Abstract

M. H. P. Bott congratulated the authors on their contribution to shelf geology and was pleased to note the general agreement with the earlier interpretations made by the speaker and his colleagues. The controversial point of the paper seemed to be the very steep linear gradient across Exmoor and the Bristol Channel assumed by the authors, which itself must be caused by a shallow structure. Another possibility is a much less steep gradient associated with thicker low density Carboniferous/O.R.S. sediments beneath the northern part of the Bristol Channel. Perhaps the true structure beneath Exmoor lay between the extremes suggested by the speaker and his colleagues (flat regional) and by the authors (steep regional).

Replying on behalf of the authors, DR Brooks agreed that the linear regional gradient of about 0.4 mgal/km is likely to have a shallow origin. Although, as pointed out in the paper, the gradient could in isolation be regarded as being caused by a slight dip on the Moho, or by a slight progressive change of overall crustal density across the survey area, the markedly different regional fields in the adjacent areas of Wales, southwest England and the Celtic Sea count against such a deep origin.

Nonetheless, with regard to the proposed thickness of Carboniferous sediments in the overthrust basin the authors consider that there is little room for manoeuvre. It is argued in the paper that the likeliest cause of the Bristol Channel low (after correction for the effect of Mesozoics) is low density Upper Carboniferous

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