Stratigraphy of the Kálla Gravel in Tapolca Basin based on multi-electrode probing and well data

Abstract

Gravel, pebbly sand, clean quartz-sand and sandstone — all formed in Lake Pannon — crop out in the vicinity of the
Keszthely Hills, Tapolca and Kál Basins; as a geological unit, this is referred to as the Kálla Formation. Up until now its
stratigraphic position, its relation to under- and overlying formations, as well as to the similar, but virtually older Kisbér
Gravel have not been determined with any degree of certainty. Based on descriptions of old wells and sedimentological and
stratigraphic data with respect to outcrops, geoelectric measurements were carried out and geological cross-sections were
compiled in order to ascertain its stratigraphic relations. Deposits underlying the Kálla Gravel consist of Triassic carbonates,
mid-Miocene limestones, pebbly limestones and the Congeria czjzeki-bearing Szák Claymarl. The Kálla Formation is
unconformably overlain by silts and sands of the Somló Formation and in some wells by the Tihany Formation (which
contains coaly clay beds).
The Late Miocene depositional history of the Tapolca Basin can be reconstructed as follows. At about 11–10 Ma, when
the lower part of the Szák Claymarl was formed, the area of the present Transdanubian Range was a large peninsula in Lake
Pannon. At its southern rim a small, tectonically preformed embayment was created. Around about 10 Ma ago the lake
transgressed northwards due to a relative lake-level rise and on the newly-flooded areas small coarse-grained deltas (Kálla
Formation) began to form due to small rivers draining the Transdanubian Peninsula. Along the rim of the peninsula several
distinct, but coeval gravelly shoreface deposits (Kisbér Gravel) may have been formed. Near to the entry points of rivers (at
the deltas) the sedimentation rate compensated and even exceeded the rate of lake-level rise; thus the gravelly-sandy deltaic
bodies prograded over the clays, which formerly were deposited below the wave-base. In front of the coarse-grained deltas,
the deposition of silts and clays continued. Therefore the Kálla and Szák Formations are partly coeval in the Tapolca Basin.
Approximately 9.5–9 ma ago a significant change occurred. Earlier, the sediments were derived from minor local sources
(i.e. from the Transdanubian Peninsula); later, following the filling up of the Danube Basin, sediments were derived from the
Alps and the Western Carpathians. The major north-western drainage system entered Lake Pannnon as extended deltaic
lobes, represented by the silty-sandy beds of the Somló Formation (prodelta and deltafront regions). Parallel with
progradation, 9–8 Ma ago the deltaplain corresponding to the Tihany Formation appeared. The overlying alluvial deposits
were removed by the Quaternary uplift of the area.
With respect to the lithostratigraphic units, it can be concluded that the Szák Claymarl, Kálla and Kisbér Gravels were
formed during the same transgressive event. The stratigraphic position of the gravelly formations is different only because
of the local variations in sediment input rates. Whereas their source area was the same and their petrographic composition is
almost identical. Within the frame of biostratigraphic resolution they are coeval at about 10 Ma and both were formed on the
margin of the Transdanubian Peninsula. It is impossible to differentiate the two deposits by geological mapping — therefore
their integration as the Kálla Formation is suggested. The Szák Formation, however, is not only a local deposit but can be
seen as part of a large clayey lithosome in the fill of Lake Pannon, corresponding to the Endrőd and Algyő Formations.
Therefore its timespan over the whole of the Pannonian Basin equals the Late Miocene.