Mineralogical and Palaeomagnetic Studies of the Sulphide-Rich Sediments of the Wash, England

Abstract

Cores of sediment from the tidal flats of the Wash have been analysed using both mineralogical and palaeomagnetic techniques. The primary objectives were to identify the sulphide minerals and any variations in their abundance, and to determine whether palaeomagnetic dating of the sediments could be achieved and if so, whether it could be used to date any diagenetic changes in the sediments. To achieve these objectives two major fields of research were undertaken, one mineralogical and the other palaeomagnetic. Mineralogical studies, involving petrography, X-ray diffraction, Mossbauer spectroscopy and atomic absorption spectrometry, have enabled identification of the authigenic iron-sulphides, greigite and pyrite. The presence of mackinawite is also inferred. Mossbauer spectroscopy has permitted the measurement, qualitatively, of the formation of pyrite, with depth, and has allowed the identification of the major iron-bearing minerals in the sediments. These are pyrite, chlorite, greigite, illite, montmorillonite, iron hydroxides and titano-magnetite. Palaeomagnetic studies of the sediments show the presence of a stable natural remanent magnetization (NRM) which is postdepositional in origin. Experiments involving the natural sediment samples and redeposition experiments show the importance of measuring the water content of unconsolidated sediments, when attempts to interpret their early magnetization are being made. Compaction, resulting in decreased water content, is identified as the principal process involved in the fixation of the NRM. Analysis of the declination and inclination records from the cores shows the usefulness of palaeomagnetic dating. This is achieved by matching the records from the cores with those of the geomagnetic record for Great Britain. The dating of the sediments can be used to date diagenetic and sedimentological events within the sedimentary sequence and to calculate sedimentation rates. Finally, rock magnetic experiments and mineralogical observations have identified multidomain titanomagnetite as the principal remanence carrier. Other minor ferromagnetic minerals are greigite, and a "hard" magnetic material (haematite or goethite).