Karst development and geochronology of cave sediments and speleothems in the Western Mecsek Mts., Hungary, Pannonian Basin

The karst region of the Western Mecsek Mts. is a hilly area with a maximum elevation of ~600 m a.s.l. The karst has developed on Triassic limestones, with part of the drainage basins on lower Triassic sandstones and conglomerates. Our main objective was to constrain the karst development in time, to identify karstification periods with respect to climate, and to obtain data on landscape evolution. Cave geometries were surveyed, and speleothems and allochthonous siliciclastic sediments were extensively sampled for U-series, cosmogenic radionuclide (CRN) burial and optically stimulated luminescence (OSL) age determination.

Cave passage orientations are dominantly controlled by tectonic fractures and bedding. Most cavities formed in the vadose zone, only a few passages show features possibly referring to the phreatic zone. No obvious geomorphic levels could be identified within the caves. Cave sediments include reworked loess (dominant), coarse gravels and sands from Triassic red sandstones, red clay, reworked speleothems (dripstones, carbonate tuff from cave streams), and fragments of the host rocks. Though several passages used to be filled with fluvially transported sediments, at present none of the spring caves issues clastic deposits, thus the deep karst system apparently acts as sediment sink.

The CRN burial age of the oldest cave sediments is ~2.5 Ma, in a sinkhole cave now in a valley-side position. The burial ages of sediments in caves with entrances at or close to present-day valley floors are Middle Pleistocene to recent.

According to the U-series ages, speleothems precipitated at least from the Middle Pleistocene to the Holocene, with most data concentrating in the last glacial maximum and in the Holocene, but several ages exceeding the limits of the measurement method. They formed under all climatic conditions, from glacials to interglacials. They underwent repeated phases of precipitation and dissolution. Widespread submerged stalagmites refer to oscillations of the karst water table; measured ages of ~3 ka and 40-50 ka indicate formation during milder climates, with still lower-than-present water tables. Based on OSL ages, loess was washed into the caves under both cold and mild climates in the Late Pleistocene and Holocene. The ages obtained by the different age determination methods do not allow to distinguish periods with different types of karst activities, they suggest that both speleothem genesis and sediment transport happened under all climate types and simultaneously.

The burial age of the clastic infill of the oldest dated cave, which now can practically lacks a drainage area, be used to infer local incision rate. 2 Ma ago it still received sediment (coarse sand and sandstone) from non-karstic rocks. Supposing a valley-floor position for that time at the recent cave entrance gives a valley incision rate of ~15 mm/ky. Pre- and post-burial surface denudation rates calculated using the CRN data vary between ~12-35 mm/ky.

Sandstone cobbles in a cave now lacking non-karstic rocks in the catchment area indicate drastic drainage changes, the capture of the upper catchment by the neighbouring watercourse.

Research was supported by PURAM, Mecsekérc Ltd. and NKFIH project FK 124807.