The Market Weighton gravity anomaly — granite or graben?

Abstract

SUMMARY The local negative gravity anomaly and associated magnetic anomalies around Market Weighton were recently interpreted by Bott, Robinson and Kohnstamm in terms of an underlying unexposed granite penetrating pre-Carboniferous basement. This interpretation was based on objective features of the anomalies which are better explained by a granite than a sedimentary basin, notably the high curvature in the minimum region and the inferred shallow magnetic basement. The graben hypothesis, revived by Arveschoug, is re-examined to test its feasibility. A central profile from NW to SE can be reproduced by such a model graben, but the total Carboniferous thickness needs to be nearly 10 km for a maximum likely density contrast of 150 kg m −3 and the Carboniferous floor needs to be relatively shallow at about 2 km depth in the adjacent regions to NW and SE. Even with this model there is some difficulty in reproducing the high observed curvature in the central region unless unknown spurious shallow effects contribute, and the magnetic anomalies would need to be explained by volcanics. Thus the graben hypothesis requires an exceptionally deep feature of unusual shape. On the basis of available evidence a granite remains the preferred hypothesis.