Field and geoelectric study of the Mesozoic (Upper Jurassic) occurrence at Kesztölc

Abstract

Kétágú Hill near Kesztölc, Pilis Mts., hosts one of the easternmost exposures of Jurassic rocks in the
Transdanubian Range (Figs 1, 2). The badly exposed succession is composed of Liassic bedded pink
pelagic limestone, crinoidal limestone, Dogger thinly layered limestone, Belemnite limestone, a Middle
Oxfordian pelagic limestone breccia horizon, a Late Oxfordian - Early Kimmeridgian red radiolarite, and
interesting formations above this radiolarite. Apparently directly above the radiolarite, in mid-slope
position on the recent hillside, a larger block of Dachstein Limestone follows (Fig. 8). Dips of the Jurassic
and Triassic successions seem to be conformable. Further downslope several isolated boulders of
Dachstein Limestone can be found. North of these and a small dry valley, massive and continuous
exposures of Dachstein Limestone are found. Former geological maps (NAGY 1969b - Fig. 3) considered
the indiviual Dachstein Limestone blocks as parts of the major Dachstein exposure at Kis-hegy. However,
the position of these blocks is so interesting, that BALOGH (1961) suggested a thrust fault between the
Jurassic outcrops and the Dachstein blocks (Fig. 4).
We remapped this area and investigated the position of the Dachstein blocks by multielectric sections.
For a brief description of this geophysical method see PALOTAI et al. (this volume). Mapping (Fig. 6)
revealed that at least the higher position Dachstein block (A on Fig. 6) is not a boulder coming from
uphill, because there is no such a rock exposed on top of the hill. Moreover, no small grainsize scree of
similar lithology was observed. Besides very similar dips, the Dachstein block has a smaller strike-slip
fault, which falls in the direct continuation of a strike-slip fault in the Jurassic succession. Even the blocks
in a lower topographic position lie well above the dry creek-bed, so it is very unlikely that they were
transported by gravity or by flush-floods. When individual lithologies were studied, only two horizons
showed redepositional phenomena. The Middle Oxfordian carbonate layer is composed of plasticlasts of
pelagic limestone (Fig. 7). Another location right above the Dachstein block "A" gave a breccia sample
with Liassic matrix.
Two parallel multielectric dip sections were designed in order to reveal the extent in depth of the
Dachstein blocks (Figures 9, 10). These lithologies are easily distinguished from the Jurassic pelagic
sediments by their relatively higher resistivity (violet in the sections) vs. low resistivities of the latter
(blue-green in the sections). The sections show that all the isolated Dachstein blocks are of limited extent
and they all float above low resistivity material. The individual ranges of blocks seem to correlate in the
parallel sections. The change in position of these rows of blocks is well explained by the differences in
topography and by the dip of the strata. Low resistivity material directly continues into the radiolarite
exposures (and their Jurassic underlayer). High resistivity material thought to be equivalent to Dachstein
Limestone rises towards the north in successive steps to arrive at the Dachstein exposures of Kis-hegy at
the northern limit of the sections. Eventual curved lower resistivity zones within the latter may
correspond to clay-filled karstic cavities. 

We can interpret the obtained geological map and the multielectric sections in two different ways. In
both varieties, the Dachstein blocks form a horizon above the Jurassic succession. In the first
interpretation Figures 9b, 10b, IT), the blocks are thought to be big clasts embedded in Late Jurassic
(probably radiolaritic and pelagic marly) matrix. These olistoliths may derive from a nearby source with
a major topographic difference. The suggested period of redeposition in this model is OxfordianKimmeridgian. In the second model Figures 9c, 10c, 12), the Dachstein boulders form part of a flat-lying
allochtchonous thrust sheet above the Late Jurassic beds. In this model, this thrust sheet is then cut up
by E-W striking strike-slip faults (with normal component). Because of the long and narrow shape of the
blocks, we suggest that this second model is more probable. The emplacement time of the suggested
allochthon is bracketed between the youngest lower deposit and the oldest cover. The youngest lower
deposit may be the Late Oxfordian - Early Kimmeridgian (DOSZTÁLY 1988) radiolarite, or a Tithonian
pelagic limestone mentioned in the literature, but not found by us. The oldest cover is a silicified breccia
composed mostly of radiolarite clasts. The age of this might be Aptian - Early Albian (analogy form the
nearby Gerecse Mts). or Eocene, or even Oligocene. In spite of the large bracket, we suggest, however,
that emplacement occurred in the Late Jurassic - Early Cretaceous.