Geological interpretation of palaeomagnetic results from Devonian rocks of Hoy, Orkney

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Abstract
Synopsis: The palaeomagnetic record of Devonian Volcanics of Hoy (∼370 Ma), occurring at the base of the Hoy Sandstone (Upper Devonian), exhibits coherent groups of remanence directions. An original shallow magnetization axis, repeatedly reported from studies of Old Red Sandstone rocks of Caithness, is confirmed, but the data suggest also that the tectonic unconformity between the Middle Devonian strata (upon which the volcanics rest) and the Hoy Sandstone originated at around the time of the volcanic activity. A secondary magnetization of inferred Permian age has been uncovered from the underlying Middle Devonian sediments (Lower Eday Sandstone). This overprinted component of magnetization, which is a pronounced feature in various Caithness rocks, is reported here for the first time for Orkney rocks. The Permian overprinting event is held responsible for the demonstrable loss of argon in the Devonian volcanics, rendering also the K/Ar ages for other igneous rocks in the region highly suspect.