Hypogenic caves of the Buda Thermal Karst developed in Pleistocene times as a result
of interaction Eocene limestones with thermal waters rising along pre-existent fractures
and discharging at the local base level of erosion. Because of subsequent tectonically
controlled uplift, most caves are now dry and far above the actual thermal-water level.
This study is an attempt to reconstruct the story of the shift of the cave from the
hypogenic (phreatic) to the epigenic (vadose) zone.
Positions (incl. altitude) of the collected speleothems were measured by means of
classical geodetics, and plotted on cave maps. Samples were studied petrographically
and classified using Frisia’s terminology (Frisia et al. 2000, Frisia 2015). Five
major groups (mammillary crusts, cave rafts, cave-coralloids/popcorns, dripstones)
were identified and their paragenetic order recorded at each sampling point. Mammillary
crusts were considered as phreatic precipitates, cave-rafts as water-table indicators,
whereas cave-coralloids/popcorns and dripstones as vadose zone features. Morphology,
mineralogy (XRD, SEM) and stable C and O isotopes were studied and age-datings were
carried out by the U/Th series method.
It turned out that the speleothems may facilitate, indeed, the reconstruction of changes
of both the water-table and temperature. Cave rafts now found at 179 m a.s.l., proved
to have been precipitated 500 kyr BP, at 37 °C on the surface of smaller or larger
thermal-ponds separated by dams in NE-SW oriented straight passages. Cave rafts settled
down to the bottom of those ponds became subject to overgrowth 390 to 410 kyrs BP.
Gradual uplift of the Buda Hills and concomitant incision of the Danube resulted in
gradual decrease of the water-table thus the level of formation of calcite rafts and
mammillary crusts also shifted downwards. At the same time, some of the previously
formed calcite rafts became overgrown by vadose-zone speleothems (eg. cave coralloids.
Age datings show that 290 to 300 kyr BP the thermal water table (by then only of 27
to 28 °C) dropped by about 8 m (its related rafts and mammillary crusts are found
now at 172 m a.s.l.) and most passages became partially air-filled (=vadose zone).
280 kyr BP the thermal water-table rose again (this time by about 5 to 6 meters) and
the rise was accompanied by a temperature-increase (related speleothems are now at
174 to 175 m a.s.l. and their temperature of precipitation was calculated as 47 to
49 °C). At about 186 kyr BP, right before having left the cave finally empty, the
temperature of the thermal water decreased already to 21 to 25 °C (related speleothems
are found now at 168 m a.s.l.). The last thermal-water speleothems were precipitated
at 160 kyr BP from luke-warm waters of 20 to 23 °C and the cave has been dry ever
since then. The swift cooling of the water during the last stages of the uplift of
the Buda Hills was most probably the result of mixing with increased amounts of infiltrating
meteoric waters from the eroding surface.