Palaeomagnetic investigation of organic-rich lacustrine deposits, Middle Old Red Sandstone, Scotland

Synopsis Organic-rich lacustrine carbonate laminites and mudstones in the Old Red Sandstone (middle Devonian) from the Orcadian Basin, Scotland, contain a magnetization that resides primarily in magnetite. After removal of a modern viscous magnetization at low demagnetization temperatures, a southerly and shallow Late Carboniferous component (before tilt: decl = 184°, incl = −5°, k = 36, α 95 = 7.3, n = 12 sites) is removed between 100 and 450°C in most specimens. A regional fold test on the shallowly dipping rocks is inconclusive, but suggests a post-folding time of remanence acquisition. A few specimens also contain a northerly and shallow component interpreted as the antipodal equivalent of the southerly and shallow direction. Rock-magnetic studies performed on representative specimens suggest that the remanent magnetic carrier is single domain, non-interacting magnetite. Partial anhysteretic remanent magnetization measurements indicate a wide range of grain sizes. The southerly component is interpreted as a Late Palaeozoic chemical remanent magnetization (CRM). The association between the CRM and the organic-rich source rocks suggests a remagnetization mechanism related to diagenesis of the hydrocarbons.