Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Spherical ferruginous concretions in Cretaceous sandstones of N Bohemia: genesis and forms of occurrence
AbstractIn the northern part of the Bohemian Cretaceous Basin, spherical ferruginous concretions were collected from several sites reported by earlier authors and from new, as yet unknown occurrences. They are hosted by quartzose sandstones of the Březno, Teplice and Jizera Formations (primary occurrences) or redeposited in eluvia or colluvia. Geological characteristics common for their occurrences are: 1) proximity of basaltic bodies, and 2) horizontal hydraulic compartmentation of the sandstone aquifer, either by siltstone interbeds or by strata with carbonate cement. Ferruginous cement in the concretions shows a complex mineralogical and textural zoning, including a bleached centre, a goethite zone, a hematite zone and a surficial goethite ± hematite rind. New data are compatible with the formation of the concretions from Fe2+-rich fluids migrating along basaltic contacts, precipitating Fe3+ at discrete nucleation centres in the host sandstone. Concretion growth was controlled by the Ostwald-Liesegang cycling with an expanding redox front.