The petrography and heavy minerals of the Toarcian black shale of the Réka Valley section of the Mecsek Hills: a pilot study

Abstract

The results of a pilot petrographic and heavy mineral study of Lower Jurassic (H. falciferum Zone) organic-rich
sedimentary rocks are presented from the Mecsek Hills (black shale interval, Réka Valley section, Óbánya Siltstone
Formation). The studied succession comprises laminated to thin-bedded black shales with intercalations of fine- to
coarse-grained turbidite beds of mixed carbonate–siliciclastic composition. Translucent heavy mineral spectra consist of
zircon and tourmaline and some garnet, apatite, rutile and staurolite. Lack of grain rounding suggests that the role of
sediment recycling was not significant. A terrigenous sediment component of the Óbánya Siltstone was sourced from a
mature, felsic continental crust made up of low- to medium-grade metapelites (e.g. gneiss, micaschist) and granitoids
exposed to erosion in the Early Jurassic. Such rocks are rather characteristic for the Görcsöny Ridge, which is thus a likely
source candidate for the Óbánya Siltstone Formation. Sedimentary provenance data from the Mecseknádasd Sandstone
Formation illustrate that a comparable rock association had already been eroded in the Pliensbachian. The latter, however
also comprised minor proportions of amphibolite facies and metabasic rocks