The geological interpretation of a gravity survey of the Alston Block and the Durham Coalfield

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Abstract
Summary: Problems of the deep structure of the Alston Block and the Durham Coalfield are outlined in relation to previous geophysical work. The gravity survey planned as the most expedient approach to these problems is briefly described. The results of a density survey undertaken in conjunction with the gravity survey are stated. The main feature of the gravity survey is a large negative anomaly over the Alston Block. The earlier conclusion, that it is caused by an unexposed granite, is upheld by the detailed physical and geological interpretation. The postulated granite approaches closest to the surface in five bosses, some of which almost certainly reach within 5000 feet of the surface. The shape of the gravity anomalies bears remarkable resemblance to the zones of mineralization and the volatile content of the coal seams, and for these reasons a post-Carboniferous age seems likely. Possible modes of origin of the postulated granite are discussed. The northward decrease of anomaly across the Stublick fault system is caused by fairly rapid thickening of the Lower Carboniferous succession, which continues eastwards across the line of the Ninety Fathom Dyke. A southward Bouguer decrease- across the Butterknowle fault is similarly interpreted as a southward thickening of Lower Carboniferous rocks. The internal structural features of the Alston Block are interpreted in relation to the assumed granite below. A study of the background Bouguer anomaly suggests a North Pennine crustal mass deficiency in addition to the granite. It is considered that the granite mass deficiency, if accompanied by an additional crustal deficiency, could account iso-statically for the present North Pennine uplift, and for the general stability since the Lower Carboniferous.

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