Palaeomagnetic and mineralogical studies of Devonian lacustrine sediments from Caithness, Scotland

A variety of lacustrine sediments from the Devonian Caithness Flagstone Group in northern Scotland has been subjected to palaeomagnetic and mineralogical analysis. In general, the sediments show little evidence of a Devonian magnetization although this is partly seen in the Spital Flagstones. The magnetization seems to be dominated by Mesozoic overprints of normal and reversed polarity. The different sub-components of the magnetizations cannot be resolved thermally because of the difficulty of applying thermal demagnetization techniques above 400°C. At and above this temperature, sulphide oxidation results in laboratory magnetizations which obscure the NRM. Mineralogical studies show that the overprint is carried by Fe hydroxides in association with pyrite and marcasite grains (dolostones) or hematite in association with Cr-spinels (Achanarras Limestone). In the Spital Flagstones the relict Devonian magnetization and the normal Mesozoic overprint appears to be carried by magnetite. Our results show the importance of mineralogical studies in making realistic interpretation of the origin of magnetizations in ancient sediments.