Mineralogy and chemical composition are presented for a chert–ironstone bed that overlies the Úrkút Mn deposit. This bed is mottled green–brown in its lower and upper parts, which are composed of quartz, goethite and celadonite. These parts of the bed are interpreted to be strongly altered tuffs, reflecting oxidic, low-temperature alteration of a hydrated, Fe-rich, Al-poor tuff, and K and Mg uptake from seawater. The middle part of the bed is a mineralized bacterial mat (quartz, goethite). Textures resembling bacterial cells and colonies are common, with wavy, bulbous laminations composed of mounds overlying a mesh-work stromatolite-like texture constructed of micrometre-size Fe oxides. This bed is concordant with the underlying Mn deposit and marks the termination of Mn accumulation. Although no genetic connection exists between the two, the rocks adjacent to the contact record the oceanographic and bottom-water conditions extant when accumulation of one of the major Mn deposits of Europe ended, when the Transdanubian Range was located in the middle of the Adria–Apulian microcontinent between the Neotethys and Atlantic–Ligurian seaways. A pyroclastic origin for part of the bed has significance for the Toarcian of Central Europe because evidence of volcanism occurring at that time is otherwise sparse.