GeolCarp_Vol53_No4_223

GEOLOGICA CARPATHICA, 53, 4, BRATISLAVA, AUGUST 2002

223—234

TECTONIC STRUCTURE OF THE KIS-FENNSÍK AREA

(BÜKK MOUNTAINS, NE HUNGARY)

MÁRTON FORIÁN-SZABÓ and LÁSZLÓ CSONTOS

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

foka@ludens.elte.hu; csontos@ludens.elte.hu

(Manuscript received July 3, 2001; accepted in revised form December 13, 2001)

Abstract: Stratigraphic and structural investigations based on geological mapping in the Kis-fennsík region (NE part of
the Bükk Mts, NE Hungary) proved the complex nappe structure of the area. According to new biostratigraphic data and
structural setting, three main tectonostratigraphic units can be distinguished: in lowest structural position Carboniferous
to Triassic deposits of the Bükk Parautochthonous Unit are forming a huge south-vergent anticline (North Bükk Anti-
cline  NBA).  The  Szarvaskő-type  Harica  Nappe  (HN,  representing  the  upper  part  of  the  Jurassic)  is  thrust  over  the
northern limb of the anticline. After the emplacement of the Harica Nappe, Middle-Upper Triassic rocks of the North
Bükk Anticline (Szeleta sliver, NBA/b) are thrust onto the HN unit, and along an E-W striking thrust fault onto the older
part of the NBA unit. In the southern part of the investigated area a narrow zone of platform carbonates and dark shales
(Vesszős Formation) was mapped. Together with the overlying thick cherty limestone succession, they are considered to
belong  to  the  NBA/b  unit.  All  the  above  mentioned  units,  affected  by  regional  dynamothermal  metamorphism,  are
overlain  by  the  Kisfennsík  Nappe  (KN),  which  consists  of  slightly  metamorphosed  Carnian  platform  limestones  and
dolomites, with some intercalating metavolcanites. Emplacement of the thrust sheet to the SE resulted in small- to map-
scale folding in the North Bükk Anticline. Thrusting is attributed to the Cretaceous tectonometamorphic evolution of the
Bükkium. Later, an intensive shortening generated well-developed, NE-SW oriented antiform-synform structures with
reverse faults in all units. Folds with NW-SE axis represent another deformation phase.

Key words: Bükk Mts, Kis-fennsík, geological mapping, Northern Hungary, tectonostratigraphy, deformation.

Introduction

The Kis-fennsík (Little High Plateau)

is situated in the north-

eastern part of the Bükk Mts (N Hungary), with the Garadna
Valley  separating  it  from  the  Great  High  Plateau  (Fig. 1).
Stratigraphy in the Bükk Mts is strongly biased by bad outcrop
conditions,  multiphase  structural  evolution  and  metamor-
phism. The intensively folded, south-vergent imbricated struc-
ture of the NE part of the mountains was already recognized
by Schréter (1916). On his map the platform limestone of Kis-
fennsík (considered to be Upper Triassic) lies as a nappe upon
Paleozoic to Lower Triassic schists, volcanites and limestones
(Schréter 1943). According to the second geological mapping
of the area by Jámbor (1959), the succession of Kis-fennsík is
folded in a syncline, and thrust over the North Bükk Anticline.
In the light of new, but still scattered biostratigraphic data (ra-
diolarian finds) Kozur (1984) proved the presence of Jurassic
in the northern slope of Kis-fennsík. A more detailed mapping
was carried out by Gy. Less. His model supposed a complex
nappe structure in the Kis-fennsík area (Less Gy. pers. com-
mun.). According to this, three nappes were detached from the
Bükkian autochthonous succession. The first detached Jurassic
shales are thrust over the northern limb of the North Bükk An-
ticline. This was followed by an Upper Triassic cherty lime-
stone-containing nappe. The finally detached nappe involving
Middle-Upper  Triassic  dolomites,  volcanites  and  the  Kis-

fennsík Limestone, is in the uppermost tectonic position. Con-
sequently, there are normal sequences within the nappes, while
thrusting resulted in an inverted setting between tectonic units,
relative  to  the  original  stratigraphic  position.  According  to
Csontos (2000), the different metamorphic-deformational con-
ditions  would  support  the  nappe  position  of  the  Kisfennsík
Limestone.  Previously  Csontos  (1988)  suggested  the  Silicic
origin of this formation.

Up to now no structural investigations were carried out to

prove the existence of nappes, and to determine the deforma-
tion characters in the Kis-fennsík area. The aim of this work is
to  answer  the  following  questions:  How  many  tectonic  units
are distinguishable? What is their stratigraphic content? How
are they deformed, and related to each other? Our investiga-
tions  are  based  on  geological  mapping  of  the  key-area  (SW
Kis-fennsík). The geological map can be found in the Appen-
dix,  the  location  is  on  Fig. 1.  The  structural-stratigraphic  as-
pects are outlined according to measured structural data, pale-
ontological-sedimentological-petrological  study  of  thin
sections, and re-investigation of borehole material. The recent
investigations of F. Velledits and the authors produced numer-
ous  new,  mainly  biostratigraphic  data  from  the  Kisfennsík
Nappe, which are presented in a separate paper (Velledits et al.
in press). In the light of new stratigraphic and structural data,
the possible Silicic origin of the Kisfennsík Nappe is also dis-
cussed in the above mentioned paper.

There is a difference in writing between terms “Kis-fennsík” (geographic name) and “Kisfennsík Nappe or Limestone”.

224 FORIÁN-SZABÓ and CSONTOS

Stratigraphy of the tectonic units

In the Alpine-Carpathian frame (ALCAPA) the Paleo-Me-

sozoic rocks of the Bükk Mts belong to the Bükkia Composite
Terrane (Kovács et al. 2000). The successions of the terrane
can be correlated with South Alpine (Carnic Alps, Ebner et al.
1991; Fülöp 1994), and NW Dinaric (Jadar Block, Sana-Una
Terrane, Protić et al. 2000) units. In the Bükk Mts three main
tectono-facial  units  can  be  distinguished:  the  lowest,  Bükk
(Nagyfennsík)  Parautochthonous  Unit  is  overlain  by  the
Szarvaskő—Mónosbél nappes (Balla 1987; Csontos 1988) and
the Kisfennsík Nappe. A new outline of sedimentary-magmat-
ic and tectonometamorphic evolution of the Bükkium can be
found in Kovács et al. (2000). The Parautochthonous Unit and
Szarvaskő nappes are folded together and form larger imbri-
cates. Boundaries of these imbricates separate minor tectono-
facial  areas,  with  slightly  different  Mesozoic  successions
(Csontos 2000). In the following, we focus on the stratigraphy
of the studied area (Fig. 1, Appendix), avoiding the repeated
referencing  of  the  geological  map  (Appendix)  in  the  text.  In
the studied area pre-Tertiary formations can be affiliated into
three  main  structural  units,  which  coincide  with  the  above-
mentioned units. The Parautochthonous Unit is represented by
its northernmost tectono-facial area: the North Bükk Anticline,
with  Carboniferous  to  Triassic  rocks.  The  Harica  and  Kis-
fennsík Nappes are overlying tectonic units. The stratigraphy
of the units and their schematic relationship are seen on Fig. 2.

The North Bükk Anticline

Formations related to this tectonofacial unit, are exposed in

the southern and western part of the studied area. In the core of
the E-W striking anticline the black, carbonate-free shales of
Carboniferous age (Szilvásvárad Formation) represent a
Variscan flysch (Hochwipfel Flysch in the Carnic Alps),

which  is  followed  by  post-Variscan  shallow  marine  molasse
sediments (corresponding to the Auernig Group, Ebner et al.
1991): grey shales with limestone mounds (Mályinka Forma-
tion). The elongated, sometimes stretched limestone bodies in-
dicate the strike of the anticline. Up to now no Variscan meta-
morphism  and/or  deformation  was  proved  in  the  Bükk  Mts
(Árkai 1983; Árkai et al. 1995). Later regional dynamothermal
metamorphism (160—120 Ma), which was followed in the NE
Bükk  by  a  Late  Cretaceous  (80—95  Ma)  metamorphic  event
(Árkai et al. 1995), resulted in axial plane foliation and ductile
deformation of different degree in most of the Paleozoic and
Mesozoic formations (Csontos 1988, 1999). The Alpine sedi-
mentary  cycle  begins  with  variegated  sand-  and  siltstones
(“Gröden  facies”),  overlain  by  sabkha  evaporites  and  dolo-
mites (Szentlélek Formation). Due to its ductile behaviour, it
occurs  in  strongly  variable  thicknesses,  sometimes  in  small
slices, or it is tectonically mixed together with Carboniferous
limestone. The overlying Upper Permian cyclic shelf deposits
(Nagyvisnyó  Limestone  Formation,  “Bellerophonkalk”)  are
widespread in the western part of the studied area. The unin-
terrupted sedimentation at the Permian/Triassic boundary con-
tinues on a carbonatic-siliciclastic ramp in the Early Triassic:
the oolithic Gerennavár Limestone is preserved in some small
areas upon the Permian, while a narrow belt of yellow-green-
ish foliated silt in the W represents the Ablakoskővölgy For-
mation. The upper part of this formation, the overlying Ani-
sian  Hámor  dolomite  and  Aniso-Ladinian  Szentistvánhegy
metaandesite  are  found  elsewhere  in  the  North  Bükk  Anti-
cline, but are missing in the closer studied area. South of the
investigated  area,  in  the  overturned  limb  of  the  North  Bükk
Anticline, the sequence is quite continuous. Here, in the nor-
mal  limb  the  Middle-Upper  Triassic  part  of  the  sequence
(Szeleta sliver, or imbricate) is overthrust along a WNW-ESE
striking front at the southern edge of Kis-fennsík onto the old-
er part of the sequence, and partly onto the already emplaced

Fig. 1. Location map. The little frame belongs to the mapped area (Appendix), while the bigger one refers to the geological map of Fig. 6.

TECTONIC STRUCTURE OF THE KIS-FENNSÍK AREA 225

Harica Nappe, too (Section 2 on Fig. 3). In a narrow zone in
the southern slope of Kis-fennsík possible Ladinian-Carnian
platform deposits (Fehérkő limestone), overthrust onto the Pa-
leozoic were mapped (Appendix). In the continuous sequences
this formation represents the re-established platform-sedimen-
tation after the Anisian-Ladinian volcanic event.

In other parts of the parautochthonous the rifting-related fa-

cies differentiation starting in the Middle Triassic caused coex-
istence of platform and basin areas (Velledits 2000). However,
in the studied area there is no relevant data about basin depos-
its  formed  synchronously  with  the  Fehérkő  platform  sedi-
ments. As a result of the present mapping, we observed, that in
a  small  belt,  the  above  mentioned  recrystallized,  slightly
cleaved  limestone  is  overlain  by  black,  bituminous,  foliated
shales and siltstones with marly and calcareous layers. These
more  competent  beds  are  frequently  cut  off  by  tectonic  con-
tacts. The same bituminous shales and marls are present at the
southern  edge  of  the  Kis-fennsík,  in  a  road  cut  near  Csókás,
and  in  lower  parts  of  Parasznya-59  and  Varbó-74  wells,  too
(Section 2 on Fig. 3). Considering the identical lithologic char-
acter, the deposits can be correlated with Vesszős Formation of
the North Bükk Anticline. Due to the lack of fossils, and the
different  structural  evaluation  of  the  type-section  in  Vesszős
Valley, near Lillafüred, the age and stratigraphic position of the
formation  is  debated  (see  different  opinions  in  Pelikán  et  al.
1993).  Regarding  the  stratigraphic  position,  our  mapping  re-
sults support the opinion of Csontos (in Pelikán et al. 1993),

that  is  the  dark  bituminous  shales  appear  between  platform
sediments and Upper Triassic cherty limestones. In addition,
this opinion is now proved by a biostratigraphic datum: a con-
odont was found at the bottom of borehole Varbó-74, at 348
m.  S.  Kovács  determined  it  unambiguously  as  Gondolella
polygnathiformis Budurov et Stefanov. This conodont is the
first biostratigraphic evidence for a Carnian age of the forma-
tion. On the basis of above mentioned data, Vesszős Forma-
tion can be related to the (Middle) Carnian “Raibl” siliciclas-
tic  event.  Its  sedimentation  in  a  partly  restricted  basin
followed the drowning of the previously existing platforms. In
the above mentioned borehole the Vesszős Shale grades up-
wards  into  thick,  basinal  cherty  limestone  (Felsőtárkány
Limestone  Formation,  Section  2  on  Fig. 3),  which,  together
with the widespread cherty limestones of the mapped area, be-
long to the North Bükk Anticline succession.

East of the mapped area, between Miskolc and Alsóhámor,

in  the  Csanyik  Valley,  Norian  conodonts  were  found  in  the
Felsőtárkány  (previously  Rónabükk)  limestone  (Kovács  S.
pers. commun.). In the North Bükk Anticline the Carnian si-
liciclastic sediments were followed mainly by basin deposits:
Felsőtárkány limestone, with locally (NE of Lillafüred) inter-
fingered crinoidal-cherty, olistolith-bearing limestone (Répás-
huta Limestone, Fig. 2)  and  platform  limestone  bodies.  Ac-
cording  to  the  present  knowledge,  in  contrast  with  the
Nagy-fennsík  facial  area  of  the  Parautochthonous  Unit,  the
possibly existing Late Carnian-Norian rejuvenated platforms

Fig. 2. Pre-Tertiary stratigraphic sequences of the tectonic units of NE Bükk, and their schematic relationship.

226 FORIÁN-SZABÓ and CSONTOS

of the Anticline must have been very much reduced. The up-
permost (Jurassic) part of the North Bükk Anticline is not pre-
served.

The Harica Nappe

On the basis of lithologic features, the brownish grey-black,

lamellar shale and siltstone with sandstone interbeds and rare
radiolarite  clasts  can  be  correlated  with  certain  elements
(Mónosbél Formation) of the succession in the Szarvaskő
(-Mónosbél) type nappes, overthrust onto the W and SW part
of the Bükk Parautochthonous Unit. The (Middle-) Late Juras-
sic  age  of  the  formation  (Fig. 2)  is  based  on  two  biostrati-
graphic data. Firstly, from a locality north of the mapped area
Kozur (1984) and Balogh et al. (1984) mention Jurassic radi-
olarians from radiolarite clasts of the shale, which, according
to L. Dosztály (in Csontos et al. 1991), could have Callovian—
Oxfordian age. Secondly, from a nearby locality (Büdös-kút)
Velledits (1998)  described  a  Norian-Rhaetian  platform lime-
stone olistolith from the shale. The olistolith contains several
generations  of  neptunian  dykes  with  foraminifers.  Several
ages could be distinguished between these fissure fillings. The
oldest  generation  was  Hettangian-Sinemurian,  while  the
youngest  most  probably  represented  the  Late-Dogger—Early
Malm period (Velledits 1998).

The Kisfennsík Nappe

The nappe outliers of this uppermost tectonic unit cover all

the other units mentioned before. The succession begins with

dolomites  (in  the  NW  corner  of  the  map  in  the  Appendix),
followed by Kisfennsík Limestone of considerable thickness.
The generally non-metamorphic, but in many places slightly
recrystallized Carnian platform limestone contains interlay-
ered  basic  metavolcanites  in  different  levels,  which  can  be
correlated  with  the  Carnian  extensional  volcanism  of  the
Bükkium (Szinva Metabasalt Formation). The separation of
Kisfennsík Limestone from the probably coexistent Fehérkő
platform limestone is based on the different tectonic position
and metamorphic grade. The age, micro- and biofacies of the
limestone, and the petrology and position of the volcanites is
given  in  detail  in  another  paper  (Velledits  et  al.  in  press)
dealing with the stratigraphy of the Kisfennsík Nappe.

Tertiary

Tertiary sediments are represented by some Miocene ter-

restrial conglomerates in the north, and by the Csókás For-
mation. The Oligocene Csókás Formation lies transgressive-
ly  upon  the  Kisfennsík  Limestone,  containing  almost
monomictic breccias of it (Less 1991). Its larger foraminifer-
al assemblage indicates a Middle Chattian horizon (SBZ 23
shallow benthic zone), and can be correlated with the Budik-
ovany Formation in Slovakia (Báldi et al. 1999). The facies
of the Csókás Formation differs from coeval, more basinal,
and  not  transgressive  developments  around  the  Bükk  Mts.
We mapped this formation over a slightly larger area, than it
was known before; its breccias and pebbles sometimes with
marks  of  bioerosion  also  appear  in  the  upper  part  of  well
Parasznya-59 (Section 2 on Fig. 3).

Fig. 3. Geological profiles across the Kis-fennsík area. Localities are indicated on Fig. 4; for key see Fig. 6, and Appendix. Thick lines
indicate boreholes.

TECTONIC STRUCTURE OF THE KIS-FENNSÍK AREA 227

Dominant observed deformations

Deformation in rocks

Deformation is observed and measured mostly in more less

competent  rocks  with  a  cleavage.  These  are  Paleozoic  rocks
and Triassic cherty limestones. In most cases only one cleav-
age  plane  is  present.  In  Paleozoic  rocks  cleavage  is  usually
parallel  to  the  bedding  (S1/S0),  as  in  the  SW  corner  of  the
area,  at  the  transition  of  Szentlélek  Formation—Nagyvisnyó
Limestone.  Only  one  cleavage  is  observed  in  cherty  lime-
stones, too. In some cases, when there are thicker marly inter-
layers, this is also parallel to bedding (S1/S0). When no such
interlayers are observed, cleavage is apparent only when cher-
ty lenses/layers help in observation, for example at the south-
ern margin of Kis-fennsík, near Magos-kő. The row of chert
lenses (i.e. bedding) forms an acute angle with cleavage and
the  long  axis  of  lenses  is  frequently  transposed  parallel  to
cleavage. Bedding/cleavage relations in cherty limestones may
indicate normal or inverted limbs of folds. In the case of Car-
boniferous limestones and shales there are two cleavage plains
observed,  but  due  to  the  lack  of  markers,  it  is  not  evident,
whether one of them is parallel to the layering or not.

All the above mentioned more ductile formations exhibit

folds with different orientations and style, which can be relat-
ed to map-scale folds. Orientation of major folds (synforms-
antiforms) is marked on the structural scheme of the investi-
gated area (Fig. 4). In the following text different structural
elements are described according to orientation groups.

Structures parallel to the main E-W strike
of the North Bükk Anticline

The strike of the antiform is indicated by the row of lime-

stone cliffs emerging from Carboniferous shales. Intersection
lineations in shales and a significant group of fold axes in Car-
boniferous-Lower Triassic rocks are parallel to the general E-
W strike of formations (stereo-plot e in Fig. 4). E-W striking
folds are generally south-vergent. A smaller syncline is linked
to the North Bükk Anticline from the north. This has E-W
strike, too (Fig. 4).

NE-SW striking structures

The relatively steeply dipping axial surface of the North

Bükk Anticline is folded in a major, map-scale Z shape fold
(Fig.  4).  The  general  E-W  orientation  of  cleavage  planes  is
also bent into the fold. In our opinion this fold has a NE-SW
striking  axis.  Following  the  arrangement  of  Carboniferous
limestone  occurrences,  several  other  Z  shaped  folds  are  also
found more to the east, for example near Ferenc spring. On the
map, the dominant structure is a folding of the NE-SW axis,
creating  a  train  of  open  synforms-antiforms  (Fig.  4).  These
folds  are  indicated  by  the  geometry  of  Kisfennsík  nappe
boundary,  as  well  as  borehole  Varbó-71  (Fig. 3,  Section  1).
The  well  penetrated  the  Kisfennsík  Limestone,  then  reached
the cherty limestone, which would have appeared much shal-
lower in case of absence of the synform. The volcanic rocks at
Barátság-kert (see Appendix and Section 1 on Fig. 3) are also

preserved in the core of a synform. After appearence of the
cherty limestone in the core of an antiform (Fig. 4), this volca-
nite continues towards the southeast, occurring in a small belt
and in borehole Parasznya-60 (Appendix). Lack of volcanites
in borehole Varbó-71 (Section 1 on Fig. 3) and in surrounding
contacts on the surface (Appendix), and the above mentioned
geometry suggests that the Kisfennsík Limestone around the
well underlies the Barátság-kert volcanites. The main Kis-
fennsík limestone body around Nyilász overlies the same vol-
canites, and contains itself smaller lenses-interlayers of volca-
nites near Válint-kereszt (Section 2, Appendix). It seems that
the sole thrust of the Kisfennsík Nappe cuts off younger and
younger formations towards the SE. The uppermost, therefore
strongly eroded Kisfennsík Nappe is mostly preserved in syn-
forms (Fig. 4).

The above mentioned map-scale folding is also evident on

the outcrop level. NE to NNE plunging axis folds were mea-
sured  in  the  North  Bükk  Anticline  (Fig.  4,  stereo-plot  e).  In
cherty  limestone  road  cuts  towards  Varbó  poles  to  cleavage
clearly  indicate  a  NE-SW  axis  folding,  in  spite  of  a  wider
range of measured fold axes (Fig. 4, stereo-plot b).

NW-SE striking structures

NW-SE oriented folding is less dominant on the map

(Fig. 4). However, such folds are measured near the borehole
Parasznya-60 (Appendix). This fold is given by cleavage poles
measured in cherty limestones (Fig. 4, stereo-plot a). The axis
of the fold dips to the SE. Outliers of the Kisfennsík Nappe are
found on the limbs of the antiform. A similar, NW-SE orient-
ed, but NW-dipping axis fold can be constructed from outcrop
dips at the southern margin of the Kis-fennsík (Fig. 4, stereo-
plot h). Oppositely dipping axes can be explained by the posi-
tion of these folds on different limbs of the NE-SW axis syn-
form (Fig. 4). The relative chronology of these folding phases
cannot be given by these data. Most fold axes measured in
cherty limestones at the southern margin of the Kis-fennsík are
also NW-SE oriented (Fig. 4, stereo-plot g), but the high scat-
ter indicates the presence of several folding phases.

Structures around important tectonic contacts

Southern margin of the Szeleta sliver

At the southern margin of Kis-fennsík, near the Vidróczki

Cave  (Appendix),  rocks  are  cross-cut  by  a  westerly-dipping
and  upwards  concave  thrust  surface,  along  which  the  cherty
limestone  is  thrust  upon  the  Fehérkő  Limestone.  Cleavage
planes are dragged along the thrust surface. West of the cave
the margin of Kis-fennsík is very complicated. Along tectonic
contacts the Vesszős Formation is often cut off. In this way,
cherty  limestone  is  often  emplaced  on  Fehérkő  Limestone
with flat, cataclastic breccia zones. Unfortunately, no reliable
shear indicator was observed in the cataclasites. On the map
(Appendix)  and  on  Section  1  (Fig. 3)  the  tectonic  contacts
within different stratigraphic members of the Szeleta sliver are
explained by south-vergent thrusts, which are otherwise typi-
cal to the Bükk Mts. However, the lack of reliable shear crite-

TECTONIC STRUCTURE OF THE KIS-FENNSÍK AREA 229

Fig.  5.  Tectonic  contacts  of  the  Kisfennsík  Limestone.  a)  At  the
stone arch of Három-kút the Kisfennsík Limestone is thrust over the
Felsőtárkány  limestone.  The  slightly  northward-dipping  nappe
boundary (indicated by a white arrow) is parallel to cleavage of the
underlaying cherty limestone. For location see Appendix. b) Section
and  stereo-plot  of  the  road-cut  near  Magas-kő,  with  the  overthrust
of cherty Limestone onto the Kisfennsík Limestone. See text for de-
tails. Location of the outcrop is indicated by letter i on Fig. 4.

ria  along  these  tectonic  contacts  makes  another  explanation
equally viable. This would explain the internal tectonic con-
tacts by major, low angle normal faulting. Shear stresses with-
in the Szeleta sliver could result in north-directed gravity glid-
ing  of  the  cherty  limestone  along  the  most  ductile  Vesszős
Shale,  which  is  cut  off  and  occasionally  completely  omitted
during the process.

In several outcrops along the southern margin, subhorizontal

slicks of unclear origin were measured in steep surfaces of dif-
ferent limestones. This feature would suggest a strike-slip fault
along the southern margin. Based on the Riedel-like orienta-
tion of these surfaces, the slicks can be related to an E-W ori-
ented left lateral shear, but more exact markers were not found
on the strongly weathered surfaces.

Structures related to the boundary of the Kisfennsík Nappe

Already Gy. Less realized that at the stone arch of Három-

kút, the Kisfennsík Limestone lies along a subhorizontal tec-
tonic  surface  on  the  cherty  limestone  (Fig.  5a,  Less  et  al.
1995).  In  the  lower  cherty  limestone,  smaller  upright  folds
with subvertical axial planes and internal ramps can be mea-
sured. The axis of these folds strikes N-S (Fig. 4, stereo-plot
f). When the contact is not parallel to cleavage of the cherty
limestone, it is accompanied by intense brecciation. In spite of
repeated  efforts  no  clear  indicator  of  tectonic  transport  was
found. On the northern part of Kis-fennsík, NW of the bore-
hole Varbó-71 (Fig 4. i) the contact of Kisfennsík and cherty
limestones is not an original nappe boundary, but a N-S direct-
ed,  easily-mappable  thrust.  Folds  visible  in  the  road-cut  and
more to the west have south-plunging axes and they have east-
erly vergencies (Fig. 5b). Poles to cleavage give a NNE-SSW
directed fold. A limb of the easternmost fold is sheared by the
thrust. It seems the cherty limestone of the western limb of the
synform traversed by Varbó-71 is thrust upon the synform of
Kisfennsík  Limestone.  Folding  and  related  phenomena  are
linked to this late thrust (Fig. 5b). Such smaller, later thrusts
are also seen NE of this exposure.

Structures around the northern boundary of the cherty lime-
stone with the Harica shales

West of well Varbó-74, near Büdös-kút-nyak, the cherty

limestone  appears  in  the  steeper  and  steeper  hillside,  topo-
graphically  above  Jurassic  Harica  Shale.  On  the  cliffs  folds
with different vergencies can be observed (Fig. 4,  stereo-plot
d).  Northerly  movement  is  suggested  by  a  broken,  sheared
northern  short  limb,  versus  the  much  less  affected  southern,
long limb. The fold affects cleavage, too. Jurassic shales occur
in a tectonic window north of Oszra-tető (Appendix). In cherty
limestone cliffs of the southern margin of this window an in-
ternal thrust surface is seen. This surface with a southerly dip
shears and drags cleavage of the footwall (Fig. 4, stereo-plot
c). Northern vergencies occur only in this area and are perhaps
related  to  thrusting  of  the  cherty  limestone  on  the  Jurassic
shales.

TECTONIC STRUCTURE OF THE KIS-FENNSÍK AREA 231

The main tectonic units in NE Bükk

The Kisfennsík Nappe

Let us review some of the arguments in favour of the exist-

ence  of  this  nappe!  According  to  mapping,  the  same  Kis-
fennsík Limestone is found upon different formations of dif-
ferent  structural  units.  On  the  other  hand,  different
stratigraphic  members  of  the  Kisfennsík  Nappe  (lower  lime-
stone,  Barátság-kert  volcanites,  upper  limestone)  lie  on  the
same cherty limestone formation. These relationships can be
explained only with a tectonic contact, found in outcrops in the
region  of  the  stone  arch  of  Három-kút  (Fig.  5a),  and  wells
(Vb-71,  Pa-59,  Pa-60).  Regarding  contacts  with  the  Harica
Unit, there is certainly an age-inversion, that is the Late Trias-
sic Kisfennsík Limestone is on Jurassic shales of Harica (Fig.
3, section 1). Slight metamorphic transformations can be dem-
onstrated  in  volcanites  and  twinning  of  calcite  veins  of  the
Kisfennsík  Nappe  (Velledits  et  al.  in  press).  However,  its
metamorphic grade is apparently less, than that of the underly-
ing structural units. It is probable, that the basinal cherty lime-
stone  tectonically  underlying  the  Kisfennsík  Nappe  was
formed  at  least  partly  synchronously  with  the  platform  Kis-
fennsík Limestone.

The Szeleta sliver

To correlate the enigmatic exposures found at the southern

margin of Kis-fennsík, Less’s NE Bükk map (Less et al. 1995)
was emended by the newly mapped area (Appendix) and some
key-points (mainly characters of boundaries) were re-evaluat-
ed (Fig. 6). According to this map, the tectonic boundary at the
southern  margin  of  the  Kis-fennsík  can  be  followed  towards
the  east  and  can  be  tied  in  the  Szeleta  tectonic  contact
(Schréter 1943), near Lillafüred. This is a tentative concept for
better understanding the structural features of the larger area.
We think that our concept (different of that of Less et al. 1995)
can interpret our new stratigraphic and structural results well,
and is not in conflict with field observations. However, other
possible  solutions  cannot  be  excluded.  Here  are  our  sugges-
tions:  In  both  areas  the  same  stratigraphic  sequence,  that  is:
Fehérkő Limestone, Vesszős Shale and cherty limestone was
found above the thrust surface (Fig. 6). West of Szeleta cave
the tectonic basis of the unit originating from the North Bükk
Anticline (and named here the Szeleta sliver) cuts into lower
stratigraphic units, namely the Szentistvánhegy metavolcanite,
and Hámor Dolomite (Figs. 2, 6), then cuts up in the sequence
of  the  Anticline.  Thus,  only  thin  Fehérkő  Limestone,  occa-
sionally  cut-off  Vesszős  Shales  and  mostly  cherty  limestone
can  be  found  in  the  mapped  area.  The  cherty  limestone  is
above  and  to  the  south  of  the  Jurassic  shales.  On  the  other
hand, the stratigraphic sequence of the Szeleta sliver is strong-
ly linked to the North Bükk Anticline, therefore overthrust of
the cherty limestone on Jurassic shales is easiest to imagine to-
wards the north-northwest. Northern vergencies on the north-
ern margin of the cherty limestone (Fig. 4, stereo-plots c, d) is
eventually related to the northerly movement of this part of the
Szeleta sliver. In case of the southern boundary of the sliver, at
the Szeleta cave and at the southern margin of Kis-fennsík, it

seems that the sliver is thrust on the North Bükk Anticline to-
wards the south (Fig. 3). As was already mentioned for tecton-
ic  reduction  of  the  Vesszős  Shale,  northward  extension  and
gliding cannot be excluded either. Another hypothesis would
suggest  that  N-S  compression  generated  not  only  south-ver-
gent thrusts, but also a backthrust of its northern margin onto
Jurassic  shales.  This  is  viable  if  ductile  shortening  is  mostly
accommodated  by  lower  structural  units  (North  Bükk  Anti-
cline and Jurassic shales), so the more rigid cherty limestone
slice can be passively thrust upon folded shales. Though back-
thrusting is most probable during south-directed main thrust-
ing,  it  can  also  be  generated  by  a  later  independent  tectonic
event.  Unfortunately  no  direct  evidence  is  available  yet  to
choose between these hypotheses. Anyway, the amplitude of
overthrusting  shows  considerable  lateral  variation  in  the
Szeleta  sliver.  In  the  north,  the  cherty  limestone  is  thrust  at
least 1—1.3 km upon Jurassic shales (Fig. 6). Towards the SE,
there is less and less stratigraphic difference between members
of the sliver and the underlying North Bükk Anticline. Finally,
east of the Szeleta Cave, the tectonic contact is gradually lost
in favour of an apparent stratigraphic contact.

The Harica Nappe

The Harica Nappe (Mónosbél Formation) is considered here

as  a  Szarvaskő-type  nappe.  In  the  northern  part  of  the  study
area the Jurassic shales of the nappe are overthrust onto differ-
ent,  eastward  younging  formations  of  the  North  Bükk  Anti-
cline: in the west the nappe is emplaced onto Permian, but in
the east, in borehole Varbó-74 we found it on Upper Triassic
cherty  limestone  with  a  clear  tectonic  contact  (Section  2,
Fig. 3). In the SW Bükk, where the underlying Parautochtho-
nous  Unit  has  approximately  the  same  age  as  the  overlying
nappes, the nappe position can be confirmed especially by the
different  geodynamic  character.  The  geochemistry  of  the
mafics in the Szarvaskő nappes indicates that they derive from
oceanic crust (Kubovics et al. 1990). These nappes lie on the
thinned continental crust of the Parautochthonous Unit (Balla
et al. 1983; Csontos 2000). Magmatites are unknown from the
Harica Nappe, but its deep-water deposits occur frequently on
Paleozoic rocks, evidently with a tectonic contact.

Structural evolution

Model

Our results suggest the following structural evolution model

for the NE Bükk. In the Bükk Mts the first, detectable com-
pressive  event  following  the  Triassic  rifting  episodes  is  the
emplacement of the Szarvaskő (-Mónosbél) nappes. Accord-
ing  to  Balla  (1987)  and  Csontos  (1999)  this  event  can  be
placed around the boundary of Jurassic and Cretaceous. These
nappes  are  emplaced  mostly  on  the  Jurassic  shales  (South
Bükk), but in some places (e.g. surroundings of Szalajka Val-
ley)  on  different  stratigraphic  units  of  the  Parautochthonous
Unit (Csontos 2000). Surface and well data indicate that there
is  a  gradual  younging  of  formations  towards  the  east  in  the
footwall of the Harica Nappe. We speculate, that different ages

232 FORIÁN-SZABÓ and CSONTOS

at the basis of the Harica Nappe are due to incipient folding
and subsequent erosion of the North Bükk Anticline. Prior to
nappe emplacement the axis of this proto-anticline was gently
dipping to the east (as at present). In this way it was possible
to thrust the Harica Nappe possibly from the north, onto this
eroded surface (Fig. 7a). In the next phase a thrust is generated
on the northern limb of the ever-tighter North Bükk Anticline.
The Szeleta sliver is tentatively derived from this thrust, but
moved towards the southwest onto older formations of the An-
ticline  and  onto  the  southern  part  of  the  Harica  Nappe
(Fig. 7b). Since the amount of overthrust is bigger in the west,
a counter-clockwise rotation is suggested, with a pole east of
Szeleta, where the most ductile formation, the Vesszős shale
thickens (Fig. 6). It is possible, that towards the east this thrust
dies out (Fig. 7b). On the southern part of the Szeleta sliver,
NW of the Szeleta Cave the map suggests a syncline parallel
to  the  thrust  boundary,  with  a  gently  SE  plunging  axis
(Figs. 6, 7c), eventually related to thrusting. The next impor-
tant  step  in  structural  evolution  was  the  arrival  of  the  Kis-
fennsík-nappe  (Fig. 7c).  As  mentioned  earlier  and  also  sup-
ported  by  the  strike  of  the  “Kis-fennsík  dolomite”  the
stratigraphic  sequence  at  the  basis  of  the  Kisfennsík  Nappe
seems to get younger and younger towards the southeast. On
the basis of these cut-off relations, it is quite probable, that the
Kisfennsík Nappe was thrust along a flat ramp to its present
position. It is generally accepted that direction of cutting up-
section  (i.e.  perpendicular  to  the  strike  of  cut-off  beds)  indi-
cates the nappe transport direction. Therefore a transport from
NW to SE (present directions) is suggested for the Kisfennsík
Nappe. The NW-SE push of the Kisfennsík Nappe was proba-
bly strong enough to bend former structures and to create large
Z shaped folds (Fig. 7c). After emplacement, the same defor-
mation  still  possibly  prevailed,  creating  the  most  dominant
NE-SW striking fold train (Fig. 7d, Section 1 on Fig. 3). On an
outcrop level, this shortening created NE-SW oriented, upright
or gently SE-verging folds. (Fig. 4, stereo-plots c, d, e). There
were several other folding events, the relative ages of which
are  not  known.  One  of  these  created  NW-SE  oriented  folds
within the Szeleta sliver (Fig. 4, stereo-plots a, g, h). This de-
formation  probably  post-dates  the  emplacement  of  the  Kis-
fennsík  Nappe,  since  tectonic  outliers  of  this  nappe  near
Három-kút—Magos-kő  and  well  Parasznya-60  show  similar
folding (Appendix, Fig. 4). The age of N-S folds near the stone
arch of Három-kút in the Szeleta sliver (Fig. 4, stereo-plot f)
and in the northern part of the area (stereo-plot i) is unknown,
and cannot be precisely separated from the former fold set.

Fig. 7. Structural evolution of the NE Bükk Mts. a) Emplacement
of  the  Harica  Nappe  roughly  from  the  north,  onto  the  incipiently
formed North Bükk Anticline. b) The Szeleta sliver extruded from
the  North  Bükk  Anticline  moves  with  rotation  onto  the  older  for-
mations of the Anticline and the Harica Nappe to the north. c) Em-
placement of the Kisfennsík Nappe to the SE, onto all other units,
deforming  the  axis  of  the  North  Bükk  Anticline  into  a  Z-shaped
fold.  d)  Present  arrangement  of  structural  units  with  the  subse-
quent formation of synform—antiform structures.

▲

TECTONIC STRUCTURE OF THE KIS-FENNSÍK AREA 233

The age of nappe emplacement(s)

The proposed Late Jurassic—Early Cretaceous emplacement

age  of  the  Szarvaskő-Mónosbél  (and  Harica)  nappes  (Balla
1987 and Csontos 1999) can neither be supported, nor denied
with our data set. The emplacement of the Kisfennsík Nappe
cannot be dated either, but wise guesses can be made. All the
structural units suffered intensive ductile deformation after the
arrival  of  this  nappe  (Fig.  7d).  Folding  seen  on  Section  1
(Fig. 3)  can  be  compared  in  intensity  to  folds  of  the  Central
and Southern Bükk Mts, which are suggested to form near the
Early Cretaceous peak metamorphism (Csontos 1999).

The Late Oligocene Csókás Formation lies transgressively

only  on  the  Kisfennsík  Limestone.  If  the  Kisfennsík  Nappe
was  emplaced  after  the  Late  Oligocene,  one  could  expect
some kind of Paleogene (including Csókás Formation) under
the nappe surface, too. Such a situation was never observed.
Later  investigations  on  the  position  of  also  transgressive
Eocene (Middle-Upper Priabonian, Less et al. 2000) deposits
in the NE Bükk (Fig. 6) may shrink the time interval available
for deformations. The striking differences between the Csókás
Formation and other, coeval facies can be explained by facies
changes on differentiated basement topography. Csókás facies
can indicate a general uplift of basement and subsequent trans-
gression (Báldi & Sztanó 2000), covering the most elevated,
shallowest  part  and  consequently  the  highest,  already  em-
placed structural unit. On the basis of the above reasoning, the
nappe  and  sliver  emplacements  and  subsequent  NE-SW  ori-
ented folding was most probably generated during Cretaceous
tectogenesis, which is anyway structurally the most intensive
period in the whole Bükk Mts.

Conclusions

On the basis of mapping and complex structural evaluation

of part of the Kis-fennsík area of the Bükk Mts, three structur-
al  units  can  be  differentiated:  the  North  Bükk  Anticline,  the
overlying Harica and Kisfennsík nappes. An intermittent sliver
originating  from  the  North  Bükk  Anticline  complicates  the
structural  edifice  (Fig.  2).  The  Jurassic  shales  of  the  Harica
Nappe  were  thrust  upon  the  northernmost,  Paleozoic-Upper
Triassic part of the Bükk Parautochthonous Unit. In contrast to
the Southern Bükk, it is supposed that this area was already af-
fected  by  incipient  folding  prior  to  the  emplacement  of  the
Harica Nappe, then they were folded together. On the southern
margin  of  Kis-fennsík  an  important  tectonic  contact  can  be
mapped,  it  is  thought  to  be  the  continuation  of  the  earlier
known Szeleta thrust (Fig. 6). Along this contact a Middle-Up-
per Triassic segment (Szeleta sliver) of the North Bükk Anti-
cline  was  thrust  onto  older  formations  of  the  Anticline  and
onto the Jurassic shales of Harica Nappe. Dark shales mapped
on this southern margin and found at the bottom of the bore-
hole Varbó-74, as well as shales east of Lillafüred are related
to the Vesszős Shales. A conodont first described from these
Vesszős Shales indicates a Carnian age for this formation. This
structural edifice was topped by the Kisfennsík Nappe, which
came from the NW. This nappe contains dolomites, Kisfennsík
platform limestone of Carnian age and several horizons of ba-

saltic  volcanites  (Szinva  Metabasalt)  intermixed  or  interlay-
ered with the limestone. Structural measurements demonstrat-
ed  intense  post-emplacement  folding  in  several  directions.
Earlier structures, such as nappe boundaries and the E-W axis
of  the  North  Bükk  Anticline  were  deformed  by  NE-SW  and
NW-SE axis, often map scale folds. Several thrusts were also
formed  possibly  related  to  folding,  but  also  dissecting  them.
Nappe  emplacement  can  be  tentatively  put  into  the  Creta-
ceous,  so  the  Csókás  Formation  of  Late  Oligocene  age  was
transgressing onto an already emplaced Kisfennsík Nappe.

Acknowledgments: We are greatly indebted to György Less,
who  participated  in  the  work  with  helpful  criticism.  Sándor
Kovács  is  thanked  for  Conodont  determinations,  Norbert
Németh for suggestions for map construction and field assis-
tance,  Pál  Pelikán  and  Felicitász  Velledits  for  useful  com-
ments;  Péter  Dövényi  for  a  quick  geophysical  survey,  and
members of the University of Salzburg, Geological and Pale-
ontological Institute for technical assistance. The authors are
thankful for the reviewers J. Hók, Gy. Less and D. Plašienka
for valuable suggestions to improve the manuscript. This work
was financially supported mainly by Péter Forián Szabó.

Appendix

Geological map of W Kis-fennsík (Bükk Mts, NE Hungary).

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