Acta Musei Moraviae

Where do the Miocene clastic sediments of the Brno region come from?

Selected samples and localities of the Ottnangian and Badenian sands at the eastern margin of Bohemian Massif have been studied to establish their provenance, origin, environments and facies: I. Ottnangian sands and gravels transported by rivers (site Brno-Bystrc). Both clasts of gravels and translucent heavy mineral suite (mainly staurolite-garnet-epidote) reflect substantial share of the Moravicum and of the Brno Massif as primary source areas. The material was transported by a paleoflow running from W or NW. The most of grains show short transport. II. Badenian sands deposited as foreshore and shoreface facies (site Brno–Oslavany). The source area was relatively small, restricted to the closest vicinity of the shore and small streams flowing from W and or from NW drained it. The detrital material originated from karby weathered rocks of the Moravicum and probably the Svratka Crystalline Unit. Heavy mineral suite consists mainly from garnets and amphiboles. The influence of older (Ottnangian) sediments had been minor at this locality. III. Badenian (“Brno sands”; site Brno-Obřany) river delta sediments derived from larger area. Translucent heavy mineral assemblage (mainly garnets, stable small share of staurolite, epidote, apatite, tourmaline and rutil) and reworked Karpatian and Ottnangian fossils reflect share of older sediments in the source. Nevertheless grains from crystalline rocks of the Moravicum and of the Brno Massif are most abundant.

There was a stable source area at the western margin of the Carpathian Foredeep during the Ottnangian and Badenian. The wider range of translucent heavy mineral assemblage in Ottnangian sands indicates derivation from products of pre-Miocene (Cretaceous and/or Paleogene) weathering. The most indikative mineral of Ottnangian deposits is staurolite. More intensive erosion caused by uplift of crystalline unit on western margin of the Carpathian Foredeep (Moravicum, Svratka unit and Brno Massif) during Badenian produced typical garnet-rich assemblages.