Petrology and geochemistry of the gravels of the Upper Permian Cserdi Conglomerate Formation (borehole No. XV, W Mecsek Mts)

Abstract

This paper presents the petrological, so-called ‘pebble-statistic study’ of the Cserdi Formation and the geochemical
evaluation of numerous rhyolite pebbles in the drillcores of the borehole No. XV (W Mecsek Mts). This borehole is the
most entire exposure of the formation.
In this formation among the pebbles the dominating rock types are granitoids-metagranitoids, metamorphites,
regional sediments, and acidic volcanogenic rocks (it was determined that the latter are from Gyűrűfű Rhyolite
Formation). During the sedimentation, the amount of volcanites was decreasing, and the amount of metamorphites and
granitoids was increasing. It is supposed that in the provenance the basement of the continent gradually exhumed and later
on predominated over the acidic vulcanites.
In the lowermost 56 metres traces of local melt formation were detected between the grains. This acidic glass suggests
— in contrast to earlier opinions — that the volcanism of the Gyűrűfű Rhyolite had not finished at the beginning of the
sedimentation of the Cserdi Formation.
The results have reinforced the earlier supposition that the Cserdi Conglomerate and the Boda Siltstone Formation
formed heteropic environments the Cserdi Formation represents the alluvial fan and Boda Siltstone the playa lake.