A RECORD OF REVERSED POLARITY CARRIED BY THE IRON SULFIDE GREIGITE INBRITISH EARLY PLEISTOCENE SEDIMENTS

Palaeomagnetic measurements were made on samples extracted from a shor t sequence of early Pleistocene estuarine clays, now exposed in a sea cliff near Sheringham on the north Norfolk coast, UK. On the basis of earlier palynological work, these clays had been ascribed a Pastonian (late Tiglian) age. The clays show marked changes in colour, from redd ish-brown at the top of the unit, to blue-grey in the middle, and grey -brown at the base. The palaeomagnetic data vary in close association with these colour changes. The top and basal brown clays show scattere d normal directions of low intensity, while the middle blue clays show strongly clustered reversed directions, of much higher intensities. S ome samples taken from the boundary between the middle blue clays and upper red clays show upon demagnetisation a normal overprint on a stab le reversed polarity. Using high-gradient magnetic extraction, magneti c concentrates have been obtained from the strongly magnetic middle bl ue clays. The presence of iron sulphide minerals in these concentrates was identified using energy-dispersive X-ray analysis during scanning electron microscopy. More specifically, X-ray diffraction identifies greigite as the only detectable ferrimagnetic mineral in the magnetic concentrates. Rock magnetic measurements show clear qualitative differ ences in the magnetic mineralogies of the three clay subunits, but abs olute identification of the magnetic mineralogy of the weakly magnetic upper and basal brown clays has not yet been possible. We interpret t he sequence as a primary reversed polarity record. This record is carr ied by the iron sulphide greigite as a chemical remanence acquired dur ing 'syn'-depositional reduction of iron via the decomposition of orga nic material in these anoxic tidal clays. Subsequently, the upper and basal subunits of the clay have been oxidised by permeation of groundw ater from the adjacent coarse-grained sediments. Most of the griegite in the oxidised margins of the clay has been altered as a result, to a new, less efficient magnetic recording material which thus carries a later, scattered, low-intensity, normal overprint.