Acta Musei Moraviae

Iron oxide concretions from the sandpit Závada near Hlučín (Silesia, Czech Republic)

Sandpit Závada near Hlučín (14 km NW from Ostrava) is extracting glaciolacustrine sediments of Saale Glacial Stage. In the upper part of the sand deposit is developed irregular horizon cemented by “limonite”, which was subject of study. Sandstones with lesser amount of the iron cement are non-coherent, larger amount of the cement causes their lithification. Locally they contain hollow or sand-filled concretions up to 15 cm. Both concretions and sandstone cement are composed of goethite with minor admixture of quartz, muscovite and kaolinite. In the inner part of hollow concretions occurs material with predominant siderite. Microscopic fissures in goethite are filled by hollandite, cryptomenale, lithiophorite, unidentified Mn-oxihydroxide and baryte, identified by EDX only. Described mineral association originate in leasing of Fe-bearing minerals during organic matter decomposition, transport of Fe²⁺ to the deeper parts of the sedimenary profile and subseqent oxidation on suitable redox barrier (interface of loess-loam and glaciolacustrine sands). Hollow goethite concretions in sandy sediments probably originate in wood or roots decomposition. They indicate larger amount of wooden mass washed into the glacial lake during the end of its existence.