The Crostolo River section in Italy yielded a detailed record of the Upper Olduvai geomagnetic polarity transition that has been used to support the hypothesis of deep mantle control on the transitional geomagnetic field. The paleomagnetic record is carried by the authigenic iron sulphide, greigite, which was interpreted to have formed shortly after deposition. Our detailed scanning electron microscope investigations indicate the presence of at least 3 generations of pyrite, which usually forms with greigite as a precursor. This suggests that the total magnetization is a complex composite that produced a smoothed record of transitional field behaviour.