Lower Carboniferous bentonites in the Bardo Structural Unit (central Sudetes): geological context, petrology and palaeotectonic setting

Abstract

The Lower Carboniferous Paprotnia beds of the Bardo Structural Unit in the central Sudetes, composed predominantly of mudstones with Upper Viséan fossils, include several bentonite layers. The bentonites, composed mainly of kaolinite, illite/smectite and smectite, with minor amounts of quartz, calcite and iron hydroxides, also contain abundant zircons, the features of which indicate their volcanic derivation. The main population of the zircons yielded a SHRIMP U-Pb age of ~ 334 Ma corresponding with, and numerically constraining, the biostratigraphic data. The field evidence, biostrati- graphic and geochronological results, together with mineralogical data from the bentonites, indicate continental margin-type sedimentation and contemporaneous volcanic (andesitic-rhyolitic) activity in the neighbouring region during the ongoing Variscan orogeny in central Europe in Late Viséan times.