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On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK
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PGEOLA-504; No. of Pages 7 Proceedings of the Geologists’ Association xxx (2016) xxx–xxx
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On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK Peter Worsley * SAGES, Wager Building, University of Reading, Reading RG6 6AB, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2015 Received in revised form 9 March 2016 Accepted 19 March 2016 Available online xxx
Two partially exposed infilled pipes hosted by chalk were revealed in excavations into a hillside some 50 m above the modern River Thames. The pipes are interpreted as collapse features induced by voids originating at depth rather than classical stream fed sinkholes. Two kinds of infill could be distinguished, sands tentatively assigned to the early Eocene Reading Formation (Lambeth Group) and gravels associated with the mid-Pleistocene Beaconsfield Terrace. The age of development can be constrained by the Thames terrace chronology and this implies major landscape changes due to fluvial erosion since initiation of the pipes some 2 Ma and a reactivation event about 1 Ma. ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Keywords: Karst subsidence Dolines Reading Formation Beaconsfield Terrace Quaternary River Thames Goring Gap
2. Setting
Kempton Park Gravel Formation, which forms a low terrace upon which most of the village of Whitchurch-on-Thames is located, the hill rises fairly uniformly from c. 50 m OD through some 80 m to merge with a plateau surface at c 130 m OD. Beyond lies a small settlement which is part of Goring Heath parish but confusingly is called Whitchurch Hill. The B471 road climbs directly up the hill and road side exposures reveal chalk bedrock at or near the surface throughout. The chalk is assigned to the Seaford Chalk Formation and typically this is a fine grained chalk of low density but of high porosity. There are numerous horizons of flint nodules. At the hill summit, a plateau extends northward and this forms part of the frequently highly dissected palaeo-Thames ‘Higher Gravel Train’ of Wooldridge (1938) which was renamed ‘Beaconsfield Terrace’ by Gibbard (1985). On the terrace, by the B471 road, a borehole proved 7.3 m of gravel on soft chalk (Squirrell, 1976). This spot thickness, which is greater than the 3–4 m norm for the terrace, probably reflects a deeper decalcification front at the chalk/ gravel interface. Some 250 m to the SW of the borehole site are two adjacent depressions 4–6 m deep in the terrace surface. These are probably sinkhole related and express relatively recent and probable ongoing differential dissolution activity.
Whitchurch hill is a south facing slope flanking the incised River Thames valley forming part of the Goring Gap (Fig. 1). From the
3. The study site
1. Introduction Studies of the geology of the Chiltern Hills karst landscapes have produced an extensive literature. Finding solutions to subsidence problems arising from dissolution of chalk are an important element of engineering geology (Matthews et al., 2000; Catt et al., 2010). In the western Chilterns of Oxfordshire alone, well over 1000 sites with natural cavities have now been identified, Edmonds (2008). The geomorphological expression of subsidence is termed either a sinkhole or doline and if surface runoff enters the hollow and becomes subterranean by exploiting a fissure then it is classified as a stream sink. Surface depressions are usually underlain by solution pipes infilled with slumped and faulted unconsolidated mainly non-calcareous sediments derived from the adjacent land surface. Normally these pipes are 1–5 m in diameter and less than 20 m deep although more extreme examples are known to be 20 m across and 50 m deep. Chalkland dolines are also common on some Dorset heaths where the Reading Formation is present, Sperling et al. (1977).
* Tel.: +44 118 984 1600. E-mail address: [email protected]
In 2014, close to the 95 m contour, about half way up the hill on private land at Site 1 (SU 636778), a 12 16 m horizontal shelf was
http://dx.doi.org/10.1016/j.pgeola.2016.03.002 0016-7878/ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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Fig. 1. Location sketch-map for the Whitchurch hill study site above the village of Whitchurch-on-Thames on the north-side of the Goring Gap. Pangbourne village, on the River Thames south bank, is located at northern end of the Sulham Gap through valley to the Kennet. The Thames flows from left to right and forms the boundary between the counties of Berkshire and Oxfordshire. Urbanised areas are crosshatched.
first excavated into the hillslope at the southern border of Firhill Plantation. As might reasonably be predicted, the chalk bedrock cropped- out beneath the hillslope covered by a thin cover of loessrich diamict. Subsequently, at Site 2 (SU 635778) at the same elevation and only 30 m west of the former, a larger shelf (15 21 m) was excavated, but this unexpectedly revealed a much more complex succession and observations on this and its interpretation are the basis of this report. Later in mid-2015, at Site 3 (SU 634778) a few metres lower and 100 m west of the second, another shelf was excavated. Although deeper the exposure had been graded before a detailed study was possible but nevertheless, the main stratigraphic elements identified concurred with those of Site 2. Prior to excavation, Site 3 had been examined in the light of the Site 2 geology yet there was no surface indication of another pipe being present in the sub-surface. At Site 2, the main face some 5 m high, paralleled the contour and was orientated ENE–WSW and at right angles to this, subsidiary faces declined from 5 m to zero downslope. The excavated floor was horizontal (Fig. 2). Most of the exposed sediments consisted of non-chalk materials and six main facies units were distinguished. In ascending stratigraphical order these units were: – 1. Chalk bedrock, the upper parts of which were variously brecciated within putty chalk. 2. A thin flint nodule-rich yellowish red sticky clay. 3. Asequence of bedded pale yellow sands and light grey silty sands. 4. A yellowish red fluvial gravel devoid of obvious sedimentary structures. 5. Grey fluvial sandy gravels restricted to part of the west face. 6. A thin cover of diamict, consisting mainly flint clasts in a silt matrix. Unit 1. The upper surface of the chalk bedrock displayed an irregular relief although in the main section this lay largely below
the base of the exposure. Both subsidiary faces displayed a number of irregular infilled dissolutional pipes ranging between 0.8 and 3.3 m deep penetrating the chalk (Fig. 2c). The top of the chalk was variously brecciated with angular chalk clasts set within a chalk paste matrix. On the freshly excavated section floor, an arcuate contact between Units 2 and 3 and the host chalk cropped out, extending from the WNW corner to within 1.8 m of the ENE corner, with its most southerly position being 5 m from the main face close to the mid-point. This pattern indicates that the horizontal excavation base was effectively a partial slice cut into a much larger circular or elongate pre-existing structure within the chalk. It is probably significant that 40 m upslope from the main face, clast-free light olive brown sand identical to that of Unit 3 was being brought up in animal burrows. Guided by comparisons with similar structural features hosted by chalk it appears that in threedimensions a mega ‘pipe’ with a possible diameter of <50 m has been partially exhumed. Unit 2. This consisted of a thin clay bed containing fresh flint nodules forming a lining over the top of the chalk upon which the later sediments were deposited. It was analogous to the insoluble material or filtration residue, rich in iron and manganese, which is frequently found associated with the dissolution front forming the chalk’s upper surface, i.e. Clay-with-flints sensu stricto. Unit 3. The largest area of the exposure consisted primarily of a 6.8 m thick sequence of mainly planar bedded sand and silty sand totally free of any larger sized clasts. Rare extremely thin clay beds and clay lumps were also present Diagenetic changes involving the deposition of irregular areas of iron-rich material tended to obscure much of the primary bedding but after several weeks, physical weathering had enhanced the size variations in the sands and a stack of distinctly cross-stratified sets up to 0.5 m thick became evident (Fig. 2d). The palaeocurrent flow was towards the south east. As several independent observers commented ‘the sand sequence had the look of the Reading Formation’. In the absence of burrows, the sands are interpreted as deeper water river channel deposits. It is just possible that the sand succession signifies aggradation within a cave void but the total absence of extraneous material at the unit margins and consistency in grain size militates against it. The entire exposed unit had been post-depositionally deformed producing an apparent dip towards the centre of the outcrop from both sides. Unit 4. Inset into the central part of the Unit 3 outcrop was a mass of gravels with a clay-silt matrix and abundant flint, quartzite and vein quartz clasts (Fig. 3a and b). Probably by chance, the base of this unit occurred within the main exposure and apart from the western limit it had a pseudo-channel form in cross section. A vertical to slightly reversed fault, at least 2.7 m deep, and striking at almost a right angle to the main face, defined the west side of the ‘channel’ and entered Unit 3 below (Fig. 3c). At the top it was truncated by Unit 6. The sands of the host Unit 3 dipped towards the fault plane at a maximum of 308. The sediment of Unit 4 appears to have been derived from the overlying Beaconsfield Terrace by progressive subsidence into a void which was developing within Unit 3. Unit 5. A very small area of grey fluvial sandy gravels, similar in lithology to Unit 4, but without the red brown weathered matrix, occupied a pocket above the chalk in the subsidiary west face but clearly post-dated the sands of Unit 3. These lay on a thin Unit 3 over a horizontal platform on the chalk (Fig. 3d). Unit 6. Across the exposures delimiting the excavation, pebbly silt-rich sediment blanketed the hillside (Fig. 3e) and is interpreted primarily as a head/hill wash sheet containing a loess component. Apart from areas of brecciated chalk, no structural evidence related to a former active layer over a permafrost table was identified. Unexpectedly, there was a total absence of ‘cryoturbation’ type structures which are usually interpreted as evidence of
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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Fig. 2. (a) Photograph of the greater part of the main section running parallel to the contour (except the extreme eastern end to the right). The arcuate contact between the host Chalk and the sands of the mega pipe infill crop-out on the excavation floor. (b) The eastern end of the main section, with Reading Formation sands and silts overlying the steeply inclined top of the host rock Chalk. This is a continuation, to the right, of the section shown in Fig. 2a. A small pipe can be seen on the right. (c) Several small dissolution pipes descending into the chalk exposed in the east (west facing) subsidiary face. Note the head sheet blanketing the slope (n.b. there is a surficial thin cover of made ground). (d) Sand facies of Unit 3. Note the cross-stratification which has an apparent dip of 308. This sediment is postulated to have collapsed into the mega pipe from in situ Reading Formation which was originally present a minimum of 30 m above the site. Scale – 20 cm in length. Modern roots are apparent. (e) Close-up view of Reading Formation sand cut by a 458 normal fault with a throw of at least 20 cm. Scale units in cm.
frost activity. Their absence might be a reflection of both the slope angle and net erosion prior to the Holocene. 4. Discussion For many years, it has been recognised that Chiltern sinkholes are predominantly located close to the edge of the Lambeth Group (Palaeogene) outliers, but also along the northern border of the main crop to the south towards the centre of the London Basin (Prestwich, 1855; Thorez et al., 1971). Relatively concentrated drainage off the mainly impermeable Palaeogene encounters the permeable Chalk
and is soon able to penetrate subsurface fractures. This process is facilitated by the drainage being slightly acidic and hence able to initiate dissolution of the carbonate bedrock, i.e. karstification. Three kinds of terrace subsidence, due to dissolutional processes in the middle Thames area, have been identified by McGregor and Green (1983); (i) truncated pipes below the base of terraces which antedate the fluvial aggradation, (ii) pipes extending though both chalk and terrace sediments but are truncated by mass movement (solifluction) material, and (iii) pipes extending down from the modern ground surface which are still underdevelopment. The latter type are the most common. Strictly, none of these types apply at the
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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Fig. 3. (a) The pseudo-channel form (in two dimensional section) of the Beaconsfield terrace derived infill (Unit 4), within the earlier subsided Unit 3 Reading Formation sands and silts. The left-hand margin is marked by a vertical fault – see Fig. 3c. Spade for scale. The right hand margin forms a steep contact with the Reading Formation which dips to the left at 458. (b) Close-up of the Unit 4 facies, clasts mainly of well-rounded ‘Bunter’ quartzites and variously shaped flint in a silty-sand matrix, all derived from the former Beaconsfield terrace above. (c) The fault marking the interface between Units 3 and 4, with the downthrow to the right. This feature defines the west limit of the inset pseudochannel shown in Fig. 3a. (d) Fluvial gravels of Unit 5 facies which post-date the Beaconsfield terrace overlying a thin representative of the Reading Formation. Note the total absence of any rubification. These gravels are evidence for a degraded former terrace which has no surface expression on the hillside. (e) A c 2 m high vertical section lying between the two mega pipes and parallel to the contour. This displays a head facies consisting mainly of derived Beaconsfield Terrace sediment over in situ Chalk. The head blankets the hillside and is probably attributable to the Last Glacial Stage (Devensian). Note the sharp horizontal boundary between the Chalk and the head sheet and an absence of any ‘cryoturbation’ structures.
study site outcrop since erosion related to valley incision has removed the terrace (see below). Surface subsidence adjacent to the Devitt Tower in Pangbourne College (SU 618754), less than 3 km to the SW of the study site, is noteworthy as a potential analogue. There in the early 1990s exploratory drilling established that there was an infilled mega pipe in chalk, some 15–20 m in diameter and 45 m depth. Below the groundwater table, water-filled cavities were found to extend
downwards for at least a further 5 m. Settlement had been triggered by a burst water main draining through the natural pipe fill sediment with an attendant reduction in its bearing strength. It appears that initially the lowermost fill material moved into cavities below the water table thereby creating a void space which was transmitted upwards to finally create the surface subsidence (Edmonds, 2008). Locally the Lambeth Group (Upnor and Reading Formations of c 55 Ma age) has a combined thickness of c 20 m and consists of
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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siliciclastic sediments ranging from clays to sands; and ‘Beds of fine to medium grained sands . . . locally reaching 7 m, occur at all levels within the [Reading] formation’ Mathers and Smith (2000, p. 10). Palaeoenvironmentally in the western Chilterns, these sediments are interpreted as representing river delta distributary systems close to the land-sea transition. The nearest clearly identified Reading Formation outcrop lies some 1.6 km to the north of the Whitchurch hill study site where it forms the Cold HarbourCrays Pond outlier. Currently there are no natural sections, but a decade ago the footings for a sports hall extension to the Oratory Junior School (SU 638802) revealed a succession dominated by pure sands and close by, in the mid 1960s, there was a small working sand pit. In the Chilterns, a diachronous mainly planar transgressive unconformity between the base of the Palaeogene and underlying chalk rises towards the NW at a very low angle (28). At the southern (nearest) end of the Cold Harbour outlier, this unconformity lies at c 155 m OD and regionally it dips to the SE. Hence, assuming no faulting, channel erosion or cave development prior to the commencement of Tertiary deposition, by extrapolation it isprobable that at Whitchurch hill, the unconformity originally had a height between 110 and 120 m. It is postulated that the Unit 3 sands within the sinkhole, were originally part of a now vanished Reading Formation sequence which extended across the area where subsequently the Goring Gap was to develop. In both sedimentological character and thickness, the Unit 3 sediments are quite unlike any other known local Quaternary sequences, especially those of the earlier Thames. Accordingly their preservation appears to be the result of major post-depositional subsidence into a pipe in the chalk prior to the erosion of much of the former Palaeogene cover. It is highly unlikely that the sands are the equivalent to those of Pliocene-Pleistocene age at Little Heath (TL 017082) (Gilbert, 1920; Catt and Maton, 2013), although this possibility cannot be totally excluded. If valid, it would require a much greater western extent than the current evidence permits. The Whitchurch hill pipe is not unique to the area since in the early 1950s a comparable sinkhole infill succession was exposed 8 km to the east in the northern part of the former Emmer Green brickworks clay pit in Caversham, Reading (SU 723773). This feature was documented by Hawkins (1956) and in typical fashion he cryptically commented ‘something out of the ordinary has happened here’. Within the southern area of the pit c 3 m of ‘Plateau Gravels’ were present; these are now classified as Winter Hill Terrace of the Thames and are firmly assigned to the Anglian Glacial Stage. These gravels lay unconformably on 5 m of undisturbed Reading Formation clays lying near horizontally over the Chalk beneath. However, in the northern part of the pit this uniformity was absent. Below the gravels, a 23 m thick sequence of Reading and London Clay Formations, dipped at 308 to the east, with the Reading Formation-London Clay unconformity lying some 12 m below its expected level. This was interpreted by Hawkins as a fragment of a large subsidence feature. He also noted that in the vicinity, the Geological Survey had accounted for variations in the height of the latter unconformity by inferring a complex of faults. However, one such predicted fault which ought to have occurred within the within the brickyard exposures did not exist and this suggests that subsidence might be responsible for the variation in height. The evidence seen in the pit exposure indicated that the land surface originally lay at over 18 m higher than present when the collapse occurred. Since this event was prior to the deposition of the river terrace gravels in his view the subsidence was ‘probably pre-glacial’ in age. Hawkins also reported that a series of trial boreholes close to the north west limits of the in advance of a construction project revealed a discrete area with over 11 m of a ‘soft silty loam’ and beds ‘obviously disarranged and weakened’. He
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speculated that a concealed sinkhole, possibly over 30 m deep, was present but had ‘very considerable antiquity’ since there was no surface expression of it. 5. Landscape context Probably the most remarkable aspect of the Whitchurch hill palaeopipes are their geomorphological position – they are inset into a 750 m long continuous slope of 10–128 which flanks the modern Goring Gap. This slope obliquely truncates the pipes and as there are two contrasting types of pipe infill, two distinct collapse phases have contributed to their infill. First, the initial formation of the main pipe voids must antedate the Beaconsfield Terrace since the erosional base of the terrace planates the chalk at an elevation below the local height of the Tertiary basal unconformity. Any former Lambeth Group sediments must have been removed by erosion before the Beaconsfield Terrace aggradation unless fragments had already been lowered by subsidence into a pipe down to an elevation below the terrace base. The fragment of the Reading Formation in both Whitchurch hill pipes was likely to have been preserved by this mechanism, with the implication that originally the sands were at least 30 m above their current level. It is remarkable that incoherent sands have moved en bloc through such a distance with little loss of their primary bedding character and this fact possibly indicates an extremely low rate of dissolution and allied settlement. Since the norm is for collapsed infills to lose their stratification, this fact argues against a stream sink suffosion with slumping into a progressively deepening pipe – ‘the egg timer effect’. Rather the void space was first created at or below the then existing ground water table and this implies significant concentrated water flow in a conduit at depth within the chalk. Second, the pipe stratigraphy shows a reactivated phase of subsidence after the formation of the Beaconsfield Terrace, as it is difficult to envisage a syndepositional input down to such a depth. Indeed, the introduction of Beaconsfield Gravels into the pipes probably occurred after terrace incision which initiated the first phase in the creation of the Whitchurch hill landform, as the gravels are rubified indicating a prolonged phase of sub-aerial post-depositional weathering of the terrace prior to any subsidence. Since the base of the Beaconsfield Terrace lies at c 125 m OD, sediment derived from it at the study site must also have been lowered by some 30 m. The faulted contact on the west side demonstrates that an accommodation space was likely to have been present below the level of the current exposure immediately prior to the initiation of fault movement. Although the partly resurveyed and revised 1/50,000 scale Reading geological sheet 268 (British Geological Survey, 1999) shows only Chalk or Beaconsfield Terrace cropping out on Whitchurch hill, the Soil Survey 1/63,360 scale sheet 268 of the same area reveals greater complexity (Jarvis, 1968). Soils reflect their parent material and consequently detailed mapping of Soil Series enables further insights into the underlying geology. Some 400 m due east of the study site the soil map shows an isolated ovalshaped soil type, just c 100 m in maximum diameter, centred on SU 638778. This is identified as Shedfield Series (sF) which consist of non-calcareous soils distinguished by weak profile development with only A and C horizons present (a B horizon is lacking). Such soils are termed ‘Rankers’ and critically, these characterise areas underlain by Palaeogene sands. On the soil map, the Ranker outcrop in question is limited to the west by the Berkhamsted Complex Series (BK’) soils on the Beaconsfield Terrace but otherwise is surrounded by chalk soils, i.e. ‘Renzinas’. It is plausible that the soil mapping has identified another Reading Formation outlier (the height approximates with that of the projected sub-Palaeogene unconformity) or alternatively the
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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exposed top of another mega pipe infill. This outcrop lies just below 120 m OD on a Chalk spur defined by Whitchurch hill to the south and the upper part of a dry valley to the north. Pipes may be classified as either a collapse sinkhole or a caprock sinkhole depending upon the initial source of subsidence (Atkinson, 1986; Waltham et al., 2005). Within the chalk the presence of high permeability zones is well known and water abstraction boreholes occasionally encounter these. Two local examples may be cited. First, 1.5 km south of the study site, the boreholes associated with former ‘Reading Corporation Waterworks’ in Pangbourne (SU 634763), yielded water flows from the ‘Chalk Rock’ greatly in excess of those predicted. Hawkins (1945) invoked a hitherto unrecognised flexure in the chalk creating open joints to account for this. Second, at Gatehampton in the bottom of the Goring Gap (SU 603800), 5 km NW of Whitchurch hill, wells produce prodigious groundwater flows from secondary fissures although this is in the Zig Zag chalk which is characterised by horizons of more indurated beds which probably create perched zones of flowing ground water (Price, 1987). Sedimentological evidence in the form of ‘sheet pipes’ indicating horizontal water flows within the Chalk of Hertfordshire was identified by Kirkaldy (1950). He interpreted an intricate contact between the chalk and Tertiary sands as evidence of a cavernous ‘stoping process’ analogous to that found in igneous intrusions. Later, near Brighton, Lamont-Black (1998) reported dissolution cavities or caves infilled by sedimentary structures ‘indicative of deposition from rapid unidirectional currents’. These observations support Edmonds’s (2008) conclusion ‘small-scale interconnected cave systems [within the chalk] may be more common than previously realised’. Indeed, the potential scale may well be greater than just small. We can infer that the down-cutting by the river Thames was likely to have been an episodic process since terraces were formed at successively lower levels down to just above the modern floodplain (Bridgland, 1994). Within the Goring Gap terraces are present although aerially they are relatively small. The presence of the nonrubified Unit 5 gravels, even if very restricted in extent, demonstrates that river depositional activity was occurring more or less coeval with the down cutting by the river at the level of the study site. Previously, without knowledge of Unit 3, there was no direct sedimentary evidence that fluvial processes had formerly operated at this level as part of the incision process contributing towards the creation of today’s Goring Gap, although this had previously been assumed.
7. Conclusion The pipes under consideration are not genetically related to sinkholes where surface streams flow off impermeable strata onto the chalk. Rather they are more likely to be linked to dissolution processes associated with a conduit operating at depth within the chalk. Certainly the observations of Hawkins at Emmer Green are consistent with this view. Development of the Whitchurch hill pipes might span over 1 Ma, with the pipes being first formed when the top of the chalk (with a cover of Reading Formation sands) lay some 30 m higher than present. Upward migration of void space from an enhanced groundwater table flow zone deep within the chalk initiated the pipe and eventually this intercepted the base of the Lambeth Group. Seemingly, blocks of sand gradually gently foundered into the voids and probably filled them. Later, after the deposition and weathering of the Beaconsfield Terrace, further settlement occurred within the pipes possibly induced a similar but now deeper enhanced groundwater flow zone. However, by this stage fluvial erosion by the ancestral Thames had created a floodplain which had reduced the height of the chalk surface to a level corresponding with the base of the Beaconsfield Terrace. Height relationships indicate a lowering of up to 30 m for sediments derived from the terrace in the rejuvenated phase of pipe infilling (see Fig. 4). It is possible that the settlement process has not yet finished.
6. Dating constraints The Beaconsfield Terrace occupies a position within the upper part of the flight of mid Thames Terraces (Sudbury Formation). Uplift modelling of these suggests that it may relate to marine oxygen isotope stage (MOIS) 32 (Westaway et al., 2002). If this estimate is valid, then an age of c. 1.1 Ma can be assigned to the terrace and as a consequence the movement of Unit 5 terrace sediment into the pipe must postdate this. Age data from Chiltern sinkhole infills per se are rare but not unknown. Site investigations in preparation for the construction of the M25 motorway (just north of the M40 interchange), unexpectedly discovered an infilled doline (sinkhole) beneath the floor of a shallow dry valley eroded through an outlier of Gerrards Cross/Lower Gravel Train gravels (Gibbard et al., 1986). A network of boreholes demonstrated that this formed a very steep sided funnel-shaped pipe of c 35–40 m maximum width and 37.5 m depth. The fill consisted of silty clay, sands and sandy gravels with a 5 m thick organic rich clay mud in the central part of the infill succession. The latter pond deposits yielded a pollen spectra characteristic of the later part of the Hoxnian interglacial (MOIS 11 of c 0.4 Ma)
Fig. 4. Cartoons showing the possible sequential development of a pipe and its infill. A–E represent the main stages of development from the early Eocene to the present. F shows a cross-section through the present hillside with the dashed lines indicating the former extent of the Beaconsfield Terrace and the upper pipe prior to the erosion which has produced the modern slope bordering the Goring Gap.
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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Acknowledgements Barry Rubery, Tony Waltham and John Whittow gave encouragement. Clive Edmunds (Peter Brett Associates), Andy Farrant and Andy Newell (both British Geological Survey) and two anonymous referees critically read the manuscript and made constructive comments for its improvement. Hilary Worsley kindly helped with the production of the figures. References Atkinson, T.C., 1986. Soluble rock terrain. In: Fookes, P.G., Vaughan, P.R. (Eds.), A Handbook of Engineering Geomorphology. Surrey University Press, Glasgow, pp. 241–257. Bridgland, D.R., 1994. Quaternary of the Thames. Chapman & Hall, London 441 pp. British Geological Survey, 1999. Reading. England & Wales, Sheet 268, Solid & Drift geology 1:50 000, Keyworth, Nottingham. Catt, J.A., Maton, C., 2013. Little Heath SSSI (TL 017082). In: Catt, J., Murton, J., Brown, E., Maton, C. (Eds.), Quaternary history of the Chiltern Plateau and scarp: Little Heath and Marsworth/Pitstone, scientific and conservation problems. Quaternary Research Association Field Guide, pp. 8–13. Catt, J., Sage, R., Edmonds, C., Banham, P., 2010. Hydrogeology: water supply, water pollution, waste disposal, engineering geology and canals. In: Catt, J. (Ed.), Hertfordshire Geology and Landscape. Hertfordshire Natural History Society, Welwyn Garden City, pp. 256–298. Edmonds, C.N., 2008. Karst and mining geohazards with particular reference to the Chalk outcrop, England. Quarterly Journal of Engineering Geology and Hydrogeology 41, 261–278. Gibbard, P.L., 1985. The Pleistocene History of the Middle Thames Valley. Cambridge University Press, Cambridge 155 pp. Gibbard, P.L., Bryant, I.D., Hall, A.R., 1986. A Hoxnian interglacial doline infilling at Slade Oak Lane, Denham, Buckinghamshire, England. Geological Magazine 123, 27–43. Gilbert, C.J., 1920. On the occurrence of extensive deposits of high-level sands and gravels resting upon the Chalk at Little Heath near Berkhamsted. Quarterly Journal of the Geological Society of London 75, 32–43. Hawkins, H.L., 1945. Field meeting at Pangbourne and Sulham. Proceedings of the Geologists’ Association 56, 228–230.
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Hawkins, H.L., 1956. Hidden swallow-holes near Emmer Green. Reading Naturalist 8, 13–14. Jarvis, R.A., 1968. Soils of the Reading District [sheet 268]. Memoirs of the Soil Survey of Great Britain. Agricultural Research Council, Harpenden 150 pp. Kirkaldy, J.F., 1950. Solution of the chalk in the Mimms Valley, Herts. Proceedings of the Geologists’ Association 61, 219–224. Lamont-Black, J., 1998. A ‘whole rock approach’ to the engineering geology of the Chalk of Sussex. In: Bennett, M.B., Doyle, P. (Eds.), Issues in Environmental Geology. Geological Society, Bath, pp. 122–172. Mathers, S.J., Smith, N.J.P., 2000. Geology of the Reading District. British Geological Survey, Keyworth 30 pp. Matthews, M.C., Clayton, C.R.I., Rigby-Jones, J., 2000. Locating dissolution features in the Chalk. Quarterly Journal of Engineering Geology and Hydrogeology 33, 125–140. McGregor, D.F.M., Green, C.P., 1983. Post-depositional modification of Pleistocene terraces of the River Thames. Boreas 12, 23–34. Prestwich, J., 1855. On the origin of sand and gravel pipes in the Chalk of the London Tertiary district. Quarterly Journal of the Geological Society 11, 64–68. Price, M., 1987. Fluid flow in the Chalk of England. In: Goff, J.O., Williams, B.P.J. (Eds.), Fluid Flow in Sedimentary Basins and Aquifers. Geological Society Special Publication 34, pp. 141–156. Sperling, C.H.B., Goudie, A.G., Stoddart, D.R., Poole, G.G., 1977. Dolines of the Dorset chalklands and other areas in southern Britain. Transactions of the Institute of British Geographers, N.S. 2, 205–224. Squirrell, H.C., 1976. The sand and gravel resources of the Thames and Kennet valleys, the country around Pangbourne, Berkshire: description of 1:25,000 resource sheet SU 67. Mineral Assessment Report Institute of Geological Sciences, 21 97 pp. Thorez, J., Bullock, P., Catt, J.A., Weir, A.H., 1971. The petrology and origin of deposits filling solution pipes in the Chalk near South Mimms, Hertfordshire. Geological Magazine 108, 413–423. Waltham, A.C., Bell, F.G., Culshaw, M.G., 2005. Sinkholes and Subsidence: Karst and Cavernous Rocks in Engineering and Construction. Springer, Praxis, Berlin 382 pp. Westaway, R., Maddy, D., Bridgland, D., 2002. Flow in the lower continental crust as a mechanism for the Quaternary uplift of south-east England: constraints from the Thames terrace record. Quaternary Science Reviews 21, 559–603. Wooldridge, S.W., 1938. The glaciation of the London basin and the evolution of the Lower Thames drainage system. Quarterly Journal of the Geological Society of London 94, 627–667.
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On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK Peter Worsley * SAGES, Wager Building, University of Reading, Reading RG6 6AB, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2015 Received in revised form 9 March 2016 Accepted 19 March 2016 Available online xxx
Two partially exposed infilled pipes hosted by chalk were revealed in excavations into a hillside some 50 m above the modern River Thames. The pipes are interpreted as collapse features induced by voids originating at depth rather than classical stream fed sinkholes. Two kinds of infill could be distinguished, sands tentatively assigned to the early Eocene Reading Formation (Lambeth Group) and gravels associated with the mid-Pleistocene Beaconsfield Terrace. The age of development can be constrained by the Thames terrace chronology and this implies major landscape changes due to fluvial erosion since initiation of the pipes some 2 Ma and a reactivation event about 1 Ma. ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Keywords: Karst subsidence Dolines Reading Formation Beaconsfield Terrace Quaternary River Thames Goring Gap
2. Setting
Kempton Park Gravel Formation, which forms a low terrace upon which most of the village of Whitchurch-on-Thames is located, the hill rises fairly uniformly from c. 50 m OD through some 80 m to merge with a plateau surface at c 130 m OD. Beyond lies a small settlement which is part of Goring Heath parish but confusingly is called Whitchurch Hill. The B471 road climbs directly up the hill and road side exposures reveal chalk bedrock at or near the surface throughout. The chalk is assigned to the Seaford Chalk Formation and typically this is a fine grained chalk of low density but of high porosity. There are numerous horizons of flint nodules. At the hill summit, a plateau extends northward and this forms part of the frequently highly dissected palaeo-Thames ‘Higher Gravel Train’ of Wooldridge (1938) which was renamed ‘Beaconsfield Terrace’ by Gibbard (1985). On the terrace, by the B471 road, a borehole proved 7.3 m of gravel on soft chalk (Squirrell, 1976). This spot thickness, which is greater than the 3–4 m norm for the terrace, probably reflects a deeper decalcification front at the chalk/ gravel interface. Some 250 m to the SW of the borehole site are two adjacent depressions 4–6 m deep in the terrace surface. These are probably sinkhole related and express relatively recent and probable ongoing differential dissolution activity.
Whitchurch hill is a south facing slope flanking the incised River Thames valley forming part of the Goring Gap (Fig. 1). From the
3. The study site
1. Introduction Studies of the geology of the Chiltern Hills karst landscapes have produced an extensive literature. Finding solutions to subsidence problems arising from dissolution of chalk are an important element of engineering geology (Matthews et al., 2000; Catt et al., 2010). In the western Chilterns of Oxfordshire alone, well over 1000 sites with natural cavities have now been identified, Edmonds (2008). The geomorphological expression of subsidence is termed either a sinkhole or doline and if surface runoff enters the hollow and becomes subterranean by exploiting a fissure then it is classified as a stream sink. Surface depressions are usually underlain by solution pipes infilled with slumped and faulted unconsolidated mainly non-calcareous sediments derived from the adjacent land surface. Normally these pipes are 1–5 m in diameter and less than 20 m deep although more extreme examples are known to be 20 m across and 50 m deep. Chalkland dolines are also common on some Dorset heaths where the Reading Formation is present, Sperling et al. (1977).
* Tel.: +44 118 984 1600. E-mail address: [email protected]
In 2014, close to the 95 m contour, about half way up the hill on private land at Site 1 (SU 636778), a 12 16 m horizontal shelf was
http://dx.doi.org/10.1016/j.pgeola.2016.03.002 0016-7878/ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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Fig. 1. Location sketch-map for the Whitchurch hill study site above the village of Whitchurch-on-Thames on the north-side of the Goring Gap. Pangbourne village, on the River Thames south bank, is located at northern end of the Sulham Gap through valley to the Kennet. The Thames flows from left to right and forms the boundary between the counties of Berkshire and Oxfordshire. Urbanised areas are crosshatched.
first excavated into the hillslope at the southern border of Firhill Plantation. As might reasonably be predicted, the chalk bedrock cropped- out beneath the hillslope covered by a thin cover of loessrich diamict. Subsequently, at Site 2 (SU 635778) at the same elevation and only 30 m west of the former, a larger shelf (15 21 m) was excavated, but this unexpectedly revealed a much more complex succession and observations on this and its interpretation are the basis of this report. Later in mid-2015, at Site 3 (SU 634778) a few metres lower and 100 m west of the second, another shelf was excavated. Although deeper the exposure had been graded before a detailed study was possible but nevertheless, the main stratigraphic elements identified concurred with those of Site 2. Prior to excavation, Site 3 had been examined in the light of the Site 2 geology yet there was no surface indication of another pipe being present in the sub-surface. At Site 2, the main face some 5 m high, paralleled the contour and was orientated ENE–WSW and at right angles to this, subsidiary faces declined from 5 m to zero downslope. The excavated floor was horizontal (Fig. 2). Most of the exposed sediments consisted of non-chalk materials and six main facies units were distinguished. In ascending stratigraphical order these units were: – 1. Chalk bedrock, the upper parts of which were variously brecciated within putty chalk. 2. A thin flint nodule-rich yellowish red sticky clay. 3. Asequence of bedded pale yellow sands and light grey silty sands. 4. A yellowish red fluvial gravel devoid of obvious sedimentary structures. 5. Grey fluvial sandy gravels restricted to part of the west face. 6. A thin cover of diamict, consisting mainly flint clasts in a silt matrix. Unit 1. The upper surface of the chalk bedrock displayed an irregular relief although in the main section this lay largely below
the base of the exposure. Both subsidiary faces displayed a number of irregular infilled dissolutional pipes ranging between 0.8 and 3.3 m deep penetrating the chalk (Fig. 2c). The top of the chalk was variously brecciated with angular chalk clasts set within a chalk paste matrix. On the freshly excavated section floor, an arcuate contact between Units 2 and 3 and the host chalk cropped out, extending from the WNW corner to within 1.8 m of the ENE corner, with its most southerly position being 5 m from the main face close to the mid-point. This pattern indicates that the horizontal excavation base was effectively a partial slice cut into a much larger circular or elongate pre-existing structure within the chalk. It is probably significant that 40 m upslope from the main face, clast-free light olive brown sand identical to that of Unit 3 was being brought up in animal burrows. Guided by comparisons with similar structural features hosted by chalk it appears that in threedimensions a mega ‘pipe’ with a possible diameter of <50 m has been partially exhumed. Unit 2. This consisted of a thin clay bed containing fresh flint nodules forming a lining over the top of the chalk upon which the later sediments were deposited. It was analogous to the insoluble material or filtration residue, rich in iron and manganese, which is frequently found associated with the dissolution front forming the chalk’s upper surface, i.e. Clay-with-flints sensu stricto. Unit 3. The largest area of the exposure consisted primarily of a 6.8 m thick sequence of mainly planar bedded sand and silty sand totally free of any larger sized clasts. Rare extremely thin clay beds and clay lumps were also present Diagenetic changes involving the deposition of irregular areas of iron-rich material tended to obscure much of the primary bedding but after several weeks, physical weathering had enhanced the size variations in the sands and a stack of distinctly cross-stratified sets up to 0.5 m thick became evident (Fig. 2d). The palaeocurrent flow was towards the south east. As several independent observers commented ‘the sand sequence had the look of the Reading Formation’. In the absence of burrows, the sands are interpreted as deeper water river channel deposits. It is just possible that the sand succession signifies aggradation within a cave void but the total absence of extraneous material at the unit margins and consistency in grain size militates against it. The entire exposed unit had been post-depositionally deformed producing an apparent dip towards the centre of the outcrop from both sides. Unit 4. Inset into the central part of the Unit 3 outcrop was a mass of gravels with a clay-silt matrix and abundant flint, quartzite and vein quartz clasts (Fig. 3a and b). Probably by chance, the base of this unit occurred within the main exposure and apart from the western limit it had a pseudo-channel form in cross section. A vertical to slightly reversed fault, at least 2.7 m deep, and striking at almost a right angle to the main face, defined the west side of the ‘channel’ and entered Unit 3 below (Fig. 3c). At the top it was truncated by Unit 6. The sands of the host Unit 3 dipped towards the fault plane at a maximum of 308. The sediment of Unit 4 appears to have been derived from the overlying Beaconsfield Terrace by progressive subsidence into a void which was developing within Unit 3. Unit 5. A very small area of grey fluvial sandy gravels, similar in lithology to Unit 4, but without the red brown weathered matrix, occupied a pocket above the chalk in the subsidiary west face but clearly post-dated the sands of Unit 3. These lay on a thin Unit 3 over a horizontal platform on the chalk (Fig. 3d). Unit 6. Across the exposures delimiting the excavation, pebbly silt-rich sediment blanketed the hillside (Fig. 3e) and is interpreted primarily as a head/hill wash sheet containing a loess component. Apart from areas of brecciated chalk, no structural evidence related to a former active layer over a permafrost table was identified. Unexpectedly, there was a total absence of ‘cryoturbation’ type structures which are usually interpreted as evidence of
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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Fig. 2. (a) Photograph of the greater part of the main section running parallel to the contour (except the extreme eastern end to the right). The arcuate contact between the host Chalk and the sands of the mega pipe infill crop-out on the excavation floor. (b) The eastern end of the main section, with Reading Formation sands and silts overlying the steeply inclined top of the host rock Chalk. This is a continuation, to the right, of the section shown in Fig. 2a. A small pipe can be seen on the right. (c) Several small dissolution pipes descending into the chalk exposed in the east (west facing) subsidiary face. Note the head sheet blanketing the slope (n.b. there is a surficial thin cover of made ground). (d) Sand facies of Unit 3. Note the cross-stratification which has an apparent dip of 308. This sediment is postulated to have collapsed into the mega pipe from in situ Reading Formation which was originally present a minimum of 30 m above the site. Scale – 20 cm in length. Modern roots are apparent. (e) Close-up view of Reading Formation sand cut by a 458 normal fault with a throw of at least 20 cm. Scale units in cm.
frost activity. Their absence might be a reflection of both the slope angle and net erosion prior to the Holocene. 4. Discussion For many years, it has been recognised that Chiltern sinkholes are predominantly located close to the edge of the Lambeth Group (Palaeogene) outliers, but also along the northern border of the main crop to the south towards the centre of the London Basin (Prestwich, 1855; Thorez et al., 1971). Relatively concentrated drainage off the mainly impermeable Palaeogene encounters the permeable Chalk
and is soon able to penetrate subsurface fractures. This process is facilitated by the drainage being slightly acidic and hence able to initiate dissolution of the carbonate bedrock, i.e. karstification. Three kinds of terrace subsidence, due to dissolutional processes in the middle Thames area, have been identified by McGregor and Green (1983); (i) truncated pipes below the base of terraces which antedate the fluvial aggradation, (ii) pipes extending though both chalk and terrace sediments but are truncated by mass movement (solifluction) material, and (iii) pipes extending down from the modern ground surface which are still underdevelopment. The latter type are the most common. Strictly, none of these types apply at the
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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Fig. 3. (a) The pseudo-channel form (in two dimensional section) of the Beaconsfield terrace derived infill (Unit 4), within the earlier subsided Unit 3 Reading Formation sands and silts. The left-hand margin is marked by a vertical fault – see Fig. 3c. Spade for scale. The right hand margin forms a steep contact with the Reading Formation which dips to the left at 458. (b) Close-up of the Unit 4 facies, clasts mainly of well-rounded ‘Bunter’ quartzites and variously shaped flint in a silty-sand matrix, all derived from the former Beaconsfield terrace above. (c) The fault marking the interface between Units 3 and 4, with the downthrow to the right. This feature defines the west limit of the inset pseudochannel shown in Fig. 3a. (d) Fluvial gravels of Unit 5 facies which post-date the Beaconsfield terrace overlying a thin representative of the Reading Formation. Note the total absence of any rubification. These gravels are evidence for a degraded former terrace which has no surface expression on the hillside. (e) A c 2 m high vertical section lying between the two mega pipes and parallel to the contour. This displays a head facies consisting mainly of derived Beaconsfield Terrace sediment over in situ Chalk. The head blankets the hillside and is probably attributable to the Last Glacial Stage (Devensian). Note the sharp horizontal boundary between the Chalk and the head sheet and an absence of any ‘cryoturbation’ structures.
study site outcrop since erosion related to valley incision has removed the terrace (see below). Surface subsidence adjacent to the Devitt Tower in Pangbourne College (SU 618754), less than 3 km to the SW of the study site, is noteworthy as a potential analogue. There in the early 1990s exploratory drilling established that there was an infilled mega pipe in chalk, some 15–20 m in diameter and 45 m depth. Below the groundwater table, water-filled cavities were found to extend
downwards for at least a further 5 m. Settlement had been triggered by a burst water main draining through the natural pipe fill sediment with an attendant reduction in its bearing strength. It appears that initially the lowermost fill material moved into cavities below the water table thereby creating a void space which was transmitted upwards to finally create the surface subsidence (Edmonds, 2008). Locally the Lambeth Group (Upnor and Reading Formations of c 55 Ma age) has a combined thickness of c 20 m and consists of
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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siliciclastic sediments ranging from clays to sands; and ‘Beds of fine to medium grained sands . . . locally reaching 7 m, occur at all levels within the [Reading] formation’ Mathers and Smith (2000, p. 10). Palaeoenvironmentally in the western Chilterns, these sediments are interpreted as representing river delta distributary systems close to the land-sea transition. The nearest clearly identified Reading Formation outcrop lies some 1.6 km to the north of the Whitchurch hill study site where it forms the Cold HarbourCrays Pond outlier. Currently there are no natural sections, but a decade ago the footings for a sports hall extension to the Oratory Junior School (SU 638802) revealed a succession dominated by pure sands and close by, in the mid 1960s, there was a small working sand pit. In the Chilterns, a diachronous mainly planar transgressive unconformity between the base of the Palaeogene and underlying chalk rises towards the NW at a very low angle (28). At the southern (nearest) end of the Cold Harbour outlier, this unconformity lies at c 155 m OD and regionally it dips to the SE. Hence, assuming no faulting, channel erosion or cave development prior to the commencement of Tertiary deposition, by extrapolation it isprobable that at Whitchurch hill, the unconformity originally had a height between 110 and 120 m. It is postulated that the Unit 3 sands within the sinkhole, were originally part of a now vanished Reading Formation sequence which extended across the area where subsequently the Goring Gap was to develop. In both sedimentological character and thickness, the Unit 3 sediments are quite unlike any other known local Quaternary sequences, especially those of the earlier Thames. Accordingly their preservation appears to be the result of major post-depositional subsidence into a pipe in the chalk prior to the erosion of much of the former Palaeogene cover. It is highly unlikely that the sands are the equivalent to those of Pliocene-Pleistocene age at Little Heath (TL 017082) (Gilbert, 1920; Catt and Maton, 2013), although this possibility cannot be totally excluded. If valid, it would require a much greater western extent than the current evidence permits. The Whitchurch hill pipe is not unique to the area since in the early 1950s a comparable sinkhole infill succession was exposed 8 km to the east in the northern part of the former Emmer Green brickworks clay pit in Caversham, Reading (SU 723773). This feature was documented by Hawkins (1956) and in typical fashion he cryptically commented ‘something out of the ordinary has happened here’. Within the southern area of the pit c 3 m of ‘Plateau Gravels’ were present; these are now classified as Winter Hill Terrace of the Thames and are firmly assigned to the Anglian Glacial Stage. These gravels lay unconformably on 5 m of undisturbed Reading Formation clays lying near horizontally over the Chalk beneath. However, in the northern part of the pit this uniformity was absent. Below the gravels, a 23 m thick sequence of Reading and London Clay Formations, dipped at 308 to the east, with the Reading Formation-London Clay unconformity lying some 12 m below its expected level. This was interpreted by Hawkins as a fragment of a large subsidence feature. He also noted that in the vicinity, the Geological Survey had accounted for variations in the height of the latter unconformity by inferring a complex of faults. However, one such predicted fault which ought to have occurred within the within the brickyard exposures did not exist and this suggests that subsidence might be responsible for the variation in height. The evidence seen in the pit exposure indicated that the land surface originally lay at over 18 m higher than present when the collapse occurred. Since this event was prior to the deposition of the river terrace gravels in his view the subsidence was ‘probably pre-glacial’ in age. Hawkins also reported that a series of trial boreholes close to the north west limits of the in advance of a construction project revealed a discrete area with over 11 m of a ‘soft silty loam’ and beds ‘obviously disarranged and weakened’. He
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speculated that a concealed sinkhole, possibly over 30 m deep, was present but had ‘very considerable antiquity’ since there was no surface expression of it. 5. Landscape context Probably the most remarkable aspect of the Whitchurch hill palaeopipes are their geomorphological position – they are inset into a 750 m long continuous slope of 10–128 which flanks the modern Goring Gap. This slope obliquely truncates the pipes and as there are two contrasting types of pipe infill, two distinct collapse phases have contributed to their infill. First, the initial formation of the main pipe voids must antedate the Beaconsfield Terrace since the erosional base of the terrace planates the chalk at an elevation below the local height of the Tertiary basal unconformity. Any former Lambeth Group sediments must have been removed by erosion before the Beaconsfield Terrace aggradation unless fragments had already been lowered by subsidence into a pipe down to an elevation below the terrace base. The fragment of the Reading Formation in both Whitchurch hill pipes was likely to have been preserved by this mechanism, with the implication that originally the sands were at least 30 m above their current level. It is remarkable that incoherent sands have moved en bloc through such a distance with little loss of their primary bedding character and this fact possibly indicates an extremely low rate of dissolution and allied settlement. Since the norm is for collapsed infills to lose their stratification, this fact argues against a stream sink suffosion with slumping into a progressively deepening pipe – ‘the egg timer effect’. Rather the void space was first created at or below the then existing ground water table and this implies significant concentrated water flow in a conduit at depth within the chalk. Second, the pipe stratigraphy shows a reactivated phase of subsidence after the formation of the Beaconsfield Terrace, as it is difficult to envisage a syndepositional input down to such a depth. Indeed, the introduction of Beaconsfield Gravels into the pipes probably occurred after terrace incision which initiated the first phase in the creation of the Whitchurch hill landform, as the gravels are rubified indicating a prolonged phase of sub-aerial post-depositional weathering of the terrace prior to any subsidence. Since the base of the Beaconsfield Terrace lies at c 125 m OD, sediment derived from it at the study site must also have been lowered by some 30 m. The faulted contact on the west side demonstrates that an accommodation space was likely to have been present below the level of the current exposure immediately prior to the initiation of fault movement. Although the partly resurveyed and revised 1/50,000 scale Reading geological sheet 268 (British Geological Survey, 1999) shows only Chalk or Beaconsfield Terrace cropping out on Whitchurch hill, the Soil Survey 1/63,360 scale sheet 268 of the same area reveals greater complexity (Jarvis, 1968). Soils reflect their parent material and consequently detailed mapping of Soil Series enables further insights into the underlying geology. Some 400 m due east of the study site the soil map shows an isolated ovalshaped soil type, just c 100 m in maximum diameter, centred on SU 638778. This is identified as Shedfield Series (sF) which consist of non-calcareous soils distinguished by weak profile development with only A and C horizons present (a B horizon is lacking). Such soils are termed ‘Rankers’ and critically, these characterise areas underlain by Palaeogene sands. On the soil map, the Ranker outcrop in question is limited to the west by the Berkhamsted Complex Series (BK’) soils on the Beaconsfield Terrace but otherwise is surrounded by chalk soils, i.e. ‘Renzinas’. It is plausible that the soil mapping has identified another Reading Formation outlier (the height approximates with that of the projected sub-Palaeogene unconformity) or alternatively the
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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exposed top of another mega pipe infill. This outcrop lies just below 120 m OD on a Chalk spur defined by Whitchurch hill to the south and the upper part of a dry valley to the north. Pipes may be classified as either a collapse sinkhole or a caprock sinkhole depending upon the initial source of subsidence (Atkinson, 1986; Waltham et al., 2005). Within the chalk the presence of high permeability zones is well known and water abstraction boreholes occasionally encounter these. Two local examples may be cited. First, 1.5 km south of the study site, the boreholes associated with former ‘Reading Corporation Waterworks’ in Pangbourne (SU 634763), yielded water flows from the ‘Chalk Rock’ greatly in excess of those predicted. Hawkins (1945) invoked a hitherto unrecognised flexure in the chalk creating open joints to account for this. Second, at Gatehampton in the bottom of the Goring Gap (SU 603800), 5 km NW of Whitchurch hill, wells produce prodigious groundwater flows from secondary fissures although this is in the Zig Zag chalk which is characterised by horizons of more indurated beds which probably create perched zones of flowing ground water (Price, 1987). Sedimentological evidence in the form of ‘sheet pipes’ indicating horizontal water flows within the Chalk of Hertfordshire was identified by Kirkaldy (1950). He interpreted an intricate contact between the chalk and Tertiary sands as evidence of a cavernous ‘stoping process’ analogous to that found in igneous intrusions. Later, near Brighton, Lamont-Black (1998) reported dissolution cavities or caves infilled by sedimentary structures ‘indicative of deposition from rapid unidirectional currents’. These observations support Edmonds’s (2008) conclusion ‘small-scale interconnected cave systems [within the chalk] may be more common than previously realised’. Indeed, the potential scale may well be greater than just small. We can infer that the down-cutting by the river Thames was likely to have been an episodic process since terraces were formed at successively lower levels down to just above the modern floodplain (Bridgland, 1994). Within the Goring Gap terraces are present although aerially they are relatively small. The presence of the nonrubified Unit 5 gravels, even if very restricted in extent, demonstrates that river depositional activity was occurring more or less coeval with the down cutting by the river at the level of the study site. Previously, without knowledge of Unit 3, there was no direct sedimentary evidence that fluvial processes had formerly operated at this level as part of the incision process contributing towards the creation of today’s Goring Gap, although this had previously been assumed.
7. Conclusion The pipes under consideration are not genetically related to sinkholes where surface streams flow off impermeable strata onto the chalk. Rather they are more likely to be linked to dissolution processes associated with a conduit operating at depth within the chalk. Certainly the observations of Hawkins at Emmer Green are consistent with this view. Development of the Whitchurch hill pipes might span over 1 Ma, with the pipes being first formed when the top of the chalk (with a cover of Reading Formation sands) lay some 30 m higher than present. Upward migration of void space from an enhanced groundwater table flow zone deep within the chalk initiated the pipe and eventually this intercepted the base of the Lambeth Group. Seemingly, blocks of sand gradually gently foundered into the voids and probably filled them. Later, after the deposition and weathering of the Beaconsfield Terrace, further settlement occurred within the pipes possibly induced a similar but now deeper enhanced groundwater flow zone. However, by this stage fluvial erosion by the ancestral Thames had created a floodplain which had reduced the height of the chalk surface to a level corresponding with the base of the Beaconsfield Terrace. Height relationships indicate a lowering of up to 30 m for sediments derived from the terrace in the rejuvenated phase of pipe infilling (see Fig. 4). It is possible that the settlement process has not yet finished.
6. Dating constraints The Beaconsfield Terrace occupies a position within the upper part of the flight of mid Thames Terraces (Sudbury Formation). Uplift modelling of these suggests that it may relate to marine oxygen isotope stage (MOIS) 32 (Westaway et al., 2002). If this estimate is valid, then an age of c. 1.1 Ma can be assigned to the terrace and as a consequence the movement of Unit 5 terrace sediment into the pipe must postdate this. Age data from Chiltern sinkhole infills per se are rare but not unknown. Site investigations in preparation for the construction of the M25 motorway (just north of the M40 interchange), unexpectedly discovered an infilled doline (sinkhole) beneath the floor of a shallow dry valley eroded through an outlier of Gerrards Cross/Lower Gravel Train gravels (Gibbard et al., 1986). A network of boreholes demonstrated that this formed a very steep sided funnel-shaped pipe of c 35–40 m maximum width and 37.5 m depth. The fill consisted of silty clay, sands and sandy gravels with a 5 m thick organic rich clay mud in the central part of the infill succession. The latter pond deposits yielded a pollen spectra characteristic of the later part of the Hoxnian interglacial (MOIS 11 of c 0.4 Ma)
Fig. 4. Cartoons showing the possible sequential development of a pipe and its infill. A–E represent the main stages of development from the early Eocene to the present. F shows a cross-section through the present hillside with the dashed lines indicating the former extent of the Beaconsfield Terrace and the upper pipe prior to the erosion which has produced the modern slope bordering the Goring Gap.
Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002
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PGEOLA-504; No. of Pages 7 P. Worsley / Proceedings of the Geologists’ Association xxx (2016) xxx–xxx
Acknowledgements Barry Rubery, Tony Waltham and John Whittow gave encouragement. Clive Edmunds (Peter Brett Associates), Andy Farrant and Andy Newell (both British Geological Survey) and two anonymous referees critically read the manuscript and made constructive comments for its improvement. Hilary Worsley kindly helped with the production of the figures. References Atkinson, T.C., 1986. Soluble rock terrain. In: Fookes, P.G., Vaughan, P.R. (Eds.), A Handbook of Engineering Geomorphology. Surrey University Press, Glasgow, pp. 241–257. Bridgland, D.R., 1994. Quaternary of the Thames. Chapman & Hall, London 441 pp. British Geological Survey, 1999. Reading. England & Wales, Sheet 268, Solid & Drift geology 1:50 000, Keyworth, Nottingham. Catt, J.A., Maton, C., 2013. Little Heath SSSI (TL 017082). In: Catt, J., Murton, J., Brown, E., Maton, C. (Eds.), Quaternary history of the Chiltern Plateau and scarp: Little Heath and Marsworth/Pitstone, scientific and conservation problems. Quaternary Research Association Field Guide, pp. 8–13. Catt, J., Sage, R., Edmonds, C., Banham, P., 2010. Hydrogeology: water supply, water pollution, waste disposal, engineering geology and canals. In: Catt, J. (Ed.), Hertfordshire Geology and Landscape. Hertfordshire Natural History Society, Welwyn Garden City, pp. 256–298. Edmonds, C.N., 2008. Karst and mining geohazards with particular reference to the Chalk outcrop, England. Quarterly Journal of Engineering Geology and Hydrogeology 41, 261–278. Gibbard, P.L., 1985. The Pleistocene History of the Middle Thames Valley. Cambridge University Press, Cambridge 155 pp. Gibbard, P.L., Bryant, I.D., Hall, A.R., 1986. A Hoxnian interglacial doline infilling at Slade Oak Lane, Denham, Buckinghamshire, England. Geological Magazine 123, 27–43. Gilbert, C.J., 1920. On the occurrence of extensive deposits of high-level sands and gravels resting upon the Chalk at Little Heath near Berkhamsted. Quarterly Journal of the Geological Society of London 75, 32–43. Hawkins, H.L., 1945. Field meeting at Pangbourne and Sulham. Proceedings of the Geologists’ Association 56, 228–230.
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Please cite this article in press as: Worsley, P., On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK. Proc. Geol. Assoc. (2016), http://dx.doi.org/10.1016/j.pgeola.2016.03.002