mainly by bar conglomerates and flood
plain siltstones. Paleocurrent data indicate transport towards
the north-west, and the climate was semi-arid. Alluvial deposi-
tion was controlled by both autocyclic and allocyclic process-
es.

2) The alluvial phase was followed by the lacustrine phase

later, mostly represented by marls and occasional coarser ma-
terial deposited from sediment gravity flows, mainly turbidity
currents. The end of the lacustrine phase, which was character-
ized by a humid climate, was characterized by prevalence of
sand deposition, in a prograding deltaic environment.

3) Synsedimentary tectonics generated subsidence of the ba-

sin, which resulted in formation and preservation of a thick

succession of flood plain deposits in the pebbly braided river
environment. Tectonic activity constantly caused lateral mi-
gration of the channel belt or progradation of the alluvial sys-
tem and was succeeded by formation of a deep lake. Prodelta
progradation took place during the period of reduced intensity
of tectonic activity by the end of the lacustrine phase.

4) The succession of fresh-water deposits, climatic condi-

tions and influence of the synsedimentary tectonics of the
Kalnik Mt area are very similar to the evolution of fresh-water
environments at the Medvednica, Psunj, Papuk and Požeška
Mts.

5) The Kalnik Mountain area represents a boundary zone

between two contemporaneous basins during the Ottnangian.
One was located in the area north-west of Kalnik Mt, and was
characterized by marine deposition. It can be correlated with
the younger part of the sedimentary succession of the Slove-
nian Oligocene Basin. In the same period, the second, North
Croatian Basin was located south-eastward of Kalnik Mt, and
was characterized by deposition in fresh-water environments.

Acknowledgments: This  paper  represents  a  part  of  the  PhD
Thesis of Davor Pavelić, which was undertaken under the su-
pervision of Jožica Zupanić (Zagreb), to which the authors are
very grateful for very useful comments and suggestions. The
review  of  the  manuscript  by  Finn  Surlyk  (Copenhagen)  and
Fritz F. Steininger (Senckenberg) is gratefully acknowledged.
We  are  indebted  to  Josip  Benić,  Ivan  Hećimović  and  Georg
Koch (Zagreb) for providing helpful suggestions. This paper
profited greately from reviews by M. Kováč (Bratislava), O.
Sztanó  (Budapest),  I.  Baráth  (Bratislava)  and  an  anonymous
reviewer. These investigations represent a part of the project:
Geological Map of the Republic of Croatia, scale 1:50,000, fi-
nanced by the Ministry of Science and Technology of the Re-
public of Croatia.

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