Resolution of superimposed magnetizations in the Devonian John O'Groats Sandstone, North Scotland

Summary. The John O'Groats Sandstone, on the north-east tip of Scotland, is the uppermost unit of a thick Old Red Sandstone sequence from the uppermost Middle Devonian. Samples from 13 sites have been treated by thermal and acid leaching techniques making use of the convergence of re-magnetization circles when stable end points were not reached. In some cases, in which the magnetic constituents were asymmetrically distributed, splitting the specimens proved useful. Two axes of magnetization have been identified. The easiest to determine, called the A axis, is present most frequently in the reversed sense, at roughly Dec 190°, Inc -40°. The normal component can also be detected as stable end points on acid leaching. The A-axis magnetization is carried mainly by the pigmented cement. The B axis, which is aligned NNE-SSW and almost horizontal, is less well defined. Both the A- and B-axis orientations cited are after making corrections for structural tilts that are present at some of the sites. The direction of the B-axis magnetization agrees well with that of other Devonian units from west of the Great Glen Fault and is interpreted as having been acquired during, or shortly after, deposition. The A magnetization is interpreted as having been acquired during the Mesozoic, probably associated with regional burial and subsequent tectonic uplift of the area during the Jurassic. Later tectonic movements to produce at least the major structural tilts post-date the A magnetization and may be related to crustal unrest at the opening of the modern Atlantic. The John O'Groats directions are compatible with similar directions found in the Duncansby Neck, a volcanic vent that penetrates the Sandstone.