Gash-Breccias of South Pembrokeshire: Fossil Karstic Phenomena?

Abstract

The paper records the results of mapping on a scale of I: 2500 of the gash-breccia pockets occurring within the Carboniferous Limestone Series of the Pembroke peninsula. Hypotheses for the 'derivation of these deposits through the medium of major cavern collapse or as former 'open-sky' scree accumulations are discussed. It is concluded, however, that the gash-breccias were tectonically produced in the first instance with preferential solution and limited collapse merely providing the finishing touches to the process of brecciation. It is suggested that some of the gash-breccias may be mid-Tertiary rather than Triassic in age. DURING the past two decades both geologists and geomorphologists have studied the nature and origin of the preglacial materials which occupy pipes and larger cavities, or otherwise disrupt the continuity of the normal bedded sequence, within the Carboniferous Limestone Series. Detailed surveys of such deposits have been undertaken in the Vale of Glamorgan,1 along the North Crop of the South Wales coalfield,2 in the Mendip-Bristol region,3 in north-east Wales4 and also in the Peak District.5 Perhaps the most clearly discernible deposits of this type to be found in the British Isles are the gash-breccias, long assumed to be of Triassic age, occurring within the cliff sections of the Pembroke peninsula. Here the detailed structure of individual pockets of gash-breccia is clearly revealed because of their common red-brown or pink colour, contrasting sharply with the prevalent greys of the Carboniferous Limestone. Prior to the present study no detailed examination of these gash-breccias had been made since that of E. E. L. Dixon on behalf of the Geological Survey.6 Dixon postulated that the gash-breccias had been derived by the solvent action of underground waters and the subsequent partial filling of caverns in the Carboniferous Limestone by the collapse of their walls and roofs. This view has been widely quoted by later writers, for example, by J. Pringle and T. N. George7 and J. Challinor.8 In the following account, Dixon's hypothesis is questioned; after considering other possible origins, it is suggested that the gash-breccias are in large measure of tectonic origin. Solution appears to have been preferentially concentrated along distinct shatter belts or at nodal intersections of fractures. While many of these disturbed zones are a direct result of the Armorican orogeny and were subsequently subjected to solution with some minor degree of collapse during Permo-Triassic times, others seem to be of later date and possibly of mid-Tertiary age. GEOLOGICAL BACKGROUND The gash-breccias of south Pembrokeshire are found only within the Carboniferous Limestone Series of the Pembroke peninsula and its outlying islands; they have not been recorded in the narrow outcrop of this formation which fringes the northern limits of the