GeolCarp_Vol66_No4_331

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, AUGUST 2015, 66, 4, 331—344 doi: 10.1515/geoca-2015-0029

Turbidites as indicators of paleotopography, Upper Miocene

Lake Pannon, Western Mecsek Mountains (Hungary)

ORSOLYA SZTANÓ

, KRISZTINA SEBE

, GÁBOR CSILLAG

and IMRE MAGYAR

4,5

Eötvös Loránd University, Department of Physical and Applied Geology, Pázmány Péter sétány 1/c, 1117 Budapest, Hungary;

sztano@caesar.elte.hu

University of Pécs, Department of Geology and Meteorology, Ifjúság ut. 6, 7624 Pécs, Hungary;

sebe@gamma.ttk.pte.hu

Geological and Geophysical Institute of Hungary, Stefánia ut. 14, 1143 Budapest, Hungary; csillag.gabor@mfgi.hu

MOL Hungarian Oil and Gas Plc., Október huszonharmadika ut. 18, 1117 Budapest, Hungary; immagyar@mol.hu

MTA-MTM-ELTE Research Group for Paleontology, P.O. Box 137, 1431 Budapest, Hungary

(Manuscript received October 31, 2014; accepted in revised form June 23, 2015)

Abstract: The floor of Lake Pannon covering the Pannonian Basin in the Late Miocene had considerable relief, including
both deep sub-basins, like the Drava Basin, and basement highs, like the Mecsek Mts, in close proximity. The several km
thick lacustrine succession in the Drava Basin includes profundal marls, basin-center turbidites, overlain by shales of
basin-margin slopes, coarsening-upward deltaic successions and alluvial deposits. Along the margin of the Mecsek Mts.
locally derived shoreface sands and deltaic deposits from further away have been mapped so far on the surface. Recent field
studies at the transition between the two areas revealed a succession that does not fit into either of these environments.
A series of sandstone a few meters thick occurs above laminated to bioturbated clayey siltstone. The sandstone show
normal grading, plane lamination, flat erosional surfaces, soft-sediment deformations (load and water-escape structures)
and sharp-based beds with small reverse faults and folds. These indicate rapid deposition from turbidity currents and their
deformation as slumps on an inclined surface. These beds are far too thick and may reveal much larger volumes of mass
wasting than is expected on the 20—30 m high delta slopes; however, regional seismic lines also exclude outcropping of
deep-basin turbidites. We suggest that slopes with transitional size (less than 100 m high) may have developed on the flank
of the Mecsek as a consequence of lake-level rise. Although these slopes were smaller than the usually several hundred
meter high clinoforms in the deep basins, they could still provide large enough inertia for gravity flows. This interpretation
is supported by the occurrence of sublittoral mollusc assemblages in the vicinity, indicating several tens of meters of water
depth. Fossils suggest that sedimentation in this area started about 8 Ma ago.

Key words: Late Miocene, Lake Pannon, Mecsek, turbidites, slope, synsedimentary folds, soft-sediment deformation.

Introduction

Sandy  turbidites  are  commonly  found  in  bathyal  water
depths,  namely  below  200 m.  On  the  other  hand,  flume  ex-
periments  showed  that  development  of  turbidity  currents  is
independent of water depth, as they are generated by density
difference between the sediment laden current and the ambi-
ent fluid, therefore the only rule of thumb is that they form
below  the  storm  wave  base  (Walker  1984).  Mud  is  usually
transported over the shelf. It has been fairly commonly stated
that prodelta sediments on shelves contain turbidites. Cases,
however, where thick sand was deposited from gravity flows
are far less common. Evidence from several prodelta to shelf
regions  proves  that  either  storm-generated,  wave-supported
turbidity flows (Nelson 1982; Fenton & Wilson 1985; Myrow
et al. 2002; Traykovski et al. 2007) or hyperpycnal flows re-
lated  to  highly  concentrated  river  plumes  during  floods
(Mulder et al. 2003; Pattison 2005; Lamb et al. 2008) are ef-
ficient  enough  in  sediment  transport.  The  former  being  of
short duration produces rather thin, while the prolonged hy-
perpycnal flows may deposit relatively thick beds.
A  more  favourable  situation  for  the  deposition  of  thick  tur-
bidite  sands  in  the  prodelta  is  when  the  delta  slope  and  the
basin  margin  slope  are  united,  as  in  the  case  of  shelf-edge
deltas.  Depending  on  a  complex  set  of  external  factors  a

wedge-shaped sandy turbiditic delta front can develop with-
out reaching the base of slope or without generating major
fans (Plink-Björklund & Steel 2005).

Both deltaic sediments and turbidites are common in the

Upper  Miocene  lacustrine  successions  (sediments  of  Lake
Pannon)  in  the  Pannonian  Basin  (Bérczi  &  Phillips  1985;
Juhász  1991;  Lucic  et  al.  2001;  Pavelić  2001;  Saftic  et  al.
2003; Krézsek & Filipescu 2005; Vrbanac et al. 2010). Del-
taic  deposits  have  been  studied  both  from  cores  (Juhász
1992; Korpás-Hódi 1998) and outcrops (Sztanó et al. 2013a).
Their architecture was imaged by high-resolution seismic sur-
veys (Horváth et al. 2010), unveiling only few tens of meters
high delta slopes, which correspond to funnel-shaped portions
mostly in gamma and/or SP well-logs, interpreted as coarsen-
ing upwards mouth bar (Juhász & Magyar 1992; Juhász 1994)
or  deltaic  units  (Sztanó  et  al.  2013a).  Mudstones  with  only
centimeter-scale  fine  sandy  interbeddings  with  current  and
wave  ripple  laminations  and  shell  lags  have  been  reported
from the few meters deep prodelta regions. They were formed
mainly by storms (Sztanó et al. 2013a).

Although turbidites are exposed on the surface in uplifted

peripheral subbasins of the Pannonian basin system, such as
the Transylvanian Basin (Krézsek & Filipescu 2005; Sztanó et
al. 2005; Tőkés et al. 2013) or the Zagorje Basin (Kovačić et
al. 2004), from the central basins they have been reported only

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from  cores,  coming  from  positions  several  km  deep  in  the
basin interiors, where molluscs also point to profundal con-
ditions (Juhász & Magyar 1992). Turbidites in the deep basin
interiors are related either to the confined basin-center accu-
mulations up to a thickness of 1000 m (Juhász 1992, 1994) or
to  the  base-of-slope  turbidite  systems  (Sztanó  et  al.  2013b;
Bada et al. 2014). The 400—600 m (occasionally 1000 m) high
and 8—10 km long basin margin slopes, bridging the morpho-
logical shelf and the deep basins, served as pathways of mass
gravity flows feeding both types of turbidite systems (Pogác-
sás  1984;  Magyar  2010;  Magyar  et  al.  2013).  The  principal
difference between the two types of turbidite systems is in the
areal  extent  of  the  major  sand  bodies  and  in  their  thickness,
the  latter  being  in  the  range  of  hundred  meters  in  deep  con-
fined  basin  centers  or  few  tens  of  meters  in  the  unconfined
base-of-slope  systems  (Sztanó  et  al.  2015).  The  individual
beds  depending  on  their  position  in  the  turbidite  systems  are
thin-bedded silty to very fine-grained sandy, or medium-bed-
ded graded turbidites of well-developed Bouma sequences in-
tercalated  with  shales  or  several  meter  thick  amalgamated,
commonly massive beds with various soft-sediment deforma-
tion structures (Sztanó et al. 2013b). In addition, features re-
lated  to  large  slumps  are  common  in  the  base-of-slope
turbidite systems (Bada et al. 2014).

Recent field studies in the SW part of the Pannonian Basin

revealed unusual lacustrine sediments on the surface, which

Fig. 1.  a  –  Simplified  paleogeographic  sketch  of  Lake  Pannon
within the Pannonian Basin about 6.8 Ma ago (drawn after Magyar
et  al.  1999).  The  north-western  part  of  the  basin  had  already  been
filled  up  with  sediments.  Slope  and  overlying  deltaic  sediments
were accumulating in the study area; b – Present-day depth of the
pre-Neogene basement shows the major depocenters which accumu-
lated  the  most  complete  lacustrine  sedimentary  successions  from
profundal  marls  to  alluvial  deposits  up  to  a  thickness  of  several
kilometers during the Late Miocene. Most of the present-day hilly
areas were parts of basement highs which got flooded only during
the  late  Late  Miocene,  and  hosted  only  some  hundred  meters
thick,  mostly  relatively  shallow-water  lacustrine  sediments.  Delta
progradation over these elevated areas is proven by sediment trans-
port directions among others. Shelf edge positions after Magyar et
al. 2013.

show characteristics of turbidites. Formerly these were
mapped as nearshore lacustrine deposits (Chikán & Budai
2005), although the occurrence of sublittoral mollusc faunas
in the vicinity indicated that deeper environments did develop
in the region. The aim of this paper is to document and inter-
pret the depositional environment of this locality and to inte-
grate it into its geological surroundings.

Geological setting

Sediments of the Upper Miocene Lake Pannon (Fig. 1)

comprise  the  major  part  of  the  basin-fill  succession  in  the
Pannonian Basin, a classic back-arc basin shaped by several
low-angle  normal  and  strike-slip  faults  (Horváth  &  Royden
1981; Horváth & Tari 1999). In this way a fairly complicated
topography had resulted by the end of the Middle Miocene,
and  it  partly  evolved  further  during  the  Late  Miocene  post-
rift  and  intervening  inversion  events  (Horváth  &  Cloething
1996). Due to these differential vertical movements the lake
floor had considerable relief, including both deep sub-basins
and elevated basement highs in close proximity.

Development of the deep basins reflected by their sedi-

mentary fills follows a uniform pattern, only their initial re-
lief (depth) and local rates of subsidence may have been
different. This succession includes profundal marls (Endrőd

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Formation),  basin-center  turbidites  (Szolnok  Formation),
slope shales (Algyő Formation), stacked deltaic successions
(Újfalu  Formation)  and  finally  alluvial  deposits  (Zagyva
Formation), and reflect gradual fill-up of these basins due to
high  sediment  supply  from  Alpine-Carpathian  source  areas
(Fig. 1) (Magyar et al. 2013). This basin-type succession was
also described from the Drava Basin (Fig. 2) near the study
area (Saftić et al. 2003).

In contrast, the basement highs, some emerging above wa-

ter level as islands or peninsulas during the early Late Mio-
cene, may have become inundated – partly or fully – only
later,  at  varying  time  points.  These  areas  were  marked  by
shoreface or deltaic deposits of local origin, usually overlain
by shales (Csillag et al. 2010). The fauna of these shales may
point to water depths of either less than 100 m (Cziczer et al.
2009) or few hundred meters (Magyar et al. 2004) showing
great spatial variations. As the deltaic to alluvial feeder sys-
tems  from  remote  Alpine-Carpathian  source  areas  reached
these  locations,  the  depositional  environment  changed,  de-
pending  on  the  water  depth  of  the  given  location.  If  water
depth reached a few hundred meters, slope shales and related
thin  turbidite  accumulations  may  have  followed,  but  these
areas  lack  thick  accumulations  of  both  profundal  marls  and
turbidites.  If  water  depth  was  shallow,  deltaic  successions
followed  without  underlying  slope  deposits  (Sztanó  et  al.
2013a).  This  latter  situation  was  perfectly  visualized  by
high-resolution  seismic  profiles  acquired  on  Lake  Balaton
(Sacchi et al. 1999; Horváth et al. 2010). Since the Pliocene
these  basement  highs  have  been  uplifted  and  partly  eroded
(Horváth  &  Cloetingh  1996;  Konrád  &  Sebe  2010),  there-
fore  in  their  vicinity  various  Upper  Miocene  lacustrine  and
older Neogene sedimentary units are exposed today (Fig. 3).

The study area west of the Mecsek Mts (Fig. 1) is transi-

tional  between  the  Mecsek,  an  emergent  Paleo-Mesozoic
basement  unit,  and  the  Drava  Basin,  where  Neogene  sedi-
mentary units up to 6 km thick cover the basement (Fig. 2).
The  Drava  Basin  is  a  well-known  hydrocarbon  prospecting
area,  where  the  lacustrine  basin-center  turbidites  and  the
overlying stacked deltaic successions could form good reser-
voirs  (Lučić  et  al.  2001;  Saftić  et  al.  2003;  Vrbanac  et  al.
2010). The architecture of the basin-fill successions, their se-
quence stratigraphy and structural features have been widely
studied (Ujszászi & Vakarcs 1993; Sacchi et al. 1999), thus
it is accepted that differential uplift of the Mecsek and large-
scale regional folding at about the Miocene/Pliocene bound-
ary  resulted  in  a  regionally  important  unconformity.  The
turbidites  of  the  Drava  basin  have  recently  been  studied  by
Uhrin & Sztanó (2011), but no detailed core analysis is pub-
licly available.

In addition to parts of the Neogene cover, Permo-Triassic

sediments, rhyolites and Carboniferous granites also crop out
along the western margin of the Mecsek (Fig. 3; Chikán  &
Budai 2005). These are overlain by Lower to Middle  Mio-
cene  sediments  dominated  by  siliciclastics,  which  occa-
sionally  acted  as  the  source  of  the  locally  derived  Late
Miocene lacustrine transgressive shoreface sands (Kálla For-
mation, Kleb 1973). Both of these Miocene sediment pack-
ages crop out in the vicinity of the Paleo-Mesozoic basement
rocks. Further from the mountains lacustrine deltaic deposits
have been mapped on the surface (Kleb 1973; Chikán & Bu-
dai  2005).  These  used  to  be  called  the  Somló  Formation,
however,  according  to  the  recently  developed  lithostrati-
graphic  schemes  they  belong  to  the  Újfalu  Formation  as  a
member (Magyar 2010; Sztanó et al. 2013a). The speciality

Fig. 2. Seismic profile from the Drava Basin showing the typical basin fill succession: profundal marls (Endrőd Formation), basin-center
turbidites (Szolnok Formation), slope shales (Algyő Formation), stacked deltaic successions (Újfalu Formation) and alluvial deposits
(Zagyva Formation). For location of section see Fig. 11.

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of the western Mecsek area is that the locally derived trans-
gressive sands might be overlain directly by the regressive
ones originating from Alpine-Carpathian sources, without
shales in between.

The other local speciality is that the sediments of the

above  distant  provenances  are  mixed  with  some  locally  de-
rived material, indicating denudation and thus a subaerial or
shallow flooded position of the Mecsek (Thamó-Bozsó et al.
2014). At the westernmost margin of the surface occurrences
of  the  lacustrine  beds,  next  to  the  village  of  Szulimán,  an
abandoned  brickyard  exposes  a  succession  that  does  not  fit
into the shallow-water deltaic environment.

Sedimentology

The abandoned brickyard is located SE of the village

(46°7’24.47” N,  17°48’47.15” E).  It  is  approximately
100 m long and 6—7 m high, a large proportion of its surface
is covered by debris at present. It exposes Lake Pannon sedi-
ments unconformably overlain by about 2 m of red clay-domi-
nated  Quaternary  deposits.  The  Upper  Miocene  lacustrine
sediments comprise alternations of muddy and sandy facies
units.  Above  clay  to  clayey  silt  beds  a  4 m  thick  series  of
fine to very fine friable sandstone occurs, and is overlain by
thin beds of silts (Fig. 4).

Muddy facies unit – it consists of 0.1—0.4 m thick lami-

nated  to  fully  bioturbated  grey  clay,  clayey  silt,  silt  and
white calcareous marl to limestone layers with intercalations
of centimeter thick graded, graded-laminated very fine or fine-
grained sand beds. Some siltstone beds are fully bioturbated,
others contain very small simple vertical mud-filled burrows.
The maximum 0.1 m thick limestone beds show very strange
characters.  Laterally  these  may  interfinger  with  “ordinary”
siltstones. They also produced roundish boudins or ball struc-
tures  with  deformed  laminations  of  the  under-  and  overlying
clays.  In  thin  section  they  are  made  up  of  micrite,  no  fossils
other  than  some  unindentified  round  features,  which  may  be
reminiscent of algae, have been found (J. Haas ex verb.)

Sandy facies unit – it is made up of 0.2—0.3 m thick,

sharp-based,  fine  to  very  fine-grained  sandstone  beds  inter-
calated with mudstones less than 0.1 m thick. Cementation is
very poor, except for two thin limestone beds also appearing
between  sandstones.  Most  sandstones  do  not  show  any
scouring or sole marking. A few have an erosional base, but
their  relief  is  not  more  than  0.2 m  (Fig. 5).  Some  beds  are
massive, graded to silt with large ball and flame structures at
the bottom. Others are graded, massive to parallel-laminated
with  sharp  contact  towards  the  overlying  silts.  Very  thin
cross-lamination  may  also  occur  together  with  the  parallel
lamination. In the uppermost beds hummocky cross-lamina-
tion  or  rather  low  in-phase  wave  lamination  was  observed.

Fig. 3. Simplified geological map of the study area (based on Budai & Gyalog 2010), also showing the inferred surface extension of Upper
Miocene sediments.

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Synsedimentary deformations occur at three levels, in beds

C, F and G (Figs. 4, 6), in thicknesses below 0.1 m. Those in
bed C show a very regular geometry laterally extending for
several  meters.  The  other  two  are  more  irregular,  although
the same vergence of folds and faults is observed. These lat-
ter are both underlain by beds with water escape features. In
all cases the core of the deformed body is a silt or limestone
layer, but the overlying sand bed was also included in the de-
formation. Folded beds show a sharp base which served as a
detachment  surface;  above  they  are  sharply  overlain  by  un-
deformed  layers.  The  main  elements  in  the  deformed  beds
are  reverse  faults  soling  in  the  basal  detachments  plane.
Fault  spacing  is  variable:  more  regular  folds  developed
above faults separated by longer distances (Fig. 6a,b), while
more  closely  spaced  thrust  planes  co-occur  with  intensely
deformed,  elongated,  flame-like  layer  fragments  (Fig. 6c),
possibly a result of less cohesive material. Fold morphology
can be upright, inclined or overturned. Back folds commonly
occur  above  the  thrust  planes.  Along  fault  planes  material
can  be  dragged  into  even  nearly  isoclinal  folds  (e.g.  in
Fig. 6a, between 2

and 3

thrust planes). Thrust vergences

range WSW—WNW, pointing to an overall westerly trans-
port direction.

Interpretation

Depositional processes

The mudstones were formed by suspension settling in quiet

waters,  below  wave  base,  most  likely  even  below  storm
wave  base.  The  limestone  layers  are  rather  enigmatic.  Due
to  lack  of  any  evidence  of  subaerial  exposure  or  pedoge-
nesis interpretation as calcretes is excluded. Based on analogy
of  other  carbonates  of  the  lacustrine  succession  (Magyar  et
al. 2004; Cziczer et al. 2009) and the algae-like structures it
is speculated that these beds formed when carbonate mud ac-
cumulated  after  the  bloom  of  some  calcareous  algae  in  the
photic zone of the lacustrine water mass. Siltstones point to
increased  suspension  input  from  distal  sources.  The  thin
graded  sandstone  beds  were  formed  from  turbulent  flows.
These  may  have  been  diluted,  small-density  turbidity  cur-
rents, or storm-induced density flows.

Although the sandy facies unit is just a few meters thick

package,  its  unusual  sedimentological  character  among  the
outcropping  Lake  Pannon  deposits  in  the  Pannonian  Basin
makes it important. A combination of massive, graded, par-
allel-  and  cross-laminated  structures  point  to  waning  flows
with  decreasing  density  and  a  change  from  turbulence  to
traction. Therefore this association evokes Bouma sequences
(Tabd, Tbc, Tbc´; Fig. 5), and refers to deposition from high-
density  sandy  turbidity  currents.  Parallel  lamination  indi-
cates  that  the  currents  were  supercritical  (cf.  Southard  &
Boguchwal  1990)  during  deposition.  The  occurrence  of
hummocky  or  low  in-phase  wave  lamination  also  indicates
rapid  flows  of  somewhat  less  velocity  or  higher  thickness
(cf. Cheel 1990; Prave & Duke 1990). The predominance of
these rapid flow deposits can also be related to hyperpycnal
flows,  and  both  may  point  to  proximal-to-source  character.
Load  and  water-escape  structures  may  indicate  event-like

Fig. 4. Lithology log and sedimen-
tary structures of the Szulimán
outcrop. Characteristic beds of the
sandy facies are indicated by capi-
tals, locations of photos in Fig. 5
by lower-case letters.

Some beds contain cm-sized rip-up mud-clasts. Secondary
soft-sediment deformations are common: not only load balls
and flames, but also dish and pipe structures occur. The most
spectacular features are, however, synsedimentary folds and
related small reverse faults (Fig. 5).

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deposition  and  rapid  burial.  In  deep  basin  interiors  these
types of structure with the same thickness of individual beds
are reported from deep-water lobes (Bérczi & Phillips 1985;
Juhász  1994;  Sztanó  et  al.  2013b),  which  usually  comprise
sand-bodies  several  tens  of  meters  thick.  On  the  margin  of
these, not only the overall thickness of the lobe deposits, but
also  bed  thickness  decreases  drastically.  On  the  other  hand,
the closest occurrence of these deep-water turbidite systems in
the  Drava  Basin  is  in  a  distance  of  10 km  and  at  a  depth  of
over  1000 m.  This  environmental  interpretation  for  the  Szu-
limán  turbidites  can  be  excluded  based  on  its  facies  and  the
geological setting of the area. Another interesting turbidite lo-
cality is situated 150 km to the west in Croatia (Kovačić et al.
2004). The succession contains large channel-fills, levee and
lobe deposits clearly of deep basin origin, uplifted to the sur-
face by large reverse faults (Tomljenović & Csontos 2001), so
they  are  not  regarded  as  analogues  to  Szulimán  either.  The
topmost part of the succession, however, contains horizontally
to cross-laminated sands, according to the description similar
in facies to our locality. These strata were interpreted as “pe-
culiar type” mouth-bar deposits in extremely shallow waters.

In the uppermost strata of the Szulimán outcrop the low in-

phase wave lamination to combined wave ripple cross-lami-
nation  may  point  to  waning  flows  generated  by  storms  (cf.
Dott & Bourgeois 1982). This current type is rather common
on  flat-lying  surfaces  of  shelves  between  fair-weather  and
storm  wave  bases.  Except  for  the  synsedimentary  folds  and
faults, the other sedimentary structures do not contradict this
possibility.  Beds,  however,  are  relatively  thick,  shell  lags,
post-storm  bioturbation  or  post-storm  mudstones  are  miss-

ing, as well as well-developed wave-ripples, which were de-
scribed  elsewhere  in  the  lacustrine  setting  (Magyar  et  al.
2006).  Therefore  it  is  not  excluded  that  storm-induced  cur-
rents  may  have  influenced  deposition  (cf.  Myrow  et  al.
2002), but were not of primary importance.

However, the most important observation which helps the

interpretation is the presence of small-scale folds and faults
verging  consistently  in  the  same  direction,  which  indicates
an  inclined  depositional  surface.  Vergence  of  thrusts  and
folds is downslope, namely in a westerly direction. The de-
formation represented by the faults and folds is a result of in-
tense  shortening,  thus  the  deformed  beds  are  regarded  as
compressional  lower  parts  of  slumps.  The  above  described
structures resemble those displayed and interpreted in detail
by Alsop & Marco (2011) from the Dead Sea region, though
the  role  of  faults  is  much  higher  here.  Their  geometry  may
reflect  late  phase  slump  translation  and  slump  cessation,
where  non-coaxial  downslope  deformation  took  place.  The
dominance of incoherent folds (sensu Alsop & Marco (2013))
indicates  strong  deformation,  also  typical  of  the  lower  por-
tions of slumps.

Depositional environment

Based on the recognition of co-existent features of turbid-

ites and inclined topography, the following depositional set-
ting is suggested. Models of experimental turbidity currents
predict that in the case of large initial volumes, high-concen-
tration  flows  on  a  1—1.5°  slope  are  likely  to  deposit  such
sand  beds  near  the  base  of  the  slope,  after  3—20 km  long
transport  (Zeng  &  Lowe  1997).  Thus  the  succession  might
have been formed on a slope, but in water depths still shal-
low  enough  for  storm-induced  currents  to  play  some  role.

Fig. 6. Syn-sedimentary folds and faults, with increasingly inco-
herent folds from (a) to (c). Sediment movement happened from
left to right in all photos.

Fig. 7. Sketch of different slope types developed in Lake Pannon.
a – few 10 s of m high delta slope prograding on the lacustrine shelf
towards the basin slope of several hundred meter height; b – transi-
tional slope of shelf-edge deltas in shallow-water areas near elevated
basement highs. The two sections are parallel, and represent coeval
deposits in the Drava Basin and in the area west of the Mecsek Mts
shown as an island in the background. The foreland gently dips to-
wards the Drava Basin as well (compare with Fig. 11).

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Therefore  several  hundred  meter  high  basin-margin  slopes
leading to the Drava Basin are excluded. However, the beds
in  the  sandy  facies  are  far  too  thick  and  may  reveal  much
larger volumes of mass wasting than is expected in prodelta
of  20—30 m  high  delta  slopes  on  the  shelf  of  Lake  Pannon.
Consequently,  these  small  delta  slopes  are  not  regarded  as
the  loci  of  deposition  either.  Instead  it  is  speculated  that  a
third type of slope had evolved. As the study area is transi-
tional between the deep Drava Basin and the elevated Mec-
sek area, it is possible that here a water depth in the range of
only 100 m developed, therefore the slope of the prograding
shelf with shelf-edge deltaic feeder systems on top could not
be higher either (Fig. 7). If the usual slope angle of 1—2° was
maintained,  this  transitional  type  of  slope  may  have  been
about 3—5 km long. It was smaller in all directions than the
usual basin slope, but could still provide large enough inertia
for  gravity  flows,  ponding  on  the  shallow  western  foreland

of the Mecsek Mts. Gradually, but rather rapidly, the width
of the shallow-water area in front of the prograding slope de-
creased  and  was  exceeded  by  the  transport  distance  of  tur-
bidity  currents.  As  a  result,  sediments  bypassed  and  were
dumped  into  the  deep  basin  in  the  south-southwest;  this  in-
hibited  the  accumulation  of  thick  turbiditic  successions  in
these transitional areas.

Evolution of deltas and slopes in the region

The depositional model of the transitional slope is also

supported by available well data. Unfortunately no cores, but
gamma-ray, standard potential and resistivity curves and ar-
chive reports on cuttings and cores are available. A geologi-
cal section partly parallel to the progradational direction of
the shelf-slope system (cf. Fig. 1b) was constructed (Fig. 8;
from cluster A to well K-65). Coarsening up-units of 20 m

Fig. 8. Correlation panel through gamma-logs of shallow wells. The progradational parasequence set of cluster A is interpreted as delta pro-
gradation on the shelf-edge. The correlative set in cluster B is interpreted as deposits of the transitional slope, with thin turbiditic lobe de-
posits mimicking the shelf-edge progradation. Note that wells of cluster A to K-65 are in dip direction, while wells of cluster B are in strike
with respect to the supposed slope. The base of the progradational set was used as a datum, measured depth is shown along the wells.
GR logs are shaded according to facies, yellow is sand, green is shale.

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thickness are interpreted as deltaic lobes, based on analogous
successions  of  the  Balaton  region  (cf.  Sztanó  et  al.  2013a).
They can also be regarded as parasequences that comprise a
progradational  parasequence  set.  This  is  best  imaged  be-
tween  20—100 m  measured  depth  in  the  well  Szulimán  B-1
situated only a few hundred meters from the studied outcrop.
They are sharply overlain by “hot” shales locally producing
the highest count on the logs. In the following 4 km distance
the progradational parasequence set is still obvious in gradu-
ally  increasing  depth  (40—120 m  MD  of  Mozsgó  K-3  and
85—170 m MD of Mozsgó M-1). The coarsening/thickening-
up set, as well as the overlying thin shales are recognized on
logs of cluster B in an even larger measured depth, at about
300 m  in  Szigetvár  K-23,  56  and  K-60.  It  means  that  a  re-
gional,  post-sedimentary  tilt  of  2.5—3°  towards  the  S  must
have happened, which is reasonable if the depth of base Late
Miocene  or  the  position  of  the  pre-Neogene  is  considered
(Fig. 2) (Kőrössy 1989; Haas et al. 2010).

Delta front to mouth bar sandstones of the youngest

parasequences reached the area of Mozsgó-1, but those of the
oldest  ones  did  not  develop  here.  Instead  their  time-equiva-
lents, several thin sandstones appear in the lower part of the
parasequence set. Similar sandstones forming sheet-like bod-
ies  are  present  in  the  wells  of  cluster  B.  Their  areal  extent
may  have  attained  5 km,  while  their  thickness  is  less  than
10 m, usually 2—5 m only. It should be emphasized that this
portion of the section (Fig. 8) is parallel with the strike of the
slope. These sandstone sheets are interpreted as small series
of  turbidites  near  the  base  of  the  slope,  in  front  of  the  pro-
grading  shelf-edge  deltas.  The  mostly  upwards  increasing
thickness of the sandstone sheets, namely their thickening-up
stacking  pattern  mimics  the  progradational  character  of  the
deltaic parasequence set.

The overlying shales, widespread all over the study area

mark a regional flooding event. As a result of this transgres-
sion, the coastal region of Lake Pannon might have stepped
back  to  the  north,  so  far  that  for  a  short  interval  only  con-
densed deposition took place here. After some time a new pro-
grading  slope  of  about  the  same  ca.  80—100 m  height  might
have evolved, thus base of slope turbidite sheets could form
again.  The  studied  succession  at  Szulimán  might  be  part  of
this  system.  The  prograding  deltas  of  this  new  phase  might
have  reached  the  Szigetvár  area  when  sand  bodies  at  about
190—200 m MD in wells K-23 and K-56 were deposited.

Actually the turbidites at Szuliman indicate a transitional

slope, which did not develop just because of the marginal
position but also because the lake level rose. Unfortunately
no biostratigraphic evidence exists on the time span of these
major floodings and the related progradational parasequence
sets. However, based on regional studies of the aggradational
to progradational character of the shelf slope of Lake Pannon
(Sztanó et al. 2013b), it is supposed that they occurred regu-
larly in about 100 kyr intervals.

Fossils

The sediments of Lake Pannon often contain fossils of en-

demic molluscs, ostracods and dinoflagellates, providing a ba-

sis for the interpretation of the depositional environment and
age of the enclosing sediments (Magyar & Geary 2012). For
instance, distinct mollusc assemblages characterize the lit-
toral, sublittoral, and profundal zones of the lacustrine envi-
ronment, thus offering a tool to estimate the paleo-water depth
(Juhász & Magyar 1992; Magyar 1995; Geary et al. 2000).

Our efforts to recover either macro- or microfossils from

the Szulimán outcrop have been unsuccessful so far. Fossil-
iferous outcrops and borehole sequences, however, were
documented in the vicinity (Fig. 3). Although these data can-
not be applied directly to the Szulimán succession, they give
us a general understanding of the age and environmental
conditions of Lake Pannon deposits in the region.

Environmental interpretation of molluscs

Two different mollusc assemblages occur in the area

(Figs. 9,  10).  The  first  one  is  characterized  by  Congeria
zagrabiensis,  “Pontalmyra”  otiophora,  Valenciennius  reussi,
and  may  also  include  Congeria  rhomboidea,  C.  croatica,
Lymnocardium  majeri,  L.  cristagalli,  L.  hungaricum,  other
cardiids  and  pulmonate  snails  (Fig. 9).  This  assemblage  is
found  in  fine-grained  sediments,  such  as  marl,  clay  or  silt,
and  is  interpreted  as  a  sublittoral  fauna.  It  has  been  found,
for instance, in surface outcrops near Ibafa (collected by G.
Chikán), Bükkösd, and in the Sh-1 borehole, at 551 m depth,
close to the bottom of the Lake Pannon sequence.

The other mollusc assemblage consists of cardiids, such as

Lymnocardium ferrugineum, L. pelzelni, L. schmidti, Proso-
dacnomya  sp.,  etc.,  dreissenids,  such  as  Congeria  triangu-
laris,  C.  balatonica  and  Dreissenomya,  and  prosobranch
gastropods,  such  as  Viviparus  and  Melanopsis  (Fig. 10).
These fossils are found in sands, and are interpreted as repre-
senting a shallow-water, littoral mollusc fauna. This assem-
blage is found, for example, in Nyugotszenterzsébet (Bujtor
1992), Cserdi, Ibafa and Bükkösd.

However distinct the two assemblages are, they do not dis-

play any clear geographical separation within our study area.
Instead, they repeatedly occur above each other in some se-
quences, for example in Bükkösd and Ibafa, making the pa-
leoenvironmental interpretation a challenge.

An unusual mixture of the two assemblages in a thin gravel

layer was recorded in borehole Nagyváty-7, at 290 m depth.
Littoral  forms,  such  as  Prosodacnomya,  Melanopsis  and
Theodoxus, occurred together with sublittoral  Lymnocardium
majeri here, obviously as a result of reworking and redeposi-
tion.  This  process  implies  the  presence  of  a  morphological
gradient,  possibly  similar  to  the  one  that  initiated  the  Szu-
limán turbidites.

Biochronostratigraphy

Apart from the deep Szentlőrinc-XII borehole where al-

most  all  Lake  Pannon  dinoflagellate  and  mollusc  biozones
were identified ((Sütő-Szentai 1989, 1991, 1995, 2000), Kor-
pás-Hódi  in  (Wéber  1982)),  the  fossiliferous  deposits  of  the
study area belong to the youngest biozones of Lake Pannon
sediments. These include the Spiniferites validus, Spiniferites
tihanyensis,  and  Galeacysta  etrusca  microplankton  zones,

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some freshwater algal ecozones, and the Congeria rhom-
boidea sublittoral and Prosodacnomya littoral mollusc zones
(Magyar & Geary 2012).

According to the distribution of microplankton zones, Sütő-

Szentai (1995) suggested that the study area was first flooded

Fig. 9. Sublittoral molluscs from the study area. a – Lymnocardium
majeri, borehole Nagyváty (Nv)-7, 290 m; b – Lymnocardium
cristagalli, Ibafa (collected by G. Chikán); c – Congeria zagrabiensis,
Ibafa (collected by G. Chikán); d – Congeria zagrabiensis, borehole
Somogyhatvan (Sh)-1, 551 m. Scale bars: 1 cm.

Fig. 10. Littoral molluscs from the study area.
a – Viviparus sp., Cserdi; b – Dreissenomya
sp.,  Bükkösd;  c  –  Congeria  triangularis,
Cserdi;  d  – Lymnocardium  pelzelni,  Cserdi;
e  –  Lymnocardium  ferrugineum,  Bükkösd;
f  –  Prosodacnomya  carbonifera,  borehole
Nagyváty (Nv)-7, 290 m. Scale bars: 1 cm.

by  Lake  Pannon  at  the  end  of  the  Spiniferites  validus  chron,
which is considered to be slightly older than 8 Ma (Magyar &
Geary  2012).  NE  (updip)  of  Szulimán,  in  Horváthertelend
(Fig. 3) and beyond, only the validus and tihanyensis zones
were found, whereas S (downdip) of Szulimán, in the bore-
holes  Szentlőrinc-XII,  Kacsóta-1,  and  Szig-K-60  (indicated
as “Szigetvár-III” in the original publications), the youngest
brackish-water biozone, the etrusca zone was also identified
above the validus and tihanyensis zones. According to Sütő-
Szentai (1995), this pattern suggests that the etrusca zone was
partly eroded from above the uplifted north-eastern part of the
study area.

The bases of the Congeria rhomboidea, Prosodacnomya

and Galeacysta etrusca zones roughly correspond to each
other, and can be dated as ca. 8 Ma (Magyar & Geary 2012).
The age of the outcropping Lake Pannon sedimentary se-
quences discussed in this paper, including the Szulimán out-
crop, can thus be estimated as 7 to 8 million years.

Regional stratigraphy and flooding events: a

discussion

A regional geological cross-section (Fig. 11) was com-

piled in order to show the position and connection of various
lacustrine formations and to understand how the deposits of
the transitional slopes can be classified. In the Drava Basin
the  deep  lacustrine  marls  and  the  following  thick  turbiditic
series, overlying and onlapping on the Middle Miocene syn-
rift sediments, occur at a depth of at least 2 km. The Szolnok
Formation  pinches  out  towards  the  basin  margin.  Therefore
the  next  Algyő  Formation  –  slope-related  turbidites  and
slope shales – overlies either the Szolnok Formation or older
Paleogene  to  Neogene  or  the  basement  near  the  basin  mar-
gins.  It  is  important  to  keep  in  mind  that  the  clinoforms  in

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Fig. 11.

Regional

cross-section

constructed

from

seismic

profile

(see

Fig.

2),

well

data

and

geological

maps

shows

various

basement

and

their

Cenozoic

cover

units

from

the

Drava

Basin

the

Mecsek

Mts.

The

Upper

Miocene

lacustrine

succession

tilted

and

thickening

towards

the

SW.

erosionally

truncated

the

/Pliocene

and

the

Quaternary

unconformities.

Progradation

of the shelf edge, namely

the boundary of the Algyő

and Újfalu Formations, is clear

from seismic profiles but

is difficult to r

econstruct from well data. The

positions of sediments indicating the

transitional

slope

discussed

this

paper

are

marked

the

wells

the

isochronous

parasequence

set.

asement

map

after

Haas

al.

2010.

this  figure  are  not  imaged  in  dip  direction
but in an oblique view. Based on a net of 2D
sections  (Uhrin  2011)  slope  progradation
happened  from  NNW  to  SSE.  It  is  evident
from  the  section  that  as  the  slope  advanced
towards the deeper parts of the Drava Basin
the  height  of  slope  increased,  from  about
150 m at the NE margin to 300—400 m in the
basin interior. The decompacted thickness of
slope shales indicates minimum estimates of
water depth up to 610 m (Balázs et al. 2015).

The black dots on the seismic profile rep-

resent  the  shelf-slope  break  marking  the
boundary between the steeply dipping slope
of the Algyő Formation and originally hori-
zontally  deposited  Újfalu  Formation,  the
product of repeated delta progradation on the
lacustrine shelf. Connecting these points in-
dicates  the  shelf-edge  trajectory,  which
shows  the  development  of  the  lacustrine
base level, a result of subsidence and climat-
ically-driven  lake  level  oscillations.  At  the
NE side of the seismic profile first a flat tra-
jectory is evident with the shelf edge of only
150 m  high  slopes  near  the  basin  margin.
This relatively small water depth is the result
of the first flooding of the basin margin. It is
followed  by  a  steeply  ascending  trajectory,
which points to a period when the lacustrine
base  level  was  rising.  Aggradation  of  the
shelf was about 200 m accompanied by only
a modest rate of progradation. About 6.8 Ma
ago (Magyar et al. 2013) the situation gradu-
ally  changed,  the  trajectory  became  almost
flat,  indicating  that  aggradation  ceased  and
progradation  became  dominant.  After  that
only minor rises of the shelf-edge trajectory
are  visible.  The  ultimate  cause  of  the  major
base-level  rise  somewhat  before  6.8 Ma  is
unknown.  The  structural  evolution  of  the
Mecsek area is complex enough to involve a
local increase in the rate of subsidence. Cli-
matic impact on lake-level rise is also equally
possible, but if it played a significant role it
must  be  evident  elsewhere  in  Lake  Pannon
sediments of the same age.

Actually this base-level rise roughly corre-

lates in time with a significant backstepping
of  the  shelf  margin  in  eastern  Hungary,
where  a  second  set  of  large  clinoforms  ap-
pears  on  seismic  sections  of  the  eastern
Great Plain (Magyar & Sztanó 2008; Magyar
2010).  This  event  also  resulted  in  the  accu-
mulation  of  extremely  thick  successions  of
the  deltaic  sediments  shown  by  well  data
(Juhász 1992, 1993), reported not only from
the  western  and  eastern  parts  of  the  Great
Plain but also from the Drava Basin (Juhász
1998). The differences in the basin fill archi-

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tectures in the Great Plain and rates of backstepping were ex-
plained  by  variations  in  sediment  supply  (Vakarcs  et  al.
1994; Csato et al. 2007). As this unit is overlain by a locally
important unconformity, much effort was done to explain the
origin of the latter, but much less attention has been paid to
the reasons for lake-level rise below. Mostly it has been em-
phasized  that  the  main  driving  force  is  differential  subsid-
ence  and  uplift  of  the  region  (Juhász  et  al.  2007).  During
about  the  same  time  interval  economic  lignite  seams  also
formed on the northeastern margin of Lake Pannon, worked
in  huge  open  pit  mines  on  the  foothills  of  the  Mátra  and
Bükk  Mts.  Their  repeated  successions  up  to  a  thickness  of
over 200 m can be followed close to the area where the del-
taic  succession  is  thick  in  the  Great  Plain  (Magyar  2010).
These  pieces  of  evidence,  together  with  multiple  paleobio-
logical data point to a period characterized by increased rates
of precipitation from ca. 7.2 Ma (Magyar 2010).

Whatever the trigger was, this period of intense base level

rise is particularly interesting because successions NE of the
seismic  profile,  shown  in  detail  by  well  data  (Fig. 8),  were
deposited during this time. As a result of flooding, lacustrine
deposits  could  extend  as  far  as  Nyugotszenterzsébet  or
Bükkösd. During this transgression wave-eroded coasts and
small  deltas  of  locally  derived  material  developed.  As  was
discussed before, fossils at several sites near the base of the
lacustrine succession – in well Sh-1, Bükkösd, Ibafa – are
not older than 8 Ma, therefore all these events must have oc-
curred after 8 Ma. As the base level repeatedly rose, these lo-
cal  transgressive  deposits  partly  got  reworked  and  eroded.
The  stacked  parasequence  sets  discussed  formerly  indicate
that  the  base  level  was  rising  continuously,  without  notice-
able falls.

The lower, dominantly shaly portion of the lacustrine suc-

cession in the Szigetvár wells (Fig. 8) may still be formed on
the same, roughly 150 m high slope which is depicted on the
seismic section, thus this part can be named the Algyő For-
mation. The overlying cyclic sequences were formed above
the shelf edge, so if they appeared on seismics they would be
called Újfalu, without doubt. Well data, with coarsening and
thickening up series alternating with shales are also features
frequently reported from the Újfalu Formation. The speciality
of the area, however, is that it can be demonstrated that not
only delta slopes, but longer and taller transitional slopes de-
veloped, and allowed the accumulation of turbidites at their
base. Although the studied outcrop is unique so far, it must
be emphasized that sedimentary characters are very different
from other facies of the Újfalu Formation. The development
of  this  third  type  of  slope  is  obviously  connected  to  the  in-
tense lake level rise, manifested in the form of stacked pro-
gradational  parasequence  sets.  The  successions  clearly
demonstrate  that  high  rate  of  sediment  supply  is  over-
whelmed by base-level rise in the short term.

Conclusions

The unique succession at Szulimán with turbidites and

synsedimentary folds with possible minor modifications by
storm-induced currents clearly indicates that an inclined pa-

leotopography, meaning some sort of slope, must have existed
within a few km of the location. The usually 20—30 m high
delta  slopes  in  Lake  Pannon  as  well  as  the  several  hundred
meter  high  basin  margin  slopes  can  be  excluded.  The  Szu-
limán  turbidites  formed  on  the  flank  of  a  basement  high,
namely  in  a  transitional  position  between  “real”  deep  basin
slopes  and  sublacustrine  basement  highs,  where  no  slopes
developed at all. However, the formation of these slopes of
transitional height is only partly a result of their spatial loca-
tion, rather it can be explained by an interplay of their mar-
ginal  location  and  of  lake  level  rise.  As  lake-level  rise
continued,  the  development  of  transitional  slopes  was  re-
peated. As there are locations where the transgressive coastal
sands are directly overlain by regressive deltaic successions,
without intercalations of thick sublittoral/profundal clays, it
is supposed that only a small temporal difference may have
existed between the flooding of some elevated areas and the
arrival  of  the  prograding  deltaic  feeder  system.  The  ampli-
tude  of  lake-level  rise  seems  to  be  unusually  high  with  re-
spect to similar events in Lake Pannon, but it is not possible
to  determine  so  far  if  the  climatic  or  structural  signal  was
stronger. As the turbidites near Szulimán are associated with
alternating  deltaic  and  open-water  transitional  slope  depos-
its, they are assigned to the Újfalu Formation.

Reasons explaining why these turbidites have gone unre-

cognized so far include the poor outcrop conditions in low-
relief landscapes surrounding the mountains and the lack of
industrial  exploration  in  these  areas.  Even  if  they  exist,  the
resolution of industrial seismic profiles is usually inadequate
to  detect  clinoforms  of  less  than  100 m  thickness,  as  is  the
quality  of  archive  core  or  borehole  documentations,  where
sedimentary  structures  are  scarcely  mentioned.  It  is  a  ques-
tion  yet  to  be  examined  whether  similar  sediments  have  a
wider distribution among Lake Pannon “shallow-water” suc-
cessions. A potential tool for their detection is the investiga-
tion of well logs and available cores, particularly from areas
where extremely thick successions of Újfalu Sand have been
reported. The hardly studied outcrops of piedmont areas also
deserve attention. The presence of turbidites linked to transi-
tional  slopes  can  also  be  indicated  by  co-existing  or  mixed
sublittoral and littoral faunas.

Last but not least, large-scale reconstructions of basin

evolution or paleogeography can benefit from detailed pro-
cess-based sedimentological studies and a combination of pa-
leontological and limited well-log data even if only very
small outcrops are present.

Acknowledgments: This research was supported by the Euro-
pean  Union  and  the  State  of  Hungary,  co-financed  by  the
European  Social  Fund  in  the  framework  of  TÁMOP  4.2.4.
A/2-11-1-2012-0001  ‘National  Excellence  Program’  (Zoltán
Magyary Grant to KS), and by the Hungarian Scientific Re-
search  Fund  (OTKA)  Projects  PD104937  and  K81530.  We
thank  Zoltán  Lantos  (MFGI)  and  Lilla  Tőkés  (ELTE)  for
helping in visualization of well-log data. Two anonymous re-
viewers  are  acknowledged  for  thought-provoking  comments
and  suggestions.  MOL  Hungarian  Oil  and  Gas  Company  is
thanked for permitting the publication of seismic data in Fig-
ure 2. This is MTA-MTM-ELTE Paleo contribution No. 215.

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