The Gash Breccias of the Pembroke Peninsula, SW Wales

Walsh, Peter; Battiau-Queney, Yvonne; Howells, Sid; Ollier, C. D.; Rowberry, Matt

doi:10.1111/j.1365-2451.2008.00676.x

Cited by 6 publications

(24 citation statements)

References 4 publications

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“…3b). Walsh et al (2008) concluded that the megabreccias were formed by more than one mechanism, and suggested a third: phreatic explosion due to upward escape of thermally-driven superheated fluids (Fig. 3d).…”

Section: Previous Work and Hypothesesmentioning

confidence: 96%

“…Ferroan dolomite nodules occur (Walsh et al, 2008) in the breccia zones at Draught (locality 13, Fig. 2).…”

Section: Carbonate Cementmentioning

confidence: 97%

“…The history of research on the Pembrokeshire megabreccias has been reviewed by Walsh et al (2008). Early research produced two genetic hypotheses.…”

Section: Previous Work and Hypothesesmentioning

confidence: 99%

“…Sibson, 1986) and phreato-magmatic explosion in igneous and hydrothermal settings. One area where all these mechanisms have been proposed for the same suite of breccias is the Pembroke Peninsula, south Wales (review by Walsh et al, 2008) where chaotic megabreccias (e.g. Fig.…”

Section: Introductionmentioning

confidence: 98%

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Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

Woodcock

Miller

Woodhouse

2014

Journal of Structural Geology

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a b s t r a c tChaotic breccias and megabreccias e locally called gash breccias e hosted within the Pembroke Limestone Group (Visean, Mississippian, lower Carboniferous) of southwest Wales are re-mapped along with spatially-related crackle and mosaic breccias. Of thirteen studied megabreccia bodies, seven lie along steep, NNW-or NNE-striking strike-slip faults originating during northesouth Variscan (late Carboniferous) shortening, though reactivated during later extension. Four bodies are conformable with EeW striking, steeply-dipping bedding, and two have irregular or indeterminate margins. The beddingparallel zones are interpreted as the dilational tips of listric normal faults, and the cross-strike faults as transtensional transfer zones. Sub-horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults. Most fissures have geometrically matched margins produced by this dilational faulting, and only locally have the indented margins indicating solutional processes. The most likely age for the main fissure extension and fill is late Triassic, based on analogous dated fills at the eastern end of the Bristol Channel Basin. The Pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones.

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Section: Previous Work and Hypothesesmentioning

confidence: 96%

“…Ferroan dolomite nodules occur (Walsh et al, 2008) in the breccia zones at Draught (locality 13, Fig. 2).…”

Section: Carbonate Cementmentioning

confidence: 97%

“…The history of research on the Pembrokeshire megabreccias has been reviewed by Walsh et al (2008). Early research produced two genetic hypotheses.…”

Section: Previous Work and Hypothesesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

See 2 more Smart Citations

Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

Woodcock

Miller

Woodhouse

2014

Journal of Structural Geology

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“…The aperture of resulting fault fissures at any one time is uncertain, but would have been only a fraction of the fault zone width. Although Mewslade is apparently the only example of this form of fissure fill on the Gower peninsula, examples may occur along strike to the west in Pembrokeshire (Dixon, 1921;Walsh et al 2008), and have been described from Carboniferous limestones in northwest England (Woodcock, Omma & Dickson, 2006).…”

Section: D Mewsladementioning

confidence: 99%

Fissure fills along faults: Variscan examples from Gower, South Wales

2009

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-The extent to which persistent, rather than transient, fissures (wide planar voids) can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbonbearing fluids. Variscan (late Carboniferous) faults cutting Dinantian (Lower Carboniferous) limestones on the Gower peninsula, South Wales, host clear evidence for fissures up to several metres wide. Evidence includes dendritic hematite growth and elongate calcite growth into open voids, spar ball and cockade breccia formation, laminated sediment infill and void-collapse breccias. Detailed mapping reveals cross-cutting geometries and brecciation of earlier fissure fills, showing that fissures were formed during, rather than after, active faulting. Fissures therefore probably formed by geometric mismatch between displaced fault walls, rather than by solution widening along inactive faults.

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The process of ghost-rock karstification and its role in the formation of cave systems

Dubois

Quinif

Baele

et al. 2014

Earth-Science Reviews

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This paper presents an extensive review of the process of ghost-rock karstification and highlights its role in the formation of cave systems. The process integrates chemical weathering and mechanical erosion and extends a number of existing theories pertaining to continental landscape development. It is a two stage process that differs in many respects from the traditional single-stage process of karstification by total removal. The first stage is characterised by chemical dissolution and removal of the soluble species. It requires low hydrodynamic energy and creates a ghost-rock feature filled with residual alterite. The second stage is characterised by mechanical erosion of the undissolved particles. It requires high hydrodynamic energy and it is only then that open galleries are created. The transition from the first stage to the second is driven by the amount of energy within the thermodynamic system. The process is illustrated by detailed field observations and the results of the laboratory analyses of samples taken from the karstotype area around Soignies in southern Belgium. Thereafter, a series of case studies provide a synthesis of field observations and laboratory analyses from across western Europe. These studies come from geologically distinct parts of Belgium, France, Italy, and the United Kingdom. The process of ghost-rock karstification challenges a number of axioms associated with the process of karstification by total removal. On the basis of the evidence presented it is argued that it is no longer acceptable to use karst morphologies as a basis with which to infer specific karstogenetic processes and it is no longer necessary for a karst system to relate to base level as ghost-rock karstification proceeds along transmissive pathways in the rock. There is also some evidence to suggest that ghost-rock karstification may be superseded by karstification by total removal, and vice versa, according to the amount of energy within the thermodynamic system. The proposed chemical weathering and subsequent mechanical erosion of limestone suggest that the development of karst terrain is related far more closely to the geomorphological development of aluminosilicate and siliceous terrains than is generally supposed. It is now necessary to reconsider the origin of many karst systems in light of the outlined process of ghost-rock karstification.

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The Gash Breccias of the Pembroke Peninsula, SW Wales

Cited by 6 publications

References 4 publications

Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

Fissure fills along faults: Variscan examples from Gower, South Wales

The process of ghost-rock karstification and its role in the formation of cave systems

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