Documentation of Mn-deposits of western Sicily

Some marine iron pisolite beds from the Toarcian of western Sicily are described for the first time. These beds, which generally overlie the Liassic platform carbonates, are usually chocolate-brown in colour; they attain a maximum thickness of about 40 cm at one locality but are often tracable as centimetre-thick remanie horizons elsewhere. Their fauna comprises Tethyan ammonites, belemnites, gastropods, rare brachiopods, fish teeth, foraminifera and crinoid ossicles.
The pisoliths themselves contain calcite, limonite (goethite), haematite and, in rare cases, chamosite; electron-probe microanalysis shows that they are enriched in certain trace elements, particularly manganese, relative to many iron ooliths. The high content of trace elements suggests deposition in a pelagic environment, as do the faunal associations of this deposit; and, since fragments of sanidine trachyte are invariably associated with this lithology - sometimes even forming the cores of the pisoliths themselves - submarine volcanism and exhalations seem a likely source for the ferruginous and manganiferous material.
The stratigraphical position of the iron pisolite, often between two stromatolitic sediments, and the traces of boring algae in the pisoliths, suggest deposition within the photic zone, probably some tens of metres deep.