The Brassington Formation of the Pennines is the most extensive onshore Miocene succession in the UK. It is preserved as outliers in Lower Carboniferous Limestone. During the Cenozoic, central England underwent uplift, with erosion of post-Mississippian strata from the Pennine axis in the Peak District. The Brassington Formation is hence significant in reconstructing Cenozoic geological history. It is non-marine, derived from Triassic sandstone and of Mid–Late Miocene age. The c. 60 outliers occur in three clusters over c. 220 km 2 . They are remnants of a sedimentary prism at least 75 m thick and with a volume of c. 10 km 3 . Suffosion of bedrock, conditioned by aggressive precursor fluids of hypogenic origin, was the major control on subsidence. The calculated volume of the fills in the Bees Nest and Green Clay outliers is 3.7–5.0 × 10 6 m 3 . Referenced to a sub-Miocene surface at c. 450 m OD in pre-subsidence times, the original volume of this subsidence complex was c. 21.8 × 10 6 m 3 and the aggregated volume of the Late Neogene White Peak palaeokarst was at least 0.66 km 3 . Subsidence was concomitant with Pliocene uplift of the Pennine Axis, suggesting suffosion accommodation of 3 × 10 6 m 3 per km 2 or 244 m 3 per annum.

Resume: L’alteration du calcaire carbonifere en Baie de Bullslaughter, Pays-de-Galles meridional : la premiere description de « roche-fantome » dans les Iles Britanniques. Le Calcaire Carbonifere de la baie de Bullslaughter offre un exemple de profonde alteration, parmi les plus significatifs des Iles Britanniques, ainsi que des elements d’une serie enigmatique de breches connues sous le nom de « Gash Breccias ». Le but de cette etude etait d’identifier les processus responsables de l’alteration du calcaire. Il est clair que l’alteration est isovolumetrique mais sa diffusion dans l’espace ne montre pas de gradient vertical et sa position en profondeur suggere que les eaux meteoriques n’ont pas contribue aux processus d’alteration. Cette derniere s’est traduite par une perte importante de calcium pouvant aller jusqu’a une decalcification presque totale. Les mineraux primaires dominants, illite-mica et quartz ont ete conserves, tandis que les mineraux argileux secondaires sont generalement absents. Le calcaire altere n’est pas un saprolite sensu stricto, car il n’a ete que peu affecte par les processus d’alteration chimique. Aussi est-il considere comme une « roche-fantome ». Ce type d’alteration resulte de la dissolution du calcaire dans la zone saturee a faible gradient hydraulique, caracterisee par une lente circulation des eaux. On avance l’hypothese que cette alteration a pu se produire tres precocement pendant les periodes d’emersion du Carbonifere, lorsque le toit des unites cyclothemique etait expose a l’erosion continentale, comme le prouvent les lacunes stratigraphiques et les formes paleokarstiques de dissolution superficielle. C’est la premiere fois que ce type d’alteration en « fantomisation » est decrit dans les Iles Britanniques.

The Carboniferous Limestone at Bullslaughter Bay hosts some of the most notable examples of deep weathering in the British Isles as well as two members of an enigmatic suite of breccias known as the Gash Breccias. The weathered limestone has been investigated thoroughly in order to identify the process responsible for the weathering. In this paper it is demonstrated that the weathering is isovolumetric but the weathering profile is not characterised by a vertical gradient and its depth suggests that meteoric waters did not contribute significantly to the weathering process. The weathered limestone has lost significant amounts of calcium and parts are virtually decalcified. It is seen that the dominant primary minerals of illite and quartz have been preserved while secondary clay minerals are generally absent. The weathered limestone cannot be a saprolite sensu stricto as it has been subjected to only restricted chemical processes. It is, therefore, interpreted as a "ghost-rock". This type of weathering results from chemical dissolution by slow moving waters in the saturated zone. It is suggested that the weathering may have taken place during periods of emergence in the Carboniferous, at the same time as the cyclothem tops were exposed to subaerial modification, as evidenced by omission surfaces and palaeokarstic solution features. This is the first time that ghost-rock weathering has been reported from the British Isles.

This paper presents the results of a systematic investigation of breccia matrices from the so-called Gash Breccias of southern Pembrokeshire, South Wales. The investigation has revealed a Visean (Brigantian) conodont assemblage preserved in the dissolution residue of a limestone breccia at Bullslaughter Bay East. The assemblage comprises eleven taxa including eight genera. Field evidence for its age suggests that the assemblage is derived from either the Asbian Oxwich Head Formation or the Brigantian Oystermouth Formation. The faunal assemblage, however, includes Vogelgnathus aff. postcampbelli, which is not known in the Asbian. It is, thus, certain that the source of the assemblage was Brigantian and the absence of Lochriea nodosa, L. mononodosa and L. ziegleri indicates that the source was probably from lower part of the Brigantian. The bearing of the assemblage on the age and origin of these controversial breccias is discussed. It is concluded that, at modern exposure levels, the Gash Breccias have always been devoid of microfossils indicative of the age of the brecciation.

Geology TodayVolume 24, Issue 4 p. 137-145 The Gash Breccias of the Pembroke Peninsula, SW Wales Peter Walsh, Peter Walsh Katedra Geomorfologii, Uniwersytet Śla̧ski, Bȩdzińska 60,41-200 Sosnowiec, Poland [email protected]Search for more papers by this authorYvonne Battiau-Queney, Yvonne Battiau-Queney Laboratoire de Géomorphologie et Gestion des Millieux Naturels, Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq Cedex, France [email protected]Search for more papers by this authorSid Howells, Sid Howells Sea Hollies, Chapel Lane, Freshwater East, Pembroke, SA71 5LB, UK [email protected]Search for more papers by this authorCliff Ollier, Cliff Ollier School of Earth and Geographical Sciences, The University of Western Australia, Nedlands, WA 6009, Australia [email protected]Search for more papers by this authorMatt Rowberry, Matt Rowberry Institute of Geography and Earth Sciences, Aberystwyth University, Ceredigion, SY23 3DB, UK [email protected]Search for more papers by this author Peter Walsh, Peter Walsh Katedra Geomorfologii, Uniwersytet Śla̧ski, Bȩdzińska 60,41-200 Sosnowiec, Poland [email protected]Search for more papers by this authorYvonne Battiau-Queney, Yvonne Battiau-Queney Laboratoire de Géomorphologie et Gestion des Millieux Naturels, Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq Cedex, France [email protected]Search for more papers by this authorSid Howells, Sid Howells Sea Hollies, Chapel Lane, Freshwater East, Pembroke, SA71 5LB, UK [email protected]Search for more papers by this authorCliff Ollier, Cliff Ollier School of Earth and Geographical Sciences, The University of Western Australia, Nedlands, WA 6009, Australia [email protected]Search for more papers by this authorMatt Rowberry, Matt Rowberry Institute of Geography and Earth Sciences, Aberystwyth University, Ceredigion, SY23 3DB, UK [email protected]Search for more papers by this author First published: 22 July 2008 https://doi.org/10.1111/j.1365-2451.2008.00676.xCitations: 5Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat 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 (1, 2), yet they have not been discussed much—until now. Figure 1Open in figure viewerPowerPoint 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. Figure 2Open in figure viewerPowerPoint 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. Suggestions for further reading Battiau-Queney, Y. 1980. Contribution a l'Étude Géomorphologique du Massif Gallois (G.B.). Thèse Lettres, Université de Bretagne Occidentale, Honore Champion, Paris . Google Scholar Dixon, E.E.L. 1921. The geology of the country around Pembroke and Tenby. Memoir of the Geological Survey of England Wales, London . Google Scholar Hancock, P.L. 1963. The relations between folds and late-formed joints in South Pembrokeshire. Geological Magazine, v.101, pp. 174–184. Google Scholar Ollier, C.D. 2006. Breccia-filled pipes: distinguishing between volcanic and non-volcanic origins Geografica Fisica Dinamica Quaternaria, v.30, pp. 1–14. Google Scholar Simms, M.J. 1990. Triassic palaeokarst in Britain. Cave Science, v.17, pp. 93–101. Google Scholar Tǎmaş, C.G. & Milési, J-P. 2003. Hydrothermal pipe structures – general features and genetic criteria – II. Phreatic breccias. Studia Universitatis Babeş-Bolyai, Geologia, v.48, pp. 55–66. Google Scholar Thomas, T.M. 1970. Field meeting of the South Wales group on the Stack Rocks to Bullslaughter Bay section of the South Pembrokeshire coast. Proceedings of the Geologists' Association, v.81, pp. 241–248. 10.1016/S0016-7878(70)80023-2 Google Scholar Thomas, T.M. 1971. Gash Breccias of South Pembrokeshire: fossil karst phenomena? Transactions of the Institute of British Geographers, v.54. pp. 89–100. 10.2307/621364 Web of Science®Google Scholar Waltham, A.C., Bell, F.G. & Culshaw, M.G. 2005. Sinkholes and Subsidence: Karst and Cavernous Rocks in Engineering and Construction. Praxis Publishing Ltd, Chichester , UK . Google Scholar Wright, V.P. 1983. The polyphase karstification of the Carboniferous Limestone in South Wales. In: K. Patterson & M.M. Sweeting (eds). New Trends in Karst Geomorphology: Anglo-French Symposium, September 1983, pp. 569–580. Geobooks, Norwich . Google Scholar Citing Literature Volume24, Issue4July/August 2008Pages 137-145 ReferencesRelatedInformation

Abstract The best preserved denudation surfaces which are benched into the margins of the pre-Permian oldland massifs of western Britain and Ireland are those comparatively close to sea-level. Such surfaces have frequently been regarded as being of late Pliocene or Pleistocene age and of marine origin. Recent discoveries reviewed in this work reinforce a growing view that these low-level planations are much older and may be of terrestrial origin. Evidence from Wales and southwest England suggests that these little-modified planation elements represent landscapes some 15 Ma old. This paper reviews the palaeobotanical, sedimentological and geomorphological evidence for the close association of several bodies of non-marine Chattian and Miocene sediment and saprolite with some of the better known planation features. In these areas, the former vegetation comprised mixed coniferous/deciduous forest, extinct species of modern north temperate genera, well known from contemporaneous deposits in north Europe. Evidently, differential relief in those times was appreciably less than that of the present and it seems probable that much of what is now the western half of the British Isles was then a wide, forested extension of a previous North European Plain, which varied in altitude by no more than a few decametres.

A fossil flora of Miocene age has been preserved by karstic agencies in a solution subsidence complex developed in a large exotic block of limestone, part of the Gwna Group melange (?Cambrian), at Trwyn y Pare, Cemaes Bay, Anglesey. The flora suggests that the Menaian Surface and the Snowdonian Mountain block were already well established landscape features by the end of the Miocene. The wider geomorphological implications of the find are briefly discussed.