Re-evaluation of archive pebble distribution data for the Upper Permian Bakonya Sandstone Member of the Kővágószőlős Formation, Hungary — a comparison with the composition of cores BAF–1 and BAF–1A
Abstract
In the second part of the 20th century the Mecsek Mts were the target of intensive mineral exploration due to the Permian uranium mineralization of the Kővágószőlős Sandstone. The mining was discontinued in the 1980s; however, in the 1990s the western part of the mountains became a focus of interest once again due to the presence of a possible highlevel radioactive waste disposal site in the Permian Boda Siltstone Formation (former name of the Boda Claystone Formation, abbreviated in Hungarian as BAF). The BAF–1, –1A boreholes were deepened in 2014 in a South Westerly direction from Boda. In its planned succession the following lithological units were recognized: Quaternary and Pannonian sediments and beneath these the Bakonya Member of the Kővágószőlős Sandstone Formation and Boda Claystone. The drilling was terminated at a depth of 474.6 m, with the Boda Claystone Formation still not having been reached. Although this mottled, coarse, dominantly terrestrial sandstone was identified as the Bakonya Sandstone Member, this was not determined unequivocally due to the absence of rhyolite pebbles in the drill core. This was due to the fact that, according to the available literature, the typical source rock (at least 20–40 %) of the Bakonya Sandstone was the Gyűrűfű Rhyolite. The most recent studies classified the dominantly red, poorly sorted, pebbly sandstone as a part of the Permian sedimentary rocks, even though Jurassic microfossils from the grey strata of the drill cores were also identified. Furthermore, the occurrence of carbonate pebbles was also a peculiarity. In order to confirm that the layers of the BAF–1 and –1A drill cores which had been reached undoubtedly belong to the Bakonya Member, work was carried out in order to investigate and compare these cores with other pebble-rich parts of the Bakonya Member occurrences in the area. For this comparison documentation concerning 11 drill cores was examined carefully. This data had been produced by geologists of MÉV (Hungarian abbreviation for Mecseki Ércbányászati Vállalat) in the 1950s and 1960s. The results of the examination related to this paper were represented in line charts and placed on the geological map of the Mecsek Mts. This enabled a comparison to be made of the spatial distribution of different types of pebbles to that of the BAF–1 and BAF–1A drill cores, and thus to observe how the debris material had changed in space and time. Based on the archive data, it is possible to show that the provenance area could have been characterised by a diverse lithology in which the debris material also varied over a relatively small area. It would imply that the Bakonya Member cannot be characterised by one distinct pebble composition. In other words, the presence of carbonate pebbles in the the BAF–1 and –1A drill cores raises uncertainties with respect to the Bakonya nature of the drilled strata.gary, West Mecsek Mts, Permian, Kővágószőlős Sandstone, pebble distribution.
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