Dark claystones; Cabeço do Peão Formation. Upper Ordovician. Trilobite fossils: Prionocheilus verneuilli.
The Cabeço do Peão Formation consists of one or two oolitic iron beds (“Camadas de Favaçal”) at the base, overlaid by micaceous pelites, with fauna rich in bryozoans and echinoderms (Queixopêrra Member).
These are followed by bioturbated micaceous silt-sandstones with intercalations of quartz-sandstone beds, 20-40 cm thick (Aziral Member). The unit ends with a thin level of ferruginous sandstones (Serra de Cadaveira Member).
The “Favaçal Layers,” which are centimeter thick, are formed by chloritized iron oolites (chamosite) and nodules with phosphate concentrates, irregularly dispersed within a fine matrix whose composition is of the same material. The chemical composition of two oolitic iron samples indicated relatively high values of Fe203 (20-30%) and P205 (< 12%).