Ichnofabrics of ?Oligocene silcretes at Kryry and Dětaň, Western Bohemia, Czech Republic

Sand-dominated residual sediments of Oligocene or pre-Oligocene age are exposed at the Dětaň locality. They form several tens of centimetres thick lenses encrusted by secondary silica cement. These silicified lenses yielded a biogenic structure, probably a nesting chamber of hymenopterid insects, which was filled with sand. A temporary outcrop of thick tabular blocks of silcrete of the same age at Kryry displayed two blocks with irregular, occasionally branching bodies of more strongly silicified sandstone, orientated mostly oblique to the silcrete surface. These bodies probably represent molds of larger tree roots. The possibility of conservation of plant root systems in sand residues by silcretization is significant from ichnological viewpoint as well as from the viewpoint of possible timing of the silicification process. The origin of insect nesting chambers is supposed to take place in the stage of weak lithification of the substrate.