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

TL;DR: In this paper, the extent and timing of earlier upland glaciations in the British Isles is studied. But the evidence for pre-Devensian glaciations is scarce.

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.

Summary

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 and MIS 2 glaciations.
• Bias in the glacial record is particularly evident in South Wales, where evidence for pre-Devensian glaciations is scarce and limited to lowland areas.
• 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.

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 eastwest orientated synclinal structure 90 km long and 25 km wide.
• 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.
• Many sinkholes, stream sinks and cave systems are known, with more than 230 km of cave passage discovered and surveyed.
• All are characterised by extensive high-level relict passages perched above more recent active streamways.
• 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).

4. Cave sediments

• Cave sediments are a conspicuous feature in parts of Ogof Draenen.
• Observation of the sediment fills in and around the northern end of the ‘Beyond a Choke’ streamway and its tributaries (Gilwern Passage, Upstream Passage, ‘The Score’ and Pen-y-Galchen Passages; Figure 3) suggest that three distinct sediment facies occur in this area.
• Minor amounts of coarse sand and fine gravel comprising mudstone and quartz occur in places, but few large clasts are present.
• This passage is part of the northward draining ‘The Score-Gilwern Passage’ conduit, one of the main drains during the evolution of the cave (Farrant and Simms, 2011).
• Moreover, despite the large quantities of sediment injected into the system, no pendants, notches, wall anastomoses, anomalous scalloping or half tubes associated with cave development under conditions of high sediment flux (known as paragenesis, Farrant and Smart, 2011) have been identified in Upstream Passage, Gilwern Passage or their tributaries.

5. Speleothem dating

• Speleothem deposition can only occur in conduits within vadose and epiphreatic (intermittently saturated) zones, because calcite precipitation is primarily driven by the degassing of CO2 from drip waters as they come into contact with the cave atmosphere/air.
• The lower CO2 partial pressure (pCO2) of the cave air allows the pCO2 of saturated groundwaters to equilibrate with the air, resulting in calcite precipitation and speleothem growth.
• The Upstream Passage sample (OD-12-08) was a small stalagmite growing on a deeply-eroded sediment bank close to the present stream level at the northern end of the passage.
• U-series analyses were performed at the Bristol Isotope Group (BIG) facilities, University of Bristol.
• Sub-samples of between 30-150 mg were obtained for 238 U234 U230 Th dating from individual growth layers comprised of clean, dense crystalline calcite.

6. U-series results

• Analytical data for all samples are provided in Table 3.
• U content also varied significantly on an intra-sample level, with stalagmite OD-12-05 (Second Inlet, Gilwern Passage) yielding concentrations between 8,721-52,570 ng g -1 .
• In some cases, the degree of intra-sample U variability can be attributed to open system behaviour resulting from U loss to the calcite crystal lattice structure.
• For the majority of samples, contributions of detrital 230 Th were minimal, resulting in only minor corrections to the final U–Th ages; however, OD-12-13 and OD-12-14 from the ‘Beyond a Choke’ streamway exhibited substantial contributions of detrital 230 Th, resulting in corrected U–Th ages with significantly increased age errors (see Figure S1 in Supplementary Information).
• Due to the limitations of the U–Th dating technique, the absolute precision on isotopic age’s decreases as samples approach the line of secular equilibrium.

7. Age of deposition

• Two strands of evidence suggest that the relict sediments are of considerable antiquity and significantly predate the last Devensian (MIS 2) glaciation.
• In addition to OD-12-05, OD-12-09, also from Gilwern Passage (Giles Barker’s Shirt), yielded a corrected age of 578 +61 -42 ka at 33 mm above base; however analyses performed at 3 and 11 mm above base yielded isotopic ratios showing clear signs of open system behaviour.
• Samples from Giles Baker’s Shirt confirm the passage is older than MIS 9.
• To incise a canyon this deep assuming a fairly typical vadose incision rate of ~5 cm per ka would require 120 ka.
• The sediment influx in the relict high level passages must have occurred a considerable time before MIS 9, most likely during the Anglian glaciation (MIS 12) between 478-424 ka.

8. Glacial geomorphology and landscape evolution

• Glacial deposits in South Wales suggest the region was glaciated on at least two occasions through the Pleistocene; during the Anglian and more recently during the Devensian (Barclay, 1989).
• Glacial till has been mapped across parts of Mynydd Llangattock and till forms an extensive sheet at c. 350-400 m asl around Brynmawr (Barclay, 1989).
• Indeed, it is debatable whether conditions even during the LGM were sufficient to generate these cirques given the short time when ice was present across the region and they may well date from earlier glaciations.
• The authors postulate that sediment-laden meltwater from an Anglian glacier flowed into the cave via inlets along the eastern margin of the Afon Lwyd valley around Blaenavon (>320 m asl).
• The upstream portions of these passages had previously been truncated by valley incision at the head of Cwm Llanwenarth, but, because they form the lowest overspill point in the cave system, they were subsequently reactivated as temporary resurgences.

9. Conclusions

• Detailed speleogenetic and sedimentological observations within the Ogof Draenen cave system has revealed a complex history of cave development, and identified several distinct sediment facies within the network of passages around Gilwern Passage, Upstream Passage, The Score and the present ‘Beyond a Choke’ streamway.
• Speleothem U-series ages show much of the Ogof Draenen cave system to be >>500 ka, with a number of samples exceeding the upper dating limit of the U–Th chronometer.
• The U-series dates imply the sediment influx occurred prior to ~350 ka, most probably during the Anglian glaciation.
• Following the emplacement of these sands, inundation and ponding occurred, probably due to ice overriding the cave and leading to the deposition of the laminated slack-water facies.
• Subsequent glacial advances were largely confined to the present valleys and did not impact significantly on the cave.

Full text

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 5° 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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Frequently asked questions

Q1. What are the contributions mentioned in the paper "Speleogenetic evidence from ogof draenen for a pre-devensian glaciation in the brecon beacons, south wales, uk" ?

In this paper, the extent and timing of earlier upland glaciations is studied for modeling the long term evolution and sensitivity of the British Ice Sheet. 

Q2. What was the reason for the vadose cave development?

The subsequent period of vadose cave development was doubtless a result of Anglian glacial incision altering base-levels, allowing resurgences to develop at lower elevations. 

Q3. What is the composition of the gravel terraces?

gravel terraces of similar composition occur up to 0.5 m above the present stream level, representing former channel stages. 

Q4. What is the way to preserve cave passages?

Away from active drainage networks, relict cave passages can be preserved untouched with little or no evidence of sub-glacial modification. 

Q5. What was the evidence for the influx of sediment?

The influx of sediment led to ponding and localized paragenesis; blocking some passages, reactivating others and, in some cases, facilitating the development of new conduits (Farrant and Smart, 2011). 

Q6. Why is the current autogenic catchment small?

The present autogenic catchment is very small because the limestone forms only a relatively narrow outcrop along the steep scarp of the Usk valley. 

Q7. What is the only unequivocal Anglian age deposit in South Wales?

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). 

Q8. What did the evolution of the drainage pattern in Ogof Draenen mean?

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. 

Q9. What is the recent evidence of glaciation in the UK?

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. 

Q10. What is the significance of the influx of sediment in the Llangattock caves?

Simms and Hunt (2007) provide evidence of sediment influx, glacial flooding and impoundment in Agen Allwedd and suggest that glacial damming and recharge from meltwater might have been a significant factor in the development of the Llangattock caves. 

Q11. What evidence indicates that the Afon Lwyd valley was already incised?

Speleothem U-series evidence presented here indicates that the Afon Lwyd valley was already incised sufficiently deeply to allow groundwater to flow south towards Pontypool prior to MIS 9. 

Q12. How did the Megadrive conduit drain south-east?

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). 

Q13. How high are the plaques of cross bedded sands?

Further upstream, plaques of cross bedded sands (Figure 5) can be seen high up on the passage walls, at least 4-5 m above the present passage floor and extending to within a couple of metres of the roof, here around 8-10 m high. 

Q14. Why is the accuracy of the U–Th dating technique so low?

Due to the limitations of the U–Th dating technique, the absolute precision on isotopic age’s decreases as samples approach the line of secular equilibrium. 

Q15. What is the plausible explanation for the sediments in Ogof Draenen?

The most plausible explanation is that the sediments were emplaced during glacial or pro-glacial conditions when glacial meltwater was able to transport significant amounts of sediment into the cave. 

Q16. What is the time required to capture the drainage?

Given the time needed to initiate, develop and incise the present streamway to sufficient depth to allow speleothem growth, the authors suggest that the incision of the Afon Lwyd valley required to capture the drainage occurred mostly during or shortly after the Anglian glaciation. 

Q17. What is the relationship between the cave and the surface landscape?

The relationship of the cave to the surface landscape indicates the eastern Brecon Beacons attained much of its present morphology during or prior to the Anglian glaciation, with relatively little modification in subsequent glacial advances.