The significance of iron ooids from the middle Eocene of the Transylvanian Basin, Romania

[Display omitted] •Iron ooids from the Eocene Transylvanian Basin intermix with nummulites.•Ooid cortex and nucleus are made of goethite. Nummulites preserve calcareous shell.•Iron ooids were transported in places of nummulite accumulation. The middle Eocene ironstone of the Transylvanian Basin, Romania provides new insight into the genesis and paleoenvironmental significance of ferruginous ooids. An horizon at the base of the Căpuş Formation in north-western Romania, well known for the spectacular nummulite banks there exposed, documents a peculiar association of large foraminifera within a matrix dominated by millimetric Fe-ooids. These red-brownish Fe-ooids are well evident through the whitish colour of the carbonatic host matrix. An integrated analysis of the ooids, carried out by Environmental Scanning Electron Microscopy, Micro-X-ray Diffraction and Raman spectroscopy, allowed its detailed chemical and mineralogical characterization. Ooids are mostly composed of continuous concentric cortical layers of goethite interlayered by a few thin layers of phosphatic phases, whereas nuclei are made of ferruginous grains. No evidence of biological activity was detected in the cortex of the ooids to support a bio-mediated genesis. The occurrence of this ironstone testifies the existence of ferruginous bottom waters in the Eocene tropical/sub-tropical shallow-marine settings of the central Neotethyan Realm. The limited extension of the iron source area suggests that also local (e.g., not global) events could have been able to trigger iron ooidal deposition throughout the Phanerozoic.