Remanent magnetism of middle Old Red Sandstone lacustrine and fluviatile sediments from the Orcadian basin, Scotland

Abstract

A palaeomagnetic survey has been made of fluviatile red beds and lacustrine flagstones in the Orcadian Basin. The results from five facies are all different in important respects and this suggests that the palaeomagnetic history is strongly influenced by short-term and long-term diagenetic processes. Red beds from below the Caithness Flagstones retain much of their original magnetization, probably due to the protection of quartz cementation. Red beds from above the Caithness Flagstones have stable magnetizations but they are apparently random in direction. This is because these porous sandstones were not protected by extensive cementation and had a long history of magnetization due to the continuous penetration of oxidizing agents. The Caithness Flagstones had varied palaeomagnetic histories and demonstrate the importance of early diagenetic events. Siltstones and dolostones in the lower Caithness Flagstones are unstable due to the presence of normal and reversed components with similar blocking temperatures. The Achanarras Limestone, a carbonate laminite deposited under strongly reducing conditions, has a stable magnetization but this is of post-Palaeozoic age and there is no original magnetization preserved. Flagstones of the Spital Group in the upper Caithness Flagstones have stable magnetizations carried by authigenic pseudo single-domain (PSD) magnetite apparently produced during early diagenesis. Throughout the Caithness Flagstones both normal and reversed components are present in many individual specimens. This makes it extremely difficult to obtain palaeomagnetically ‘clean’ directions and casts doubt on the use of the Caithness Flagstones for precise palaeomagnetic work.