Hidden volcanoes under our feet

krispiessens Geo-energy Theme Posts and Events, GeoConnect3d Posts

This post is part of the GeoConnect³d blog.

For the Alpidic and Carpathian mountain ranges are typical huge crystalline massifs and white limestone cliffs but less people know that a large part of the Carpathians – in contrary to the Alps – is built by Neogene volcanoes and their products. But even among geologists is less known that there are also volcanoes hidden from our eyes, deeply buried below the young sedimentary cover of the basins. In this short article we will focus only on volcanoes in the Slovakian part of the Danube Basin, however many similar bodies were detected also in Hungary and in the Transcarpathian Basin.

Although tuffitic rocks and intercalations were known from a number of wells, nobody expected whole volcanoes until the 60-ies. The first one – the Šurany volcano – was hit by a drill only in 1961 (well Š-1, 2810 m deep), the second, largest one – the Kráľová volcano – in 1966 (well Kr-1, 2974 m) and the last one was discovered in 1977 near Bratislava, the capital of Slovakia in the village Rusovce (well HGB-1, 1493 m). Since the Danube Basin was presumed a hydrocarbon reserve mostly in the ’70-ies also an extensive geophysical research had begun applying almost all the methods available in this time: 2D seismic profiles, gravimetry and measurements of magnetic anomalies (fig. 1). Strangely enough, however the volcanic bodies are proven by wells and nicely fit on the magnetic anomalies, they show no gravitational anomaly at all – this phenomenon can be caused by the high degree of compaction of sediments at these deep structural levels.

Fig. 1. Above: magnetic anomaly (based on Seiberl et al., 1998), 1 – Gabčíkovo anomaly 2 – Kráľová volcano 3 – Šurany volcano. Below: Complete Bouguer anomaly map (after Šujan et al., 2021).


All three known volcanic bodies – Šurany, Kráľová and Rusovce – are deeply buried (about 1000, 2000 or even 2500 m below the surface). In the case of the largest Kráľová volcano the well did not hit the basement below the body, in Šurany and Rusovce volcanic rocks lie directly on the crystalline basement. All three volcanoes are of lower Badenian age, so the belong to the oldest phase of Neogene volcanic activity in the Carpathians (fig. 2).

Fig. 2. Simplified well logs from wells Š-1 (Šurany), Kr-1 (Kráľová) and HGB-1 (Rusovce).


Since we have no seismic profile from the Rusovce area, we can construct cross sections only through the Kráľová and Šurany bodies (fig. 3). Both, but especially the Kráľová volcano lie on normal fault zones, or crossing of those zones, in the case of the Rusovce body we can suppose similar structural predisposition.

Fig. 3. Simplified cross-sections through the Kráľová (after Kronome et al., 2014) and Šurany (after Gaža, 1963) volcanic bodies.


As we saw on fig. 1, the magnetic anomalies are lined up into „zones“ of NW-SE and NE-SW directions. Here we need to mention about the enigmatic Gabčíkovo magnetic anomaly. However this is the largest such anomaly in the whole Danube Basin it is localized in the deepest part of the Danube Basin below the Slovak – Hungarian border and thus even the deepest wells – about 3000 m – ended in the Pannonian, so nobody knows exactly how deep that part of the basin is, but usually estimated about 8 km (!). The source of the anomaly can be an other „supervolcano“ and/or a deep ultramafic basement, as it was suggested by some authors.

Well, the „boring“ plains and lowlands sometimes hide really exciting secrets.



Balazs Kronome
SGIDS – State Geological Institute of Dionyz Stur




Literature:

Bondarenková, Z., Zbořil, Ľ. and Motlíková, H. (1977): Hydrogeologický prieskum Bratislava – Rusovce, vrt HGB-1. Report, Geofond ŠGIDŠ, p. 1-237 (in Slovak)

Gaža, B. (1966): Geologické, zhodnotenie štruktúrnostratigrafického vrtu Kráľová-1. Report, Geofond, Bratislava (in Slovak)

Gaža, B. (1963): Geologická správe o hlbinnom prieskume v oblasti Surany-Pozba v r. 1960 – 1962. Report, Geofond, Bratislava (in Slovak)

Kronome, B., Baráth, I., Nagy, A., Uhrin, A., Maros, Gy., Berka, R. and Černák, R. (2014): Geological Model of the Danube Basin; Transboundary Correlation of Geological and Geophysical Data. Slovak Geol. Mag., 14, 2, p. 17-35

Seiberl, W, Oberlecher, G., Kovácsvölgyi, S., Schonvinszky, L., Pancsics, Z., Firo, M. and Kubeš, P. (1998): Magnetic AT anomaly map 1:500000. Geol. Inst. of Hungary

Šujan, M., Rybár, S., Kováč, M., Bielik, M., Majcin, D., Minár, J., Plašienka, D., Nováková, P. and Kotulová, J. (2021): The polyphase rifting and inversion of the Danube Basin revised.







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