Az eperkés-hegyi felső-jura képződmények áthalmozott tömbjei

Abstract

The Eperkés Hill exposures are famous for the international scientific community. The Jurassic
formations exposed in two artificial exposures: the "Long trench" and the "Big excavation" challenge
geological interpretation. In spite of the relatively good exposure conditions, the successions are far from
being unambiguous. Besides well-bedded conformable Middle-Upper Jurassic pinkish limestone one
can find bigger, sometimes apparently continuous blocks of light coloured Dachstein and Kardosrét
Limestone (Upper Triassic and Lower Liassic platform carbonates). These were either interpreted as the
emerging tops of smaller horsts (FÜLÖP 1969; CSÁSZÁR et al. 1988), or as oHstoliths (megabreccia of GALÁCZ
1988, GALÁCZ & VÖRÖS 1989), or a combination of the two (MIZAK 2002; CSÁSZÁR et al. 2002). We revisited
the exposures and give here a detailed field description of the two trenches, helped by several
geophysical sections.
In both exposures the pinkish, thinly bedded Kimmeridgian-Tithonian limestone contains metre-size
blocks of light coloured platform limestone. Rare orientation data from these (observed algal mats;
oncoid-layers; calcite-filled palaeo-voids) suggest that each block has its own bedding which differs from
that of the other blocks, and which is not compatible with that of the pelagic limestone. Moreover, 1:300
mapping of the Big excavation (Fig. 3) revealed that these blocks are of different origin and mixed. The
rocks of the Big excavation are unconformably covered by Aptian - lowermost Albian crinoidal limestone.
Smaller undulations in the dip of the pelagic limestone can be interpreted as a consequence of gentle
folding, or locally as draping above the bigger clasts. The Long trench exposure (Fig. 5) is dissected by
two fault zones. These faults (with observed left lateral slickenslides) can explain why the three
compartments of the exposure have a slightly different composition. In the westernmost compartment
there is a succession of Middle Jurassic radiolarite, Kimmeridgian pelagic marly limestone and an Upper
Kimmeridgian-Tithonian Hierlatz-type limestone. This latter crinoidal and organodetrital limestone was
interpreted as a submarine talus cone (GALÁCZ 1988; VÖRÖS & GALÁCZ 1998). In the central part of the
exposure the light coloured carbonates dominate. However, the pinkish pelagic limestone is observed as
non-regularly bedded layers or as infiltrations among bigger blocks. Smaller grain size breccias were
observed at two sites. Z . LANTOS (pers. comm.) determined one as a probable fissure filling, the other as
a probable sedimentary breccia. Both had Upper Jurassic pelagic limestone matrix. The eastern part of
the exposure is composed of Tithonian pinkish limestones without bigger blocks.
In order to extend the surface observations at depth we used multielectric geophysical sections. This
method is a classical resistivity measurement, with a computer-controlled variation of the A-B and M-N
electrode distances and configurations. The resulting resistivity measurements give cumulative
information from different depths and paths. These raw data are then inverted (Res2DInv Geotomo
software) to give the resistivity of a certain point at a certain location. The depth penetration is strongly
dependent on electrode distance. We choose shorter electrode distances (1 m and 2 m in sections E2-3 and El, respectively) because of our main interest in the shallowest horizons. From former experience
the massive platform limestones have a higher, while the pinkish pelagic limestones have a lower
resistivity. Cherts have a relatively low resistivity, but higher than the pinkish limestones.
The electric sections show higher resistivity patches swimming in low resistivity material. This case is
best seen on sections E2 and 3 (strike and dip sections; Figures 7 and 8). The higher resistivity patches
show a very good agreement to the mapped individual boulders. Slight variations in resistivity also
suggest the dip of the matrix and that of the Jurassic-Aptian unconformity observed identically at
surface. The electric sections strongly suggest that the pelagic matrix is more than 10 m thick, and bears
at least metre-size boulders in several horizons. This view strongly supports the original olistolithinterpretation (GALÁCZ & VÖRÖS 1989). The El section (Fig. 9) parallel to the "Long trench" is also in very
good agreement with the observed surface section. The gentle curvature in lower resistivity material at
the western part of the section corresponds to the slight variation in dip of the Middle Jurassic -
Kimmeridgian part of the section. This compartment is separated from the central one by a steep fault,
which is projected to the surface section by its measured dip. The central compartment is characterized
by a relatively shallow, high resistivity block, which we interpret as an emergent remain of Late Triassic
- Liassic platform limestone. This interpretation is supported by a nearby mapping well (Ot-86, CSÁSZÁR,
pers. comm.). However, above the eastward dipping top of this platform, low resistivity pelagic
limestone is deposited. This contains isolated smaller spots of high resistivity material that correspond to
the boulders observed in the surface section. It seems that this olistolith-horizon is offset by a normal
fault. Finally, low resistivity pelagic material without boulders ends the section.
Detailed geological mapping and the multielectic sections helped us demonstrate that the Upper
Kimmeridgian - Lower Tithonian pelagic limestones support occasionally metre-size boulders, i.e.
olistoliths of shallow platform origin. The redeposition of these bigger boulders needs a steep palaeotopography and a nearby source. Naturally, the succession immediately below the olistolith-bearing
horizon cannot be the source, the much more complete and continuous Jurassic successions found more
to the west (Lókút sections), neither. The area east of Eperkés Hill is mainly made up of Upper Triassic,
and locally preserved Lower Jurassic platform remains covered by Aptian - Lower Albian crinoidal
limestone. Although there certainly was pre-Aptian erosion, we speculate that the uplift of this eastern
area might have begun as early as Late Jurassic, to provide the necessary clasts. Steep palaeo-topography
could have been produced either by normal faulting, or by thrust faulting. The first, classical solution
was suggested by GALÁCZ & VÖRÖS (1972) and was generally accepted. However, we would like to draw
the reader's attention to the description of very similar redeposited material in the Middle - Upper
Jurassic of the Salzkammergut area in the Northern Calcareous Alps (NCA). Austrian authors explained
these redeposition phenomena by the forward propagation of a nappe system (GAWLICK et al. 1999;
MANDL 2000) or by the activation of a major transpressional strike slip fault system (FRANK & SCHLAGER
2006). Their arguments were based on observed structural positions and the polimict nature of the clasts.
The NCA have long ago been considered a close relative and counterpart of the Transdanubian Range
(HANTKEN 1868; VÖRÖS & GALÁCZ 1998), so these analogies might be seriously considered when
interpreting the origin of the Upper Jurassic olistoliths at Eperkés Hill.