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Journal ArticleDOI

Speleogenetic evidence from Ogof Draenen for a pre-Devensian glaciation in the Brecon Beacons, South Wales, UK

20 Nov 2014-Journal of Quaternary Science (Wiley)-Vol. 29, Iss: 8, pp 815-826

Abstract: The British Isles have been affected by as many as 30 glaciations during the Quaternary. However, the evidence for pre-Devensian glaciations in upland regions is scarce. Understanding the extent and timing of earlier upland glaciations is essential for modelling the long-term evolution and sensitivity of the British Ice Sheet. Caves, being protected from surface erosion and weathering, can preserve evidence of earlier glaciations in the form of speleothem and sediment archives. The ∼70-km-long Ogof Draenen cave system in South Wales, UK, contains multiple cave levels related to changes in the surface topography and drainage during the past 0.5 Ma. The cave contains evidence of massive influxes of sediment that were sufficient to choke the cave and alter the underground drainage. Analysis of the cave sediments, passage morphology and geometry suggests the cave once acted as a subterranean glacial spill-way before being overridden by ice. Speleothem U-series data demonstrate that this sediment influx occurred before Marine Isotope State (MIS) 9, probably during the Anglian glaciation (MIS 12). Evidence from Ogof Draenen indicates the impact of subsequent glaciations on the landscape evolution of the region was minimal and that much of the surface topography dates from the Anglian.
Topics: Cave (58%), Speleothem (57%), Ice sheet (50%)

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SPELEOGENETIC EVIDENCE FROM OGOF DRAENEN FOR
A PRE-DEVENSIAN GLACIATION IN THE BRECON
BEACONS, SOUTH WALES, UK
Andrew R. Farrant
1
, Christopher J. M. Smith
2, 3
, Stephen R. Noble
4
, Michael J. Simms
5
,
David A. Richards
2, 3
1. British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK. E-mail: arf@bgs.ac.uk
2. Bristol Isotope Group (BIG), Wills Memorial Building, University of Bristol, Queen’s Road, Bristol,
BS8 1RJ, UK.
3. School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, UK.
4. NERC Isotope Geosciences Laboratory (NIGL), British Geological Survey, Keyworth, Nottingham,
NG12 5GG, UK.
5. Department of Geology, National Museums Northern Ireland, Cultra, Holywood, Co. Down, BT18
0EU, Northern Ireland.
Abstract
The British Isles have been affected by as many as 30 glaciations during the Quaternary.
However, the evidence for pre-Devensian glaciations in upland regions is scarce.
Understanding the extent and timing of earlier upland glaciations is essential for modelling
the long term evolution and sensitivity of the British Ice Sheet (BIS). Caves, being protected
from surface erosion and weathering, can preserve evidence of earlier glaciations in the form
of speleothem and sediment archives. The ~70 km long Ogof Draenen cave system in South
Wales, UK, contains multiple cave levels related to changes in the surface topography and
drainage during the past 0.5 Ma. The cave contains evidence of massive influxes of sediment
that were sufficient to choke the cave and alter the underground drainage. Analysis of the
cave sediments, passage morphology and geometry suggests the cave once acted as a
subterranean glacial spill-way before being overridden by ice. Speleothem U-series data
demonstrates that this sediment influx occurred before Marine Isotope State (MIS) 9,
probably during the Anglian glaciation (MIS 12). Evidence from Ogof Draenen indicates the
impact of subsequent glaciations on the landscape evolution of the region was minimal and
that much of the surface topography dates from the Anglian.
Keywords: speleothem, glaciation, Wales, U-series dating, U–Th, landscape evolution, Ogof
Draenen
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(Note: Welsh terms used in this paper: Ogof = Cave, Afon = River, Cwm = Valley, Mynydd
= Mountain)
1. Introduction
Most of the upland karst areas in the north and west of the UK have been glaciated multiple
times during the past million years, with the greatest advances during Marine Isotope Stage
(MIS) 12 (Anglian) and MIS 2 (Devensian) glaciations. Until recently there was evidence for
only a small number of glaciations in the UK (Bowen, 1999; Bowen et al., 1986; Clark et al.,
2004). Now perhaps as many as 30 glaciations are known (Böse et al., 2012; Lee et al.,
2012; Lee et al., 2011; Thierens et al., 2012; Toucanne et al., 2009), dating back about 2.6
Ma, although the timing of many remains equivocal. Equally, recent work has shown that the
climatic thresholds required to build glaciers in Britain were much lower than previously
considered with glaciers existing throughout the Little Ice Age (LIA), from the mid-16
th
to
mid-19
th
centuries (Harrison et al., 2014; Kirkbride et al., 2014). Collectively, they indicate
the British Ice Sheet (BIS) was as dynamic and responsive as other Northern Hemisphere ice
sheets, and highly responsive to even subtle changes in climate.
Frequently, the evidence for pre-Devensian glacial activity in many upland areas is often
lacking, and is often inferred only from exotic clasts in river terrace deposits (Whiteman and
Rose, 1992). Typically this absence is attributed to the erosional effect of Devensian ice
sheets removing any evidence of former glaciations, particularly during the Late Glacial
Maximum (LGM). Bias in the glacial record is particularly evident in South Wales, where
evidence for pre-Devensian glaciations is scarce and limited to lowland areas. The
Llanddewi Glacigenic Formation on the Gower Peninsula is the only unequivocal Anglian
age deposit in South Wales, and represents the margins of the Welsh ice sheet at this time
(Gibbard and Clark, 2011).
Based on geomorphological analysis and dating of cave sediments and speleothems, it is clear
that cave systems in upland areas of the UK often pre-date the last glaciation (Waltham et al.,
1997) and, in some cases, extend back to the early Pleistocene (Lundberg et al., 2010; Rowe
et al., 1988; Waltham and Lowe, 2013). These caves can preserve evidence of surface
processes, including glacial activity over long timescales. Glaciations can have profound and
complex effects upon karst landforms and their underlying aquifers, and may destroy, inhibit,
preserve, or stimulate karst development (Ford, 1987; Ford et al., 1983; Ford and Williams,
2007). Glacially-induced valley incision can instigate major changes to underground
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drainage systems as the conduits adjust to new, lower base levels. These modifications are
recorded within cave systems by changes in passage morphology and geometry, and are
analogous to fluvial terraces as recorders of base-level change (Palmer, 1987). Some caves,
depending on local circumstances are affected by glacial meltwater, a modern example being
Castleguard Cave in Canada (Ford, 1983). Sub-glacial water flow can be considerable,
especially in active, wet-based ice streams, and at the margins of glaciers and ice sheets.
Where these are in contact with karstified aquifers, there is scope for significant input of
allogenic meltwater into pre-existing cave systems (Lauritzen, 1984, 1986), injecting fluvio-
glacial sediment deep underground. These caves act as sediment repositories, protected from
subsequent weathering and surface erosion processes on timescales up to 10
6
years. Away
from active drainage networks, relict cave passages can be preserved untouched with little or
no evidence of sub-glacial modification.
Crucially, caves also host speleothem deposits, which can be accurately dated using uranium-
series (U-series) methods (Meyer et al., 2009; Richards and Dorale, 2003). These are often
interbedded with or overlie cave sediments, thus allowing both the timing of cave formation
and sediment deposition to be constrained over the last 500 ka, and with suitable samples,
beyond 500 ka using U–Pb methods (Richards et al., 1998). Given the lack of preserved and
datable surface material in glaciated upland areas, cave systems offer some of the best
prospects for preserving evidence for pre-Devensian landscape evolution. In this study, we
present evidence from speleothem U-series dating, cave sediment analysis and speleo-
morphological data for pre-Devensian glacial activity in upland areas of South Wales, an area
where the preservation of evidence for earlier glaciations is limited.
2. The study area
The Brecon Beacons in southern Wales is a large upland area (900 km
2
) situated on the
northern edge of the South Wales coalfield (Figure 1), which occupies a large elongate east-
west orientated synclinal structure 90 km long and 25 km wide. The Brecon Beacons are
composed predominantly of Devonian sandstone (the ‘Old Red Sandstone’), which dips
gently (between and 20°) to the south. These are overlain by Lower Carboniferous
limestones and a thick sequence of Upper Carboniferous siliciclastics, including the Twrch
Sandstone Formation ('Millstone Grit') and the ‘Coal Measures’, a cyclical sequence of
sandstones and mudstones with some coal seams (Barclay, 1989). The Lower Carboniferous
limestones outcrop around the coalfield, locally forming a relatively narrow but well
developed escarpment, especially along the north-eastern edge of the syncline.
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The limestones are well-karstified, particularly on the northern edge of the coalfield. Many
sinkholes, stream sinks and cave systems are known, with more than 230 km of cave passage
discovered and surveyed. Eight of these cave systems each contain over 8 km of passage
(Table 1). Together they represent some of the best examples of interstratal cave systems in
the UK (Waltham et al., 1997). All are characterised by extensive high-level relict passages
perched above more recent active streamways. Most of them contain copious amounts of
silty or sandy sediment preserved in the higher level relict passages long abandoned by active
streams. This is true of Ogof Draenen, the caves beneath the adjacent Mynydd Llangattock
(Agen Allwedd, Daren Cilau and Craig yr Ffynnon; Smart and Gardner, 1989) and Ogof
Ffynnon Ddu, 40 km further west (Smart and Christopher, 1989). This study is focused on
Ogof Draenen, where a detailed examination of the cave geomorphology (Farrant and Simms,
2011; Farrant and Smart, 2011) coupled with U-series dating of speleothems from the cave,
has enabled a detailed chronology of the cave’s formation and sedimentary history to be
constructed.
3. Ogof Draenen
Ogof Draenen [51.79966ºN, 3.09439ºW] is a complex multiphase intrastratal cave system
located near Blaenavon, 6 km south-west of Abergavenny, South Wales (Figure 1). It
currently stands as one of the longest cave systems in the UK, with ~70 km of surveyed
passages spanning a vertical range of >150 m (Stevens, 1997; Waltham et al., 1997). The
cave underlies Gilwern Hill, The Blorenge and Mynydd y Garn-fawr, which together form
the interfluve between the deeply-incised Usk valley and the smaller Afon Lwyd valley. The
cave has a long and complex history (Simms et al., 1996; Waltham et al., 1997) which is
discussed in detail in Farrant and Simms, (2011). Speleogenesis combined with valley
incision and base-level lowering has left a vertically-stacked series of relict passages
preserved in the limestone beneath the Twrch Formation cap-rock. The highest, and
therefore the oldest cave levels are preserved up to 150 m above the present cave stream with
progressively younger, lower passages developed sequentially down dip to the west. Tracer
tests show the cave stream resurges 6 km beyond the present southern limit of the cave in
Pontypool (Maurice and Guilford, 2011). A relative chronology of cave evolution has been
constructed from speleo-morphological observations throughout the cave, including passage
geometry, dimensions and morphology, and the analysis of palaeoflow directions from
dissolutional scallops, stratified cave deposits, cross bedding and ripple marks. Other
observations such as the transition from vadose to phreatic passage morphologies have
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enabled palaeo-watertable elevations to be fixed. Analysing the relationship between aquifer
geometry, surface topography and the various active and relict conduits in Ogof Draenen has
enabled us to relate these palaeo-watertable elevations and cave levels to changes in the
surface landscape (Simms and Farrant, 2011).
Ogof Draenen comprises four vertically stacked, genetically-separate cave systems linked by
phreatic under-captures (passages developed in the phreatic zone by water draining from an
existing conduit into a newer conduit), shaft drains, chance passage intersections and invasive
vadose inlets. Only the lowest level is hydrologically active today although some relict
passages contain misfit streams. The present autogenic catchment is very small because the
limestone forms only a relatively narrow outcrop along the steep scarp of the Usk valley.
Consequently, recharge throughout the cave’s history has been predominantly allogenic,
derived principally from numerous small streams draining the Upper Carboniferous
siliciclastics that overlie the cave. Streams draining the sandstone feed into a series of
conduits that drain initially down dip and then trend approximately along strike to resurge at
springs in the surrounding valleys. The oldest relict underground drainage system is
represented by the Megadrive conduit and the associated War of the Worlds conduit (Figure
2a). This conduit system drained south-east, roughly along strike to former resurgences at c.
360 m above sea-level (asl) in the Usk valley (Farrant and Simms, 2011). This was
abandoned when the drainage was captured southward to a suite of progressively lower
resurgences at 360-320 m asl following incision in the Afon Lwyd valley. Continued
landscape evolution led to a second major change in the underground drainage pattern, this
time in response to valley incision in the Clydach Gorge to the north, effectively reversing the
hydraulic gradient. This allowed the development of a new, lower level series of passages,
the ‘The Score-Gilwern Passage’ conduit to develop down dip to the west. This drained
northwest to a former resurgence in the Clydach Gorge at 320-300 m asl (Figure 2b).
Renewed incision in the Afon Lwyd valley caused a second reversal in flow direction, this
time to the south. Ultimately, new springs developed 10 km to the south near Pontypool at
120 m asl (Figure 2c) to which the ‘Beyond a Choke streamway presently drains. Ogof
Draenen thus represents a hydrological see-saw, with successive conduits at progressively
lower elevations each draining to different resurgences in response to incision in three
separate valleys. This sequence of events is thought to span much of the Middle to Late
Pleistocene, possibly extending back over a million years into the Early Pleistocene (Simms
and Farrant, 2011).
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Cites background from "Speleogenetic evidence from Ogof Dr..."

  • ...Ford (1989), Klimchouk & Ford (2000a, b), Klimchouk (2007) and Banks et al. (2015). Hypogene karst comprises two components: these are hydrogeological recharge of soluble formations from below and deep-seated sources of geochemically aggressive karst water....

    [...]

  • ...Ogof Draenen, South Wales, UK The longest cave in Wales and the second longest in the UK (70 km) not calculated Farrant et al. (2014)...

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  • ...Ford (1989), Klimchouk & Ford (2000a, b), Klimchouk (2007) and Banks et al....

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Abstract: CHAPTER 1. INTRODUCTION TO KARST. 1.1 Definitions. 1.2 The Relationship Between Karst And General Geomorphology And Hydrogeology. 1.3 The Global Distribution Of Karst. 1.4 The Growth Of Ideas. 1.5 Aims Of The Book. 1.6 Karst Terminology. CHAPTER 2. THE KARST ROCKS. 2.1 Carbonate Rocks And Minerals. 2.2 Limestone Compositions And Depositional Facies. 2.3 Limestone Diagenesis And The Formation Of Dolomite. 2.4 The Evaporite Rocks. 2.5. Quartzites And Siliceous Sandstones. 2.6 Effects Of Lithologic Properties Upon Karst Development. 2.7 Interbedded Clastic Rocks. 2.8 Bedding Planes, Joints, Faults And Fracture Traces. 2.9 Fold Topography. 2.10 Paleokarst Unconformities. CHAPTER 3. 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SPELEOGENESIS: THE DEVELOPMENT OF CAVE SYSTEMS. 7.1 Classifying Cave Systems. 7.2 Building The Plan Patterns Of Unconfined Caves. 7.3 Unconfined Cave Development In Length And Depth. 7.4 System Modifications Occurring Within A Single Phase. 7.5 Multi-Phase Cave Systems. 7.6 Meteoric Water Caves Developed Where There Is Confined Circulation Or Basal Injection Of Water. 7.7 Hypogene Caves: (A) Hydrothermal Caves Associated Chiefly With Co2. 7.8 Hypogene Caves: (B) Caves Formed By Waters Containing H2s. 7.9 Sea Coast Eogenetic Caves. 7.10 Passage Cross-Sections And Smaller Features Of Erosional Morphology. 7.11 Condensation, Condensation Corrosion, And Weathering In Caves. 7.12 Breakdown In Caves. CHAPTER 8. CAVE INTERIOR DEPOSITS. 8.1 Introduction. 8.2 Clastic Sediments. 8.3 Calcite, Aragonite And Other Carbonate Precipitates. 8.4 Other Cave Minerals. 8.5 Ice In Caves. 8.6 Dating Of Calcite Speleothems And Other Cave Deposits. 8.7 Paleo-Environmental Analysis Of Calcite Speleothems. 8.8 Mass Flux Through A Cave System: The Example Of Friar's Hole, W.Va. CHAPTER 9. KARST LANDFORM DEVELOPMENT IN HUMID REGIONS. 9.1 Coupled Hydrological And Geochemical Systems. 9.2 Small Scale Solution Sculpture - Microkarren And Karren. 9.3 Dolines - The 'Diagnostic' Karst Landform? 9.4 The Origin And Development Of Solution Dolines. 9.5 The Origin Of Collapse And Subsidence Depressions. 9.6 Polygonal Karst. 9.7 Morphometric Analysis Of Solution Dolines. 9.8 Landforms Associated With Allogenic Inputs. 9.9 Karst Poljes. 9.10 Corrosional Plains And Shifts In Baselevel. 9.11 Residual Hills On Karst Plains. 9.12 Depositional And Constructional Karst Features. 9.13 Special Features Of Evaporite Terrains. 9.14 Karstic Features Of Quartzose And Other Rocks. 9.15 Sequences Of Carbonate Karst Evolution In Humid Terrains. CHAPTER 10.THE INFLUENCE OF CLIMATE, CLIMATIC CHANGE AND OTHER ENVIRONMENTAL FACTORS ON KARST DEVELOPMENT. 10.1 The Precepts Of Climatic Geomorphology. 10.2 The Hot Arid Extreme. 10.3 The Cold Extreme: 1 Karst Development In Glaciated Terrains. 10.4 The Cold Extreme: 2 Karst Development In Permafrozen Terrains. 10.5 Sea Level Changes, Tectonic Movement And Implications For Coastal Karst Development. 10.6 Polycyclic, Polygenetic And Exhumed Karsts. CHAPTER 11. KARST WATER RESOURCES MANAGEMENT. 11.1 Water Resources And Sustainable Yields. 11.2 Determination Of Available Water Resources. 11.3 Karst Hydrogeological Mapping. 11.4 Human Impacts On Karst Water. 11.5 Groundwater Vulnerability, Protection, And Risk Mapping. 11.6 Dam Building, Leakages, Failures And Impacts. CHAPTER 12. HUMAN IMPACTS AND ENVIRONMENTAL REHABILITATION. 12.1 The Inherent Vulnerability Of Karst Systems. 12.2 Deforestation, Agricultural Impacts And Rocky Desertification. 12.3 Sinkholes Induced By De-Watering, Surcharging, Solution Mining And Other Practices On Karst. 12.4 Problems Of Construction On And In The Karst Rocks - Expect The Unexpected! 12.5 Industrial Exploitation Of Karst Rocks And Minerals. 12.6 Restoration Of Karstlands And Rehabilitation Of Limestone Quarries. 12.7 Sustainable Management Of Karst. 12.8 Scientific, Cultural And Recreational Values Of Karstlands.

1,986 citations


"Speleogenetic evidence from Ogof Dr..." refers background in this paper

  • ...Glaciations can have profound and complex effects upon karst landforms and their underlying aquifers, and may destroy, inhibit, preserve or stimulate karst development (Ford et al., 1983; Ford, 1987; Ford and Williams, 2007)....

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BookDOI
16 Mar 2007

1,183 citations


"Speleogenetic evidence from Ogof Dr..." refers background in this paper

  • ...Glaciations can have profound and complex effects upon karst landforms and their underlying aquifers, and may destroy, inhibit, preserve or stimulate karst development (Ford et al., 1983; Ford, 1987; Ford and Williams, 2007)....

    [...]


Journal ArticleDOI
Arthur N. Palmer1Institutions (1)
Abstract: Limestone caves form along ground-water paths of greatest discharge and solutional aggressiveness. Flow routes that acquire increasing discharge accelerate in growth, while others languish with negligible growth. As discharge increases, a maximum rate of wall retreat is approached, typically about 0.01-0.1 cm/yr, determined by chemical kinetics but nearly unaffected by further increase in discharge. The time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width, discharge, gradient, and PCO2. Most caves require 104 - 105 yr to reach traversable size. Their patterns depend on the mode of ground-water recharge. Sinkhole recharge forms branching caves with tributaries that join downstream as higher-order passages. Maze caves form where (1) steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or (2) discharge or renewal of undersaturation is uniform along many alternate routes. Flood water can form angular networks in fractured rock, anastomotic mazes along low-angle partings, or sponge-work where intergranular pores are dominant. Diffuse recharge also forms networks and spongework, often aided by mixing of chemically different waters. Ramiform caves, with sequential outward branches, are formed mainly by rising thermal or H2S-rich water. Dissolution rates in cooling water increase with discharge, CO2 content, temperature, and thermal gradient, but only at thermal gradients of more than 0.01 °C/m can normal ground-water CO2 form caves without the aid of hypogenic acids or mixing. Artesian flow has no inherent tendency to form maze caves. Geologic structure and stratigraphy influence cave orientation and extent, but alone they do not determine branch-work versus maze character.

845 citations


Journal ArticleDOI
David R. Bridgland1Institutions (1)
Abstract: Staircases of large-scale aggradational river terraces are a notable feature of many valleys in the temperate lattitudes, particularly in areas beyond the reach of the erosive activities of Pleistocene ice sheets. It is now recognized that the cyclic fluctuations of climate during the Quaternary have driven the generation of terraces, through the direct and indirect influence of both temperature and precipitation on fluviatile activity. Where fossiliferous deposits are preserved within terrace sequences it is often possible to date these and to correlate them with the oceanic record, thus providing an important framework for the evidence of environmental change on land. Middle and Late Pleistocene terraces in different areas can commonly be seen to have formed in synchrony with glacial–interglacial cycles or with longer-periodicity megacycles. Climatic forcing alone is insufficient to cause terraces to form, however; uplift is also necessary, so that terrace sequences can provide a useful record of crustal movement. In northwest Europe, where some of the best known studies of river terrace sequences have been carried out, the fluviatile deposits are also an important repository for Palaeolithic artefacts, from which a record of early human occupation can be reconstructed.

368 citations