THE INSTITUTE OF GEOLOGICAL SCIENCES BOREHOLE AT BECKERMONDS SCAR, NORTH YORKSHIRE

Abstract
SUMMARY: The Beckermonds Scar Borehole passed through 260 m of Lower Carboniferous strata of Asbian to Arundian age. These are mostly limestones but also include an attenuated representative of the Ashfell Sandstone. Underlying Ordovician strata were penetrated to 260 m rod length and consist of steeply dipping magnetite-rich siltstones, mudstones and greywacke sandstones. These contain acritarchs of probable Arenig age from a single horizon. Mineral flats consisting of barite, fluorite, calcite and chalcopyrite occur within late Asbian limestones. The homogenisation temperature of the fluorite was 190°C, an unusually high value for the Askrigg Block. The magnetic susceptibility of the Ordovician sediments is equivalent to a magnetite content of about 1% and is capable of producing magnetic anomalies of several hundred nanoTesla amplitude. Remanent magnetisation is not likely to contribute significantly to the magnetic anomalies as its intensity is only about 20% of the induced magnetisation. The sediment samples have a plane of maximum susceptibility parallel with the bedding planes, lending support to the idea that the magnetite is an original constituent. The magnetite-bearing sediments are likely to extend near to or even reach the basement surface along the northern, western and southern sides of the concealed Wensleydale Granite. A similar origin cannot be ascribed with confidence to adjacent anomalies which appear to indicate more deep seated magnetic bodies, but the results of the Beckermonds Scar Borehole cast some doubt on the conventional geological interpretation of aeromagnetic anomalies in the UK in terms of igneous or metamorphic bodies.

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