Post-Conference excursion: Tectonics, sedimentation and
magmatism along the Darnó Zone
László Fodor1, Gyula Radócz1, Orsolya Sztanó2, Balázs Koroknai1, László Csontos2, Szabolcs
Harangi2
1
Geological Institute of Hungary (MÁFI), Budapest, 1143 Budapest Stefánia 14, Hungary
2
Eötvös University, Budapest, 1117 Pázmány Péter sétány 1/C, Hungary
Introduction
The Darnó Line or Darnó Zone is an important NNE-SSW trending structural element,
which is located in north-eastern Hungary and reaches the southernmost part of the Slovak
Republic (Fig. 1). Although the tectonic zone played important role in structural evolution of
the Gemer–Bükk area, and despite numerous works dedicated to different aspects of its
characteristics, understanding of the geometry, kinematics, and temporal evolution of the line
still needs further research. In our excursion guidebook we intend to contribute mainly to the
structural and sedimentological aspects of the ‘Darnó Zone problem’. Because of close
geographic location to the CETeG-3/2005 Meeting, and because of available abundant
information, the excursion will focus on the middle segment of the long Darnó Zone.
As a guidebook, we will not present a complete and coherent analysis. Our main
intention is to present some relatively new structural and sedimentological data and
interpretations, which can help to better understand the complex deformation history of the
zone as well as its role in sedimentation and magmatism. we describe this evolution in time
slices with the help of simplified maps of surface formations and structures.
The introductory chapters give a short summary of basic data in a more available
English version, because some good works about the Darnó Zone exist only in Hungarian (for
example, the last very good and detailed one edited by Szebényi, 2003). During this
compilation, we inevitably shortened and eventually reinterpreted the original Hungarian
works.
Definition of the Darnó Zone and related structures
In this chapter we give a temporary definition of the main different structural and/or
paleogeographical elements, and we describe their major characteristics. The Darnó Line was
defined in the surroundings of the Darnó Hill by Telegdi-Roth (1937) after the unpublished
work of Rozlozsnik and Schréter (carried out in 1936-1937, published 1942). In this work we
use the term Darnó Fault to refer to this peculiar, well-defined structural element. On the
surface it can be followed from the Darnó Hill up to Bükkszék. On the Darnó Hill the fault is
between Mesozoic and early Miocene formations while more to the north, the eastern fault
block is younger.
The Darnó fault is a moderately dipping thrust, putting Mesozoic onto Oligocene
sediments. The overthrust relationship was demonstrated in boreholes (Majzon 1940; TelegdiRóth 1951; Schréter 1951) and in subsurface galleries (Szentes 1951a). The western footwall
of the fault is characterised by gentle folds or WNW dipping monoclinal fexures (Jaskó
1946). The reverse fault and the folds are clearly related to each other (Szentes 1951b). The
reverse separation can reach 1 km at least. Telegdi-Roth (1951) noticed that the reverse
displacement was changed to normal (or oblique-slip) motion, after the Eggenburgian; this
reactivation explain the younger rocks in the hanging wall of the Darnó Thrust.
On the other hand, it is to note that “horizontal crustal displacement” was supposed
along the Darnó Fault already at the beginning of the research (Jaskó 1946). Supported by
several small-scale observation and map-scale structural analysis, the strike-slip character of
the Darnó Line was emphasised (Zelenka 1975; Szalay and Zelenka 1979; Zelenka et al.
1983).
The Uppony Fault (or Thrust) bounds the Uppony Hills on the NW side and separates
Paleozoic formations from Triassic, Oligocene and early Miocene rocks (Schréter 1945; Pantó
1954, 1956). This fault has very similar character than the Darnó Fault, dipping moderately to
steeply south–eastward (Schréter 1945; Pantó 1954). The internal geometry is discussed in the
description of Stop 3.
These two elements share the same tectonic role and can be connected as Darnó Line
(DL) (Pantó 1956) up to the Rudabánya Mts. Further NNE-ward the DL is running along the
eastern margin of the Rudabánya Mts. up to southernmost Slovakia (e.g., Pantó 1956; Radócz
1987; Szentpétery 1997; Vass 2002). Alternative definition would place the Darnó Line at the
western boundary of the Rudabánya Mts. or at both margins of the Rudabánya stripe
(Hernyák 1977; Grill et al. 1984; Less et al. 1988). Radócz (1987) described the role of the
DL as separating more than 1km thick Oligocene to Eggenburgian (lower Burdigalian)
sequence at the western side from the reduced or completely lacking sediments of the same
age. He also noticed a graben at the western side of the DL filled with thick Ottnangian coalbearing suite. The thicker sedimentary pile of the western fault block is clearly reflected by a
negative Bouguer anomaly belt stretching from the Mátra to the Rudabánya Mts (Szabó and
Sárhidai 1989). The eastern, strait boundary of this belt seems to correspond to the Darnó
Line.
The West Borsod ridge is a newly defined term unifying the Uppony–Szendrő and
Darnó ridges of Radócz (in Szebényi 2003) (Fig. 1). The ridge is located east from the Darnó
Line and marked with the complete lack of marine early Oligocene and the sporadic
occurrence of restricted brakish to marine early Miocene (late Egerian? to Eggenburgian)
sedimentation (Radócz 1964; Radócz in Szebényi 2003, Báldi 1986). The ridge is also
characterised by terrestrial sedimentation occurring in several levels from late Egerian? to
Eggenburgian (before the ‘lower tuff level’). The ridge was later covered by the ‘lower
rhyolitic tuff level’ (Eggenburgian/Ottnangian) and/or by brakish to marine Ottnangian–
Karpatian and middle Miocene sediments.
The eastern extension of the ridge is variable. In the south, near the Darnó Hill the
ridge is narrow and is bounded by the Southern Bükk Paleogene depression. The ridge widens
to the western margin of the Bükk Mts., and narrow again northeast from the Uppony Hills,
constrained by the East Borsod Oligocene Depression. It is still open question, at what extent
the Bükk Mts. were covered by marine early Oligocene sediments. However, the possible
sedimentary cover was removed before the Egerian transgressive events (Báldi and Sztanó
2000).
The Nekézseny Fault (Schréter 1945, 1953) separates the Uppony Paleozoic from the
Bükk-type Paleozoic–Mesozoic formations at the southern margin of the Uppony Hills, while
the Tapolcsány Fault (Fodor et al. 1992) is located along their south–eastern boundary. The
two faults differ in fault kinematics and timing of activity (Fodor et al. 1992).
The term Darnó Zone can be used in a wide or in a restricted manner. The Darnó
Zone (DZ) sensu stricto can incorporate the Darnó Line, the Darnó Hill, Uppony Hills with
their southeastern boundary fault (Tapolcsány Fault), and finally the Rudabánya Mts. The
Darnó Zone sensu lato may contain all the deformed area, which are geometrically loosely
related to the Darnó Line. We adopt here the term Darnó Deformation Belt (DDB,) slightly
modifying the term Darnó Fault Belt of Vass (2002) for this wide deformation zone. This
definition may permit all structural elements of Tertiary age to be included in the belt. the
DDB extends form faults near Ózd and Bükkszék up to the Bükk Mts (Fig. 1).
The Darnó Zone can be divided into three segments; the southern part is located below
and south of the Mátra Mts. The middle segment stretches from the Mátra Mts. to the
southern tip of the Rudabánya Mts. (north from the Sajó/Slaná river), while we understand
northern segment of Darnó-related structures north form southern tip of the Rudabánya Mts.
In our guidebook we will mainly concentrate to the middle segment, while discussing briefly
information derived from the two extremities.
The southern segment of the Darnó Zone is covered with late Miocene to Quaternary
sediments and the middle Miocene volcanic suite of the Mátra Mts. Its south-southwestern
termination is not precisely defined, presumably it joints the ENE trending Balaton–Tóalmás
shear zone (Tari et al. 1992; Fodor et al. 1999). From the northern slope of the Mátra Mts.
surface outcrops, borehole and geophysical data permit to define relatively precisely the
(map-view) geometry of the middle segment of the zone. The northern segment was mapped
in the 80’s (Less et al., 1988). Together with borehole data, some other publications (Grill et
al. 1984; Szentpétery, 1997; Less 2000) discuss structural aspects of the zone. In contrast to
the middle segment, coverage with geophysical data are weaker.
Several volcanic intercalations are known in the Darnó Deformation Belt. The ‘lower
rhyolite tuff level’ has been dated as ~19Ma (Márton and Pécskay 1998) and referred as latest
Eggenburgian or earliest Ottnangian (Vass et al. 1988; Gyalog 1996). An informal ‘middle
Burdigalian’ nomination can also be used. The ‘middle rhyolitic level’ in fact incorporate
dacite tuffs and volcaniclastic flows. These volcanic events occurred in the latest early
Miocene (late Karpatian) or mainly in earliest Badenian (Radócz 1975, 2004). We integrated
all volcanic units to the ‘upper tuff level’ occurring in the late middle Miocene (Sarmatian) to
early late Miocene (early Pannonian).
Tertiary paleostress fields
Bergerat, Fodor and Csontos (1988, 1989) made observations on mesoscale fractures,
which permitted the reconstruction of Tertiary evolution of stress field. Expect for few sites,
most of the data were not published only summarised in unpublished reports of Csontos
(1988b) and Fodor et al. (1992). The sites are located partly in Paleozoic of the Uppony Hills,
partly in Senonian, few are from the Jurassic of the Darnó Hill area, while one third of the
sites are in Tertiary sediments. Tertiary sites do not always yielded well-constrained stress
axes, however, they are important for the time constraints of the determined stress field.
WNW-ESE compression and perpendicular tension is characterised by conjugate
strike-slip faults. Dextral strike slips are trending WSW-ENE, while sinistral ones are oriented
NW-SE (Fig. 3). The corresponding structures are represented by 1-10 cm wide deformation
belt in the Eggenburgian sandstone (see Stop Ózd). While this stress field affected the
Eggenburgian sandstone, it was still active at the Eggenburgian-Ottnangian boundary. Using
constraint from paleomagnetism (see later) the end of this stress field might have occurred in
the early Ottnangian (~19–18,5 Ma). Lower time constraint cannot be given, but analogy from
the Buda Hills may suggest persistent stress field from late Eocene (Fodor et al. 1999).
NW-SE compression was only observed in the Paleozoic and Senonian rocks of the
Uppony Hills (Fig. 3). While σ1 was always horizontal, σ3 could be vertical or horizontal,
giving a sort of transpressional character to this stress field. The main conjugate strike-slip
faults often have reverse component of slip. Sinistral faults are ~N-S, dextral faults are ~E-W
tending. It is difficult to give narrow time span for this stress field, apart from its clear postCampanian age. It is also delicate to separate it from either the stress fields with WNW-ESE
and NNW-SSE σ1; NW-SE compression can be a local deviation from either of the two other
stress fields. Stop 2. may give a hint that this phase was younger than WNW-ESE
compression.
NNW-SSE compression – ENE-WSW tension characterised a phase of strike-slip
faulting. Locally, only normal faults or joints developed parallel to the maximal horizontal
stress axis. The dominant sinistral strike-slip faults are trending NNE-SSW, parallel to the
Darnó Zone. Dextral faults are oriented NW-SE. Strike-slip faults affected Paleozoic,
Mesozoic, and Senonian rocks. In Sirok a very good example of faulted ‘middle tuff’ was
published (Fig. 3) (Bergerat et al. 1984). Radócz (1964, 2001) observed gently dipping
slickenside lineations in several boreholes, probably related to sinistral slip, in Ottnangian to
Badenian sediments. The Sirok site (STOP 6) also gives the minimal upper time constraint for
the deformation; Ottnangian to Karpatian sediments and the ‘middle tuff level’ were involved
in the faulting. Chronology of paleomagnetically determined rotations would suggest an early
Ottnangian (~18,5 Ma) beginning and middle Badenian (~15 Ma) termination of sinistral
strike-slip faulting (Márton and Fodor 1995). This time span would agree with the main
rifting phase of the Pannonian basin (Fodor et al. 1999).
ESE-WNW tension is characterised by horizontal tensional axis trending ESE-WNW
to SE-NW (Fig. 3). The maximal stress axis σ1 is generally vertical but locally horizontal,
leading to local strike-slip-type deformation. The main outcrop-scale structures are NNESSW to NE-SW trending normal faults. Outcrop-scale conjugate strike-slip faults occur
locally but not well represented on map scale. This stress field marks the youngest
deformation phase in the area. While Ottnangian-Karpatian (late Burdigalian) sites show the
effects of two stress fields, middle Miocene sites were affected only by this tension stress.
This youngest tension was established in the Badenian. More precise timing cannot be given
in the Darnó Deformation Belt, but projection from the southern Bükk, and comparison with
paleomagnetic data (Márton and Fodor 1995) would suggest the change in stress field in the
middle Badenian (around ~15 Ma).
Paleomagnetic data and stress field evolution
There is only one paleomagnetic data from the Darnó Deformation Belt. However,
The area both NW and SE from the DDB are well covered with Tertiary sites of successful
paleomagnetic analysis (Márton and Márton 1996). Two phases of counterclockwise rotation
were documented in the Miocene and no rotation was detected in the late Paleogene. The first
rotation affected the ‘lower rhyolite tuff level’, while the second occurred after the ‘middle
tuff level’, while the ‘upper tuff level’ was not rotated. Radiometric ages of volcanic levels
(Balogh et al. 1993; Márton and Pécskay 1998), their magnetostratigraphy (Radócz et al.
1999) and biostratigraphy of intercalated sediments (Radócz 2001, 2004) suggest that the first
rotation occurred between 18-17 Ma (~Ottangian), while the second between 15 and 14 Ma
(Badenian) (Márton and Márton 1996). It is to note that no rotation was detected in rocks
younger than 14 Ma, although the number of sites is small.
Márton and Fodor (1995) correlated rotations with changes of the stress field. They
concluded that time spans of rotations are similar to time spans of stress field change. In
addition, rotation angle is very similar to the change in the direction of maximal horizontal
stress axes; only the sense of rotation is opposite for block rotation than for stress field
change. They suggested that the change in stress field was mainly apparent, and rotation
induced an apparent change in stress field, while the real, far-field stress field did not change
significantly from ~N-S compression and/or ~E-W tension. If this supposition were true, the
changes in stress field should occur at the same time as the well-constrained rotation events.
Therefore, the chronology of paleomagnetic events is used here to constraint the time of
change in stress field and thus deformation style.
Magmatism
In the Darnó Deformation Belt, several Miocene volcanic horizons are present
(Radócz 2001). Some of these levels can be connected to the south Bükk volcanic rocks and
to the Salgótarján basin (Harangi, 2001; Harangi et al. 2005), while correlation with the
Central Slovakian volcanic field or to the Tokaj Slance Mts. is also possible. In this chapter
we give a short description, concentrating on the position of the volcanic horizons, after the
works of Schréter (1929), Jaskó (1952), Pojják (1963), Radócz (1975, 1993, 2001), Radócz et
al. (1999).
One volcanic event can be suspected in the Eggenburgian Pétervására sandstone on the
basis of altered montmorillonitic clay horizon and coincides in time with the
Burdigalian/Eggenburgian maximum flooding. In the post-Eggenburgian sediments, three
main and several smaller volcanic intercalations can be determined. The ‘lower rhyolite tuff
horizon’ deposited on a slightly undulating denudation surface. Welded ignimbrite with
fiammes are lacking in the DDB, in contrast to the South Bükk volcanic field, where they are
frequent (Cappacioni et al. 1995; Szakács et al. 1998; Harangi et al. 2005). In the southern
part of the DDB, 30-40m thick ignimbrite/surge deposits occur with fluidal bombs in
boreholes and in a few outcrops. The volcanic rocks are variably altered and show redeposited
character northward. Although the thickness of the unit is generally decreasing, but in the
Felsőnyárád synform it is more than 100m; in this case the large thickness is due to
redeposition of volcaniclastics in terrestrial environment and do not represent original
pyroclastic flow or air-fall deposits. These observations suggest that the eruption centre could
be located in the southern DDB (Schréter 1929; Radócz 2001).
The ‘middle rhyolite tuff level’ is dominantly dacitic in composition. The volcanic
sequence is interfingering with lower Badenian strata (tuffitic sandstone, marl, clay) (Radócz
2004). The thickness of the volcanic level is some tens of metres; the maximal thickness of
70m occurs near the Bükk Mts. (Szilvásvárad). Away from this site, the decreasing thickness,
size of pumice, and the increasing redeposited character may also suggest that the eruption
centre could be close.
Sarmatian magmatism is marked with rhyolite tuffs and andesitic volcanoclastic rocks.
The rhyolite tuffs are classified as the ‘upper tuff level’ although they are not strictly coeval
and occur at several stratigraphic positions. Segregation pipes are abundant (Stop 1).
Andesitic tuffs and agglomerates are generally 20-50m thick, although locally they
thicken to 100-400m. Volcanic dykes, inclusions of Paleogene or basement rock lithoclasts
are frequent. Volcanic centres (pipes) were identified near the Sajó valley (Radócz 2001). It is
possible that the ascending magma could use fault branches of the DDB, without large
displacement along the faults themselves. Lahar-type deposits also occur, and volcaniclastics
are interfingering with fluvial gravels, redeposited, hypersthene-bearing sand layers, finegrained tuffitic marl layers with leaf prints.
Finally, at the base of the late Miocene (Pannonian) the ‘uppermost rhyolite level’
can be detected, mainly NE and N from the East Borsod Depression.
Geochemistry and genesis of the magmatic rocks
The Neogene volcanism of the Carpathian-Pannonian Region started with repeated
explosive eruption of silicic magmas. They resulted mostly in non-welded and partially to
densely welded ignimbrites, which cover large areas in the Pannonian Basin. Most of these
deposits are covered by Late Miocene to Quaternary sediments, however, excellent outcrops
almost all of the pyroclastic horizons occur in Northern Hungary, particularly at the South
Bükk (Bükkalja) region (Capaccioni et al. 1995; Szakács et al. 1998; Lukács et al. 2002;
Harangi et al. 2005). Since this volcanism occurred during a long time interval, from 21 Ma to
13.5 Ma (Márton and Pécskay 1998), these pyroclastic deposits have great stratigraphic
importance, as well as, they provide valuable information about the magmagenetic processes
during the formation of the back-arc basin area. Trace element composition of the glasses and
the major element composition of the phenocrysts proved to be good discriminative tools to
correlate the scattered outcrops of the ignimbrite units. Based on these data, Harangi et al.
(2005) distinguished four ignimbrite units in the Bükkalja Volcanic Field, Northern
Pannonian Basin. Each of these units is characterized by specific geochemical fingerprints.
The Th, Nb, Y and the rare earth element concentration of glasses are effective discriminator
elements. The modal composition of mineral phases (occurrence or lack of certain minerals)
and chemistry of plagioclases and biotites are also good correlation tools, especially the Fe,
Mg and Ti contents of biotites. The in-situ trace element composition of glasses, representing
the liquid part of the erupted magma, can be also used to constrain the petrogenesis of the
rhyolitic magmas. The trace element ratios such as La/Nb, La/Y and Th/Nb suggest the
importance of minor (e.g., hornblende and ilmenite) and accessory (e.g., zircon, allanite)
minerals controlling the composition of the erupted melt. The rhyolitic magmas could evolve
from a mantle-derived metaluminous andesitic parental melts via fractional crystallization.
Crustal anatectic origin is not supported by the composition of specific minerals (e.g.,
orthopyroxene and biotite; Harangi et al. 2005) and the zircon morphology data (Szabó and
Harangi, 2001). Syn-eruptive magma mingling was detected in the genesis of the Middle
Ignimbrite Unit, based on the strong intra-sample geochemical variation both in the glasses
and in the phenocrysts (Czuppon et al. 2001; Harangi et al. 2002). Mingling of rhyolitic
magmas was pointed out by Lukács and Harangi (2002) in the Harsány ignimbrite (Upper
Ignimbrite Unit). Sr and Nd isotope ratios of pumices and cognate lithoclasts show a gradual
change from the pre-extensional to the syn-extensional ignimbrites suggesting a decrease of
crustal involvement in the genesis of the magmas (Harangi 2001). It is noteworthy that during
the late-stage of the silicic volcanism (about 13-14 Ma) eruption of different rhyolitic magmas
occurred fairly close to one another (Lukács and Harangi 2002; Harangi et al. 2005).
Darnó Deformation Belt through time
Mesozoic
The possible role of the Darnó Zone in the Mesozoic structural evolution is somewhat
controversial. Zelenka et al. (1983) clearly demonstrated that the same Mesozoic rocks occur
on both sides of the Darnó Fault, at least in the Darnó Hill–Recsk area. However, in the
northern segment, the branches of the Darnó Zone sensu stricto juxtapose different Mesozoic
and Paleozoic successions. This is the case between the Aggtelek and Bódva units in the
Aggtelek-Rudabánya Mts., between Rudabánya Mesozoic and the Uppony and Szendrő
Paleozoic. This juxtaposition clearly post-dates first-order nappe stacking (Grill et al. 1984;
Less 2000), and can be the results of top-to-NW trust or strike-slip on the Darnó Zone faults,
and the combined, selective erosion of the eastern fault block. In consequence, the Darnó Line
does not represent first-order nappe/terrain boundary during the late Jurassic-Cretaceous
orogeny.
On the other hand, some structural data may indicate that the Darnó Line, or a
precursor structural element might have accommodated considerable deformation during the
late Mesozoic or earliest Tertiary. All stratigraphic units and structural elements of the
western Bükk Mts., Szendrő Hills show a curvature approaching the Darnó Line (Zelenka et
al. 1983; Csontos 1988a, b). This deformation was interpreted as a sinistral drag due to the
Darnó motion. Zelenka et al. (1983) attributed late Paleogene to middle Miocene age of this
structure. However, the torsion seems to be accommodated by ductile or brittle-ductile
structures, it might have happened only at considerable burial before middle Eocene (Csontos
1999).
Ductile deformation within the Darnó Deformation Belt were studied in the Uppony
and Szendrő Hills built up by low-grade Early Paleozoic sequences, futhermore in the (very)
low-grade Permo-Mesozoic metasediments of the Rudabánya Mountains (Torna unit) (Fodor
and Koroknai 2000; Koroknai 2004). According to available geological and geochronological
data all units mentioned above suffered (Late Jurassic?–) Cretaceous metamorphism related to
crustal thickening during the Alpine orogeny (Árkai et al. 1995). The earliest deformation
(D1) is represented by the formation of a bedding-parallel fist foliation (S0-1) associated with
intensive flattening in the bedding plane as a consequence of an early folding and/or nappe
thrusting.
The most characteristic deformation (D2) is resulted in the folding of the beddingparallel first foliation (S0-1) into upright to moderately inclined, close to tight (locally
isoclinal), sub-horizontal to gently plunging, NW-vergent F2 folds. A well-developed,
generally SE-dipping, penetrative axial plane foliation (S2) was formed during this event. Fold
axes trend mostly to NE-SW, or ca. E-W. The bedding-parallel foliation is heavily transposed
into the „main” S2 foliation in many outcrops, suggesting intensive flattening during
progressive deformation.
In the Uppony and Szendrő units ductile simple shear occurred after/or at the late stage
of F2 folding, resulting in N(W)-vergent thrusting, generally with a slight sinistral component.
Coeval NE-SW trending sinistral strike-slip movements and thrusting suggest a
transpressional tectonic regime during deformation. However, no large-scale mylonitic zones
could be identified in the units studied. Structures related to the relatively high-temperature
D1 and D2 events are predominant in all units. The parallelism of ductile elements (sinistralreverse shear zones) and the Tertiary Darnó Line makes possible that Tertiary faults were
initiated along a weakened zone herited from the Cretaceous ductile deformation.
Eocene to early Kiscellian (early Rupelian – 37-30 Ma)
There is little if any differences in the latest Eocene to early Oligocene formations on
the two sides of the Darnó Line/West Borsod ridge; the same formations of comparable
thickness can be found in the Buda Hills–Mátra Mts. and in the other side, in the Southern
Bükk and East Borsod Paleogene areas (see comparative stratigraphic columns of Báldi,
1986). However, no such sediments are preserved on top of the West Borsod ridge and thus it
cannot be excluded that the Darnó Deformation Belt (the zone sensu lato) represented minor
and partial paleogeographic barrier already at that time.
The Darnó Line is close but not coinciding with the latest Eocene to mid-Oligocene
(Less in Szebényi 2003) Recsk andesite body. Is is possible that the onset of magmatism was
often related to deformation along the Darnó Deformation Belt (Varrók 1962; Zelenka 1975).
Late early Oligocene to early Miocene (late Kiscellian to Eggenburgian,
30-18,5 Ma)
Sedimentation
The Darnó Line represents abrupt thickness changes for the late Oligocene to earliest
Miocene (Egerian to Eggenburgian/Chattian to early Burdigalian), as described in previous
chapters (Radócz 1987). This was revealed already by the first exploratory wells (TelegdiRoth 1937, 1951), stratigraphical analysis (Báldi 1986), seismic reflection profiles (Szalay
and Zelenka 1979; Albu et al. 1983, 1985; Sztanó and Tari 1993), and the Bouguer anomaly
belt (Szabó and Sárhidi 1989).
The Darnó Line represents an original paleogeographic boundary of the Eggenburgian
basin (Sztanó and Tari 1993; Sztanó 1994). The eroding eastern margin was the source area of
clastic input represented by pre-Tertiary rocks of the West Borsod ridge, including some
ofiolithic nappes (Sztanó and Józsa 1996). The sediments reached the basin via fan-deltas,
which were arranged along the Darnó Fault (Sztanó 1994, Stop Kis-hegy). Some softsedimentary deformation (water-escape structures) may also point to a seismically active area
in the vicinity of the Darnó Line (Sztanó 1994; and Stop 4).
The Darnó Line separated marine Eggenburgian sedimentation area and sporadic
terrestrial sediments (gravels, conglomerates, coal seams and variegated clays) occurring over
the West Borsod Ridge. Uplifted position of the ridge is further indicated by restricted
sedimentation area. The Felsőnyárád synform was characterised by terrestrial to brakish
sequence on top of the ridge (Radócz 1964).
Map-scale deformation pattern, stress field, fault kinematics
Oligocene to early Miocene structural elements of the Darnó Zone can be best studied
with subsurface data geophysical methods because of later sedimentary cover and structural
reactivation. Interpretation of seismic reflection profiles permits the description of the
Oligocene to early Miocene basin west of the Darnó Line. The basin can be considered as a
large asymmetric syncline with gentle WNW and steep ESE limb (Fig. 4) (Petrovics et al.
1992; Sztanó and Tari 1993). The syncline is bounded by top-to-WNW reverse faults on its
ESE limb (Szalay and Zelenka 1979; Albu et al. 1983, 1985; Braun et al. 1989; Petrovics et
al. 1992; Tari et al. 1993; Sztanó and Tari 1993). Some reverse faults are covered by younger
sedimentary layers, while the easternmost one has surface rupture. In fact, this is the fault
what is considered as Darnó Line on the surface. This branch seems to bound different
basement units as well, while having Triassic rocks in footwall and Paleozoic in its hanging
wall and incorporate the surface-rupturing Uppony and Darnó Thrust. On top of the hanging
wall block the Felsőnyárád synform could represent a piggy-back syncline folded during
Eggenburgian sedimentation, as revealed by cross section and facies distribution (Fig. 4, 5)
(Radócz 1964). More to the SE, the East Borsod Oligocene basin could be interpreted as
situated on the back-limb of the major anticline or bordered by SE-vergent backthrust or
monocline.
This geometry is in agreement with the sections based on boreholes in the Darnó Hill–
Bükkszék area. Here geological maps also indicate a large monocline or syncline, because the
oldest Kiscellian rocks are cropping out near the Darnó Fault and formations show a
stratigraphical younging toward NW (Fig. 2, 4). Small anticlines in the close vicinity of the
Darnó Fault can be secondary structures related to the main reverse faults. The large syncline
along the Darnó Line widens in the area of Ózd, north from the Darnó Hill. We suggest that
the widening belt are also marked by folds. Related to this, the north-trending faults from the
Darnó hill to Ózd could be reverse faults in that 30-18,5 Ma time span, but reactivated later by
normal slip. The present-day faults reactivated segments of a salient, which gradually turn
away from the Darnó Line. Further to the north, below the Rimava depression Vass et al.
(1989) and Vass (2002) described the Šafarikovo high and the Abovce depression; in our
interpretation these elements could belong to similar folds (anticline and syncline or flexure)
like in the Hungarian side of the basin.
It is possible that the reverse faulting included the Senonian of the Uppony Hills.
Thrusting of the Bükk Paleo-Mesozoic onto the Uppony unit might have occurred along the
Nekézseny and Tapolcsány faults, although other timing can also be given. This deformation
was marked by NW-SE compression (Fig. 4).
WNW-ESE compression can clearly be documented for this time span in the DDB
(Fodor et al. 1992, Márton and Fodor 1995). NW-SE compression is poorly dated, but may
represent local deviation from WNW-ESE compression. Both stress fields are in agreement
reverse faulting and folding along the Darnó Zone. The resolution of the stress field
determination is not precise enough, saying if the motion was clearly reverse, or associated
with strike-slip component. Noticeable sinistral slip was suggested on the basis of
Eggenburgian sedimentation (Stop 4) and slight obliquity of folds with respect to the main
Darnó Line (Fodor et al. 1992 and stop 4); however, all these signs can be considered as
indications and not a definite proof.
Petrovics et al. (1992) and Sztanó and Tari (1993) realized that the sediments
gradually downlap on the gently tilted NW limb of the syncline and the depocentres seem to
migrate northwestward. On the base of this geometry, Sztanó and Tari (1993) suggested that
reverse displacement of the Darnó Line and related subsurface faults resulted in loading of the
western block and its subsidence as a foreland-type depression. Thus, the Darnó Line can be
defined as a surface-rupturing, late Kiscellian to Eggenburgian reverse fault bounding a small,
asymmetric flexural basin. Our suggestion is that the moderate west-dipping geometry of the
eastern margin can be connected to a sort of ramp anticline, related to a blind reverse fault,
having its flat part at or below the base-Tertiary surface (Fig. 4).
Seismic sections demonstrate a more or less continuous contractional deformation
along the Darnó Line. The first motion could occur in the late Kiscellian (around 30 Ma) as
revealed by deformed sedimentary wedge on seismic sections. Internal unconformities and
covered reverse faults suggest more or less continuous evolution during the late Oligocene
and early Miocene. On the surface we have several arguments for Eggenburgian activity of
the DDB. These observations demonstrate that the Darnó Line is clearly syn-sedimentary
structural element from the late Kiscellian to the end of Egggenburgian. Initiation of the
Darnó Line in late Kiscellian invoke the possible connection to the Tóalmás-Balaton and
Periadriatic Line. However, the kinematics of the two fault system were different; dextral
along the Pal-Balaton, reverse along the Darnó Line.
Ottnangian–middle? Badenian (18,5-15 Ma)
Eggenburgian/Ottnangian lower rhyolite level, and the overlying marine Ottnangian–
Karpatian–early Badenian (late Burdigalian–early mid-Miocene) succession covers major
displacement of the Darnó Line. It is clearly shown by the cross sections of Radócz (1964)
(Fig. 5). Thus, the ‘lower rhyolite level’ constraints the main Darnó displacements before ~19
or ~18,5 Ma.
During the ~18,5-15 Ma time span the strike-slip type stress field induced sinistral
faulting along the Darnó Line and sinistral-normal faults, which are slightly oblique to DL
(Fig. 6). Either the sinistral slip reactivated SE dipping, pre-existing Darnó-type reverse faults
or strike-slip faults cut across earlier reverse faults. It is possible that very minor shortening
continued in some folds, particularly along restraining bends of the Darnó Line. Small-scale
reverse faults in coal seams (Radócz 1964) are oblique to the Darnó Line, in agreement with
its dominant sinistral slip and NNW-SSE compression.
This phase may accommodate the largest Tertiary sinistral displacement, as suggested
by earlier workers (Zelenka et al. 1983; Vass 2002). The best displacement markers are
described in the northern segment of the DZ. Along the eastern margin of the Rudabánya Mts.
narrow stripe of steeply dipping Eggenburgian conglomerate (Szuhogy Fm) is close to coeval
or slightly older marine shoreface limestone and siltstone. Tectonised character, and pebble
composition of this body was interpreted to suggest strike-slip motion (Szentpétery 1997). On
the north–eastern side of the Rudabánya Mts. the isolated subsurface occurrence of marine
Egerian? sediment (Tbr-1 borehole) was connected to the marine basin west of the DL. In this
case, 10-20km sinistral separation can be estimated between the E-W trending shoreline in the
Aggtelek and Rudabánya Mts. and the intra-basinal sediments (Szentpétery 1997) although
the exact value is questionable (Fig. 6).
It is to note, however, that the Miocene displacement did not necessarily accommodate
all sinistral shear reflected by the bending of the pre-Tertiary structures. On the other hand,
we suggest that the most of the vertical slip component of the Darnó Line was accommodated
before the sinistral slip. This two-stage evolution of the Darnó Line is more complex than
previously suggested “simple” sinistral strike-slip (Zelenka et al. 1983; Grill et al. 1984).
Moderate thickness variations of the Ottnangian-Karpatian to early Badenian marine
succession occur across N to NNE-trending faults of the Darnó Deformation Belt (Radócz,
1964, 1975; Báldi 1988). The displaced Eggenburgian sediments of the Rudabánya area and
the deformed ‘middle tuff level’ suggest pre-late Badenian timing of the strike-slip faulting.
The second CCW rotation (~15-14 Ma) post-dated this phase.
Late Badenian–recent (~15-0Ma)
Coeval with the second CCW rotation, the stress field changed to tensional with a σ3
oriented ESE-WNW, perpendicular to the Darnó Line. This stress induced normal slip of
Darnó-parallel faults within the entire DDB. As mentioned earlier, this resulted in normal
reactivation of earlier SE-dipping reverse faults (Fig. 2). In some cases, new normal faults
could form. Analysis of surface and subsurface maps, borehole data from and mining galleries
permitted the establishment of several hundred faults of 1-500 m separation (Radócz 1966,
1994; Juhász, 1966), partly resulted form the strike-slip, partly from the tensional
deformation.
Map-scale fault are oriented NNE-SSW to NE-SW. This deformation enhanced the
NNE-SSW trending graben system, which is sub-parallel to the Darnó Line. Grabens are
generally asymmetric and have a dominant dip (tilt direction). The changing polarity of
graben margin faults suggests the presence of accommodation zones and relay ramps. Such
grabens extend into the whole Darnó Deformation Belt, and continue south–eastward. The
normal faults disrupt the Mesozoic formations of the south-eastern Bükk Mts.; one bounding
normal fault is running near (below?) the conference location. NE-trending normal or
sinistral-normal faults occur in the East Borsod depression and along the southern margin of
the Szendrő Hills.
This phase involved the Ottnangian to early Badenian marine sequence. On the other
hand, Sarmatian andesitic and rhyoitic volcanic rocks and overlying late Miocene sediments
cover some of the faults of the Darnó Deformation Belt, in other cases show minor
displacement. North of the Sajó river, some faults of the Darnó Deformation Belt displace late
Miocene terrestrial formations (Radócz 1994; Less and Mello 2004). These examples suggest
modest latest Miocene to Pliocene faulting.
Stops of the excursion
STOP 1. Sarmatian rhyolitic tuff, Lénárddaróc, Kakarcsó Hill
The quarry on the Kakarcsó Hill, east of Lénárddaróc exposes the ‘upper rhyolite tuff
level’, while smaller outcrops show the associated clastic Sarmatian sediments (Fig. 7). A
nearby borehole Lénárddaróc Ld-2 revealed the stratigraphic position of the rhyolite tuff.
Above marine Badenian 6m gravel and tuffitic clay occurs, probably of Sarmatian age. The
overlying, 16m thick rhyolite tuff to lapilli tuff is similar to the outcropping rock. In the
borehole terrestrial clastic sediments cover the volcanoclastic rock.
The quarry exposes two volcanic units. The lower part of the quarry exposes massive
fine-grained lapilli tuff representing pyroclastic flow deposit. It contains pumice clasts with 13 cm size. Charcoal fragments up to 20cm long indicate terrestrial deposition of the
pyroclastic flow. This unit is overlain by a pyroclastic surge deposit exposed in the upper
level (Ilkey-Perlaki et al. 2001). This deposit shows cross bedding with truncation surfaces
and undulations. The upper unit contains large amount of accretionary lapilli suggesting a
possible phreatomagmatic mechanism of the volcanic eruption. They are particularly
concentrated in segregation pipes.
Few brittle structures permitted the reconstruction of a transtensional stress field (data
of Fodor and F. Bergerat, Paris). The largest strike-slip fault had 1m vertical separation
between the tuff and sediments. This faulting represents the youngest deformation of the DDB
(Márton and Fodor 1995). The site yielded a good paleomagnetic result. The bulk rotation
was practically zero (Márton and Márton 1996). All structural observation is agreement with
Sarmatian position of the tuff. Looking eastward from the quarry, morphological effect of the
Darnó Line can be seen on the forested slope of the Határ-tető.
STOP 2. Senonian conglomerate, Csokvaomány, road junction
The small quarry is located near the road junctions Lénárddaróc and
Csokvaomány, close to the railways tunnel. Clifton et al. (1984) and Kovács and Haas (in
Szebényi 2003) summarised the major stratigraphical and sedimentological character of the
Senonian Nekézseny Fm, which is considered as Gosau-type sediment. The sequence of the
outcrop is composed of 1-3 m thick beds bounded by erosional surfaces. Beds often start with
variably cemented conglomerate passing upward to granule or (pebbly) sandstone. Locally
slight inverse-to-normal grading can be observed (Fig. 8). Some bedding contacts are not
sharp, conglomerate, sandstone and pebbly sandstone alternate between erosional surfaces.
Maximum size of gravels is about of 20cm and they often show imbrication (Fig. 8). Normal
graded, inverse-to-normal graded, imbricated layers were deposited from high-density
turbidity currents. Strata without gradation or imbrication could be formed by freezing at the
base of the turbidity currents or by grain flows. The sediments may have been originally
deposited on gravely deltas and redeposited by sedimentary gravity flows into a deeper basin.
The age of the unit is Campanian or Santonain-Campanian.
Pebble composition is important for paleotectonic reconstruction (Kovács and Haas in
Szebényi 2003). Senonian conglomerate mainly contains local Paleozoic rocks form the
Upony Hills, intra-Senonian clasts (sandstone, conglomerate and Rudist-bearing limestone),
Triassic rocks from the Bódva unit, quarzite and metamorphosed rocks out from the Uppony
unit. The lack of Bükk-type Paleo-Mezosoic rocks indicate that either the Bükk Mts were far
from the deposition of the conglomerate or were covered by units, which were completely
eroded. Mišík and Sikora (1980) published shallow-water upper Jurassic (Tithonian)
limestone pebble from this outcrop. The formation of this platform-carbonate is interpreted as
the sign of the closure of the Meliata ocean in the Gemer-Bükk region.
Conjugate oblique-slip normal faults dissect the moderately dipping beds (data of
Bergerat, Fodor, 1989). The intersection line of the faults is perpendicular to bedding plane
(Fig. 9). This suggests that the fractures were originated as strike-slip faults at horizontal bed
position. The back-titled faults show WNW-ESE compression and perpendicular tension. The
tilting itself was due to folding of the Senonian layers; nearby outcrops suggest NW-SE
compression during folding. The oldest pre-tilt faults and the folding could be correlated to
the 30-18,5 Ma phase, although pre-Eocene timing is also possible. The folded beds were also
affected by ENE-WSW tension, presumably from the 18,5-15 Ma-old phase.
STOP 3. Uppony Thrust, Uppony, entrance of the Csernely valley
This stop is located south from Uppony village, at the north-western margin of the
Uppony Hills. In the section at the entrance of the Csernely valley we can observe the general
geomorphic character of the Uppony Thrust and study structures in pre-Tertiary rocks of the
Darnó Zone (Fig. 10).
(1) The nortwestern boundary fault (Uppony Thrust, Schréter 1945) of the Darnó Zone,
expressed by a marked topographic change between the southeastern Paleozoic and the
northwestern Miocene, Oligocene and Triassic rocks. The thrust surface can not be observed
directly, but it was beautifully documented by Pantó (1954, 1956) in the subsurface iron
mining shafts (Fig. 10). His profile clearly demonstrates the chaotic character of this fault
zone: in the northwestern part of the profile variable, sheared blocks of metasomatized
(ankeritic) Triassic carbonates and marls are (partly) embedded in Miocene clay-matrix, and
this “assemlage” is overthusted by the Paleozoic (Middle-to Upper Devonian) formations
along a rather steeply SE dipping fault plane. This configuration was confirmed by the
borehole Uppony U-12 located at the creek (Fig. 10), which first penetrated an overturned
Paleozoic sequence. Below the Paleozoic rocks a heavily tectonized zone was reached
containing of blocks of Paleozoic, Triassic, Oligocene and Miocene formations.
(2) The strongly folded internal structure of the Uppony Paleozoic sequence can be studied at
the NW part of the valley (Fig. 10). The light grey, massive, platform-facies Uppony
Limestone Fm. (Middle-to Upper Devonian) is exposed in a narrow (~ 20-30 m) stripe. To the
SE, the wider stripe of the fine-grained, dark bluish-gray, well-bedded, pelagic Lázbérc Fm.
(Middle Carboniferous) is exposed with wonderful tight folds. Further to SE, in the
abandoned quarry again the light grey, massive Uppony Limestone Fm. is cropping out. This
profile was interpreted in different ways: Kovács (1982) regarded all major lithological
contacts as reverse faults, hence an imbricated structure was suggested with minor
(drag)folding (Fig. 10). In contrast, Koroknai (2004) interpreted this section as a map-scale
overturned syncline with parasitic folds observed in the core of the syncline (Fig. 10), which
was strongly overprinted by subsequent Miocene brittle tectonics.
STOP 4. Giant convolution, Pétervására Sandstone, Ózd-Somsály
Geological position
The Pétervására Sandstone (PS) is exposed in north Hungary and south Slovakia (Fig.
11). The PS consists mainly of medium- to coarse-grained cross-stratified glauconitic and
fine-grained weakly bedded to massive sandstones with conglomeratic intercalations.
Throughout the whole formation there is a fairly good correlation between grain size and
scale of bedforms (Sztanó 1994). Towards the west and the north various facies units show
deepening and the PS interfingers with coeval shallow bathyal sediments (Szécsény Schlier;
the correlative names are Fil’akovo Sandstone and Lučenec Schlier in Slovakia; Vass et al.
1988). In the east the formation is bounded by the Darnó Zone. Studies of molluscs (Báldi
1986) and of nannoplankton (Nagymarosy and Báldi-Beke 1987) indicate an Early Miocene
(Eggenburgian) age for the sandstones. The conglomeratic intercalations are not confined to
the top of the formation, as previously Báldi (1986) suggested, but occur in the lower units, as
well (Sztanó 1994).
Sedimentary facies
The dominant sedimentary structure is cross-stratification up to several meters in set
thickness, both in sandstones and in conglomerates (Fig. 12). Close to the supposed palaeoshoreline (ie. Darnó Zone) a fair amount of gravel-sized material occurs in the cross-bedded
sandstone, and also sets consisting purely of conglomerates appear. Coarse-grained material
always contains a great amount of bioclasts, mainly shell debris (Ostrea, Balanus, shark
teeth). Both the composition of pebbles (Fig. 13) and the mollusc assemblage shows close
relationship to the Darnó Conglomerate (see Stop 5).
Deformation of foresets caused by water escape is found at several localities, but the
largest (up to 4 m high) occurs in this outcrop (Fig. 14). Both vertically and laterally, zones of
massive gravelly sand with irregular lower and upper boundaries alternate with zones of
strongly deformed foresets, in which escape pathways can roughly be followed (Fig. 14.
Structures are dishes and pipes of extraordinarily large size and irregular spacing. The
deformational structures indicate that pore-water migrated along the foresets before bursting
out. Rearrangement of grains was confined only to areas where the semi-consolidated
sediment had become liquefied and mixed with the liberated pore-fluid. The original bedding
can usually still be recognized. Grain size in these foresets ranges from coarse-grained sand to
granules, and up to large pebbles. Sorting is fairly good within individual foreset beds, which
have a relatively large intergranular space. No clay-sized material is present, which could
have blocked the free movement of the pore-water and thus contributed to the generation of
over pressure. In the range of grain sizes, which are present, intergranular water moves freely.
The forceful dewatering therefore is explained as the result of very rapid sedimentation and
tectonically triggered liquefaction.
Depositional environment
Based on the occurrence of bundle sequences, reactivation surfaces and mud-drapes
(not visible in this outcrop), the large-scale cross-bedded sets are interpreted as tidal sand
waves driven northwards by strong ebb currents (Sztanó and de Boer 1995). The crossstratified conglomerate or pebbly sandstone units in the PS are lobate bodies with steep slip
faces interbedded within the field of sand waves. They built out northwards, parallel with the
main transport pathways determined by the strong tidal (ebb) currents (Sztanó 1994).
Petrographically the pebbles of the PS closely correspond to the Darnó Conglomerate (Sztanó
and Józsa 1996), and their source area must have been the same. With other words the clasts
first deposited on steep slopes of the fan deltas along the Darnó Zone, slightly later got
reworked and dispersed by the strong northward-directed tidal currents. Due to these
unidirectional currents such deltaic lobes might have been highly elongated and asymmetrical.
Parts of these lobes had been detached from their parent delta during the ongoing slow
relative sea-level rise (cf. Sztanó and Tari 1993), and thus become subject of mixing within
the sandwave field of the PS.
Coeval tectonics
An increasing amount of ophiolite-derived detritus was documented towards the
northeastern part of the PS (Sztanó and Józsa 1996). The strong shore-parallel (Darnó Line-
parallel) tidal currents forced the conglomeratic lobes to migrate from south to north, and
spread the sediments in a northward direction. Although this process may help the
accumulation of the ophiolite-derived detritus, it can be understood, when the lateral shift of
the source area with respect to the marine basin is also considered. Due to the lateral strikeslip of the Darnó Fault, every 'new' influx of sediment arrived into the basin north of the
previous lobe (Fig. 15). In this way bed-load accumulation in the northern part of the basin
was further promoted by the activity of the basin-margin faults. This interpretation of the
northward accumulation of the ophiolite-derived clastics points to a left-lateral strike-slip
activity of the Darnó Fault System during the Eggenburgian (Fig. 15). Alternatively, it can
reflect the north-northeast-ward propagation of the reverse Darnó Line, which resulted in
gradual exposure of the source hanging wall.
Post-sedimentary deformation
One of the most striking features of the PS is the varying rate of cementation. Usually
finer the grain-size the sandstone is more friable. Facies units made up of thin-bedded mostly
fine- grained sandstone with decimetre-scale cross bedding and mud-drapes makes elongated
horizontal nodules. Facies of large-scale cross-bedding is characterized by oblique nodules
(parallel to foresets). The hardest cement occurs at cross points of bedding with joints or
faults, like in the Ózd outcrop. In the outcrop, 1-10cm wide cemented zones can be
considered as deformation bands. Along the deformation bands, 1-15cm displacement can be
observed, using sedimentological markers. Within the deformation bands, 1-3mm thin zones
can be observed with different cementation and/or clay content and could accommodate the
bulk of the displacement. The deformation bands and the joints constitute a conjugate fracture
set. This geometry, the sense of displacement, and few, poorly developed striations point to
WNW-ESE compression and NNE-SSW tension (Fig. 16). The direction of compression is
perpendicular to the Darnó Zone, thus may indicate its reverse character.
STOP. 5 Darnó Conglomerate, Szajla, Kis-hegy
Geological position
The outcrop is located SE from Szajla village on the western side of the Kiss Hill.
Coarse conglomerates crop out only in a narrow zone between a post-early Miocene normal
or strike-slip and an older reverse fault bounding the Darnó Hill in the northwest (Fig. 11) and
were described as littoral in origin and Early Miocene in age (Schréter 1940, 1952). These are
disconformably overlying on pillow basalts of Triassic–Jurassic age (Dosztály and Józsa
1992), which provide the source of the bulk of the pebbles. The rest of the clasts are
radiolarites, siliceous slates and cherts, also cropping out nearby, east of the Darnó Fault (Kiss
1958). Faunal assemblage found in the conglomerate (Chlamys-Ostrea-Anomia) confirms the
Eggenburgian age (Early Miocene Loibersdorf-type fauna; Báldi 1986). Balanus-Ostreabearing strata reflect deposition in the highly agitated littoral water, while Ditrupa-bearing
horizons indicate depths below the wave base.
Sedimentary facies
The architecture of the steeply-dipping conglomerate beds is mostly unknown due to the poor
outcrop conditions. The thickest vertical section observed is 20 m, but in another outcrop beds
can be followed over 70 m in dip-direction. The conglomerates are badly sorted with grain
sizes ranging from very coarse sand to cobbles. Some of the beds show clast-supported fabric,
in the majority, however, clusters of gravel are floating in a very coarse sandy matrix. Faint
inverse grading is present in several beds, however major coarsening- or fining-upward
successions cannot be detected. The shape of the particles is determined by their parent rock:
many of them are flat (red slates, radiolarite, various kinds of schists and even diabase).
Roundness is mainly poor, angular clasts are common. Well-rounded pebbles and cobbles –
indicating either long enough fluvial transport or staying in the agitated coastal waters – were
found only in a few beds. Clusters of flat pebbles usually show a weak imbrication, dipping
upslope or are aligned subparallel to the bedding, pointing to mass flows. Others dip
downslope with a 10° greater angle than the dip of the beds, which may point to sliding of
pebbles. Beds of conglomerates dip dominantly towards the northwest with a wide deviation.
Dip angles are around 30°. Low angle differences between individual strata of 0.2-1.5 m thick
are common. A solitary package of oppositely dipping beds was observed at the Kis-hegy
section (Fig. 17). Elsewhere an asymmetrical internal erosional surface indicates gradual
infilling of a scour.
Depositional environment
Pebbly sands and conglomerates must have been deposited by sub-aqueous gravity flows on
the steep slopes. The great dip angle, the varying amount of sandy matrix, the clusters of
imbricated clasts suggest a certain strength and internal shear within the transporting medium,
thus high- concentration debris flows or grain flows are supposed. The lack of mud-sized
material has increased slope instability, low angle erosional features, backsets are, however,
the rare exceptions. The primary sedimentary structures of the Darnó Conglomerate suggest
that it was formed on steep slopes of a small coarse-grained delta, with steep profiles fed most
likely by a gravelly alluvial cone or by a line source of unstable bedload channels of alluvial
fans (Sztanó and Józsa, 1996).
Relative base level changes, coeval tectonics
Outcrop conditions, as well as its structural position prohibit the reconstruction of the delta
morphology. The Darnó fan-delta might have been closely related to the basin-margin fault
scarps of the Darnó Fault, which might have been active during deposition, and thus
determined the position of the palaeo-shoreline at that time. The coeval tectonic activity is
most obviously shown by the distribution of specific assemblages of pebbles and minerals
(i.e. the northward-increasing rate of accumulation of the ophiolite-related detritus) in the
sediments situated westward of the Darnó Fault System (Fig. 13, see discussion at the Ózd
outcrop, Stop 4). The close petrographic similarity with the conglomeratic intercalations in
the lower aggradational part of the Pétervására Sandstone (Sztanó and Tari 1993; Sztanó and
Józsa 1996) indicate that sources of coarse clastics were already present along the Darnó Fault
before intense relative sea- level rise took place during the Eggenburgian, when the formation
of the Darnó Conglomerate (sensu stricto) took place. The presence of a voluminous fluvial or
alluvial feeder system along the Darnó Fault can be excluded. The pebbly material shed into
the basin was roughly the same all along the Darnó Fault. At present, these rocks are known
only from a relatively narrow region east of the tectonic zone. However, it is supposed that
during the Early Miocene part of the West Borsod ridge, and eventually the Bükk Mts. were
still covered with nappes consisting of the above-mentioned rock varieties of ocean-floor
origin (Meliata-Szarvaskő nappes, Csontos 1988a, 1999).
Stop 6. Sirok, Castle Hill
The medieval castle above Sirok is built into and on top of the rhyolite tuff, generally
considered as early Badenian ‘middle level’ (Less and Mello 2004). No detailed modern
petrological, volcanological, geochemical work has been done yet. Fault pattern of the
outcrops in the village was studied independently in two groups. Both Bergerat et al. (1984)
and Fodor et al. (1992) showed NNE-trending sinistral strike-slip faults, having been formed
by NNW-SSE compression and perpendicular tension.
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Legend for Figures
Fig. 1. Position of the Darnó Deformation Belt in north–eastern Hungary and southernmost
Slovak Republic, with geographic names and stops of the excursion (map is after Balogh
1964; Less and Mello 2004). The fault pattern mainly shows the result of the youngest late
Badenian(?) to recent faulting phase. Note curvature of Cretaceous structures in the Bükk
Mts. Section A, B, C, D correspond to Fig. 2, 10, 5, respectively.
Fig. 2. Cross section of Telegdi Roth (1951) showing the reverse then normal character of the
Darnó Fault. Note folded Oligocene beds.
Fig. 3. Stereograms of mesoscale fault pattern and stress axes determined in the DDB (after
Fodor et al. 1992, Bergerat et al. 1984).
Fig. 4. Late Kiscellian to Eggenburgian structures of the Darnó Deformation Belt.
Stereograms indicate stress field characterising this deformation phase (after Csontos 1989;
Fodor et al. 1992). Subsurface distribution of some peculiar Oligocene–early Miocene
formations are shown with ornamented white patches.
Fig. 5. Cross sections in the northern part of the Darnó Deformation Belt. Note the general
synclinal structure and its eastern reverse faults zone. Surface-rupturing fault corresponds to
the Darnó Line. Dashed faults give one possible solution. The western part of the section C is
compiled the seismic section Szuha-1, (Braun et al. 1989; reinterpreted by Sztanó and Tari
1993). The eastern part is based on the cross section of Radócz (1964), simplified. Note that
vertical is scale is different on the western (two-way travel time, in seconds) and eastern end
(metres). Due to this, vertical scale is not the same as horizontal. However, this would not
modify considerably the upper ~600—800m of the section. The section D is after Albu et al.
(1985), modified.
Fig. 6a. Ottnangian—middle Badenian (18,5–15 Ma) strike-slip faults in the Darnó
Deformation Belt. Fig. 6b. Late Badenian to recent fault pattern, kinematics and surface
occurrences of Sarmatian–early Pannonian volcanic rocks.
Fig. 7. Location of the quarry near Lénárddaróc, formations of the borehole Lénárddaaróc-2
and perspective view of the quarry, after the unpublished observation of Radócz (1997). The
rhyolite tuff belongs to the ‘upper’ level of Sarmatian age.
Fig. 8. Photographs of the Senonian Nekézseny conglomerate at Csokvaomány. A) normally
or inverse-to-normal graded conglomerate units passing to sandstone units. Note erosive base
of a scour. B) imbricated pebbles in the conglomerate.
Fig. 9. Stereograms from the quarry at the road junction and railways cut near Csokvaomány,
Senonian clastics. Note symmetrical fault pattern of Phase 1 when beds are back-tilted to
horizontal. The relative local chronology from Phase 1, 1b and 2 correspond to clockwise
change of the compression. Grey arrows near the stereogram indicate estimated stress axes.
Small square, circle: projection of pole to fault, bedding plane. Dashed line: projection of
bedding plane.
Fig. 10. Cross sections in the Csernely valley, near Uppony village, in the Uppony Hills show
reverse displacement of folded Paleozoic rocks over Triassic and Paleogene to early Miocene
sediments (Koroknai 2004). Detailed section of Pantó (1956, Fig. C) shows the chaotic
character of the fault zone. Paleozoic formations show ductile deformation (Fig. B, after
Csontos 1988b), which indicate sinistral transpressional deformation.
Fig. 11. (A) Distribution of the Pétervására Sandstone in NE Hungary and southern Slovakia
(Sztanó, 1994). (B) Geological map of the Darnó Hill, the only place where the Darnó
Conglomerate is exposed (modified after Kiss 1958; Félegyházi and Vecsernyés, 1969).
Fig 12. Sedimentary section at Ózd–Somsály. Large-scale cross-bedding occurs in mediumgrained glauconitic sandstone at the lower part of the outcrop, overlain by pebbly to granular
sandstones with giant convolution above. Arrows show transport directions with respect to
north (up) (Sztanó 1994).
Fig. 13. Pie diagrams show differences in composition of ophiolite-derived detritus at (1) Kishegy, (2) Ózd–Somsály. The percentages indicate the share of ophiolite-related clastics
(volcanites and radiolarite together) in the total amount of clastic material. Between the heavy
minerals some source-specific minerals were also found, (P. pumpellyite, A. aktinolite)
(Sztanó & Józsa, 1996).
Fig.14. Large-scale water-escape (convolution) structures in coarse to granular sand near Ózd.
Notice zones of full and partial dewatering (Sztanó, 1994)
Fig. 15. Model of accumulation of ophiolite-derived clastics in combination with the leftlateral strike-slip along the Darnó Fault. All sediment was transported from south to north by
the tidal currents. Moreover, due to the strike-slip activity, every new influx of sediment is
distributed northward of the previous one, thus resulting in a northward increase of the
amount of ophiolite-derived detritus (Sztanó and Józsa 1996).
Fig. 16. Stereogram of fractures (deformation bands, striated faults, joints) in the
Eggenburgian sandstone of Ózd-Somsály outcrop. Note compression sub-perpendicular to the
Darnó Line.
Fig. 17. Between the westward-dipping large-scale foresets some beds dip in the opposite
direction at Kis-hegy (Darnó Hill). These beds are interpreted as backsets (Sztanó 1994).
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