GRANITE BENEATH THE PENNINES IN NORTH YORKSHIRE

Abstract

( Short communication ) A cored borehole drilled in Raydale, Yorkshire, has proved granite at 495.05 metres depth directly beneath Lower Carboniferous (Visean) beds. The site (SD 9026 8474) lies on the south-east side of Raydale Beck some six kilometres SSW of Bainbridge and 350 metres N 20° E of Raydale House at an altitude of 267.9 metres above O.D. The bore was drilled as part of the research programme of the Institute of Geological Sciences. The Alston Block and the Askrigg Block in the north Pennines have long been recognized by geologists as tectonic units, in which the comparative lack of folding and major faulting affecting the Carboniferous strata was due to some feature of the underlying basement rocks which gave them rigidity. The discovery on the Alston Block of a negative gravity anomaly (Bott and Masson-Smith, 1957), with minima corresponding closely with the postulated emanative centres of mineralization, indicated the existence of low-density rocks and prompted the drilling by the University of Durham Department of Geology of the Rookhope borehole which proved the presence of a granite batholith (Dunham et al. , 1965). This has been named the Weardale Granite, and its age has been determined as 410 ± 10 m.y., by the rubidium-strontium isotope method (Holland and Lambert, 1970), corresponding to an end-Silurian age of emplacement. These results encouraged further studies of the negative gravity anomaly already proved to exist on the Askrigg Block by a gravity traverse extending from Kettlewell in Wharfedale to Aysgarth in Wensleydale (Whetton et al. , …