The Gash Breccias of the Pembroke Peninsula, SW Wales

Abstract

The thickest development of Carboniferous Limestone in Great Britain (about 1200 m) is in the Pembroke Peninsula of SW Wales. In various places, the regularity of the normally well-stratified limestone is broken by zones of disturbance, which are spectacularly displayed in magnificent near-vertical cliff sections. The zones generally occupy the whole of the 50 m-high cliffs and are up to 300 m wide. The chief component of these zones is a chaotic, clast-supported breccia, composed of angular limestone fragments welded together with varying degrees of firmness by sparry calcite veining or a normally sparse, red-pink sandy or silty matrix. The breccias are very easy to distinguish and form a striking contrast to the grey cliff scenery hereabouts (Figs 1, 2), yet they have not been discussed much—until now. Figure 1. Location map, showing localities mentioned in the text, the outcrop of the Carboniferous Limestone, the chief synclinal axes and the distribution envelope of the Gash Breccias (after Thomas 1971). 1. Flimston Bay; 2. Bullslaughter Bay (west); 3. Bullslaughter bay (east); 4. Trevallen; 5. Box Bay; 6. Draught; 7. Whitesheet Rock; 8. Lydstep Point; 9. St Margaret's Point; 10. Den Point; 11. Valleyfield Top; 12. Pembroke Castle. Download figure to PowerPoint Figure 2. The St Margaret's Point Gash (SS 119 974), viewing ESE along the Variscan strike direction. The contact is planar and faithfully follows a bedding plane in the Carboniferous Limestone succession. An exposure of the ‘undersurface’ of the breccia mass, examined by boat, displays a faint oblique slickensiding, which is inclined to the east at c. 30°. The height of the cliff at the junction is about 30 m. The breccia can be traced intermittently along the northern coast of St Margaret's Island for a distance of 220 m but it is not known where lie the northern, eastern and western boundaries beneath Caldey Sound. The air photograph reveals that either the southern boundary of this Gash is irregular, with sharp, near-right-angled indentations, or the Gash is affected by later cross faulting. Dixon records that the breccia mass is at least 45 m ‘thick’ so the implied minimum volume of this mass is c. 300 000 m3. Download figure to PowerPoint