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Home , Alpine orogeny, Lambeth Group, London Basin, London Clay

The geological context and evidence for incipient inversion of the Basin Contexte géologique et arguments en faveur de l’inversion naissante du bassin londonien R.C. Ghail*1, P.J. Mason2 , J.A. Skipper3 1 Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK 2 Department of Earth Sciences & Engineering, Imperial College London, London SW7 2AZ, UK 3 Geotechnical Consulting Group, 52A Cromwell Road, London SW7 5BE, UK

ABSTRACT A reappraisal of ground investigation data across London reveal that a range of unexpected ground conditions, encountered in engineering works since Victorian times, may result from the effects of ongoing inversion of the . Site investigation bore- hole data and the distribution of river terrace deposits of the Thames and its tributaries reveal a complex pattern of block movements, tilting and dextral transcurrent displacement. Significant displacements (~10 m) observed in Thames terrace in borehole TQ38SE1565 at the Lower Lea Crossing, showing that movement has occurred within the last ~100 ka. Restraining bends on reactivated transcurrent faults may explain the occurrence of drift filled hollows, previously identified as fluvially scoured pingos, by faulting and upward migration of water on a flower structure under periglacial conditions. Mapping the location of these features constrains the location of active transcur- rent faults and so helps predict the likelihood of encountering hazardous ground conditions during tunnelling and ground engineering. RÉSUMÉ Une réévaluation des résultats provenant d’études de terrain à travers Londres a révélé une diversité de phénomènes géologiques inattendus. Rencontrés lors de travaux d’ingénierie depuis l’époque victorienne ils pourraient être causés par les suites de l’inversion du basin londonien en cours. Les données provenant de forages et la répartition des sédiments de la terrasse fluviale de la Tamise et ses affluents révèlent un schéma complexe de mouvements de blocs et de failles de cisaillement inclinées ou dextres. Les mouvements significatifs (~10m) observés dans le forage TQ38SE1565 des graviers de la terrasse de la Tamise au niveau du Lower Lea Crossing indiquent un âge inférieur à ~100ka. Des courbes de retenue sur des failles de cisaillement réactivées pourraient expliquer l’existence de creux remplis de sédiments, précédemment catégorisés en tant que pingos érodés par des processus fluviaux, qui seraient plutôt créé par des failles et la migration ascendante d’eau sur des structures en fleur en conditions périglaciaires. En cartographiant ces caractéristiques, la localisation de failles de cisaillement pourra être contrainte et ainsi aider à prédire la probabilité de rencontrer des terrains hasardeux lors de percement de ou de travaux d’ingénierie.

1 INTRODUCTION collision and North Atlantic ridge push (Musson, 2007), but these forces appeared not to have affected The London Basin, a wedge-shaped region ex- the London Basin. However, a reappraisal of the data tending from Newbury to Rochester and Great Yar- show that the geology of the London Basin, its pat- mouth in southern , is one of a number of tern of drainage, and the nature of its periglacial fea- post-Variscan sedimentary basins (Busby and Smith, tures, all strongly indicate recent and ongoing inver- 2001) NW-SE trending basement fractures that were sion of the basin. reactivated as dextral transcurrent faults at various points in the Palaeogene and Neogene, causing inver- sion of the and basins (Blundell, 2 GEOLOGY AND TECTONICS 2002; Chadwick, 1993) the Weald, for example, has been uplifted by more than 1500 m (Jones, 1999a) The London Basin (Figure 1) is one of a number since the . The inversion is likely a result of Permo-, post-Variscan, sedimentary basins of the combined compressive effects of the Alpine (Busby and Smith, 2001) many of which were reac- tivated during Neogene inversion (Chadwick, 1993). were tentatively added (Sumbler, 1996). The many They opened above basement Variscan transcurrent instances of unexpected ground conditions encoun- and thrust faults (Chadwick, 1993) and while each tered in London since Victorian times (Chandler et opened at a slightly different time, all were generally al., 1998; Lenham et al., 2006; Mortimore et al., depositional during the . However, the 2011; Newman, 2009) point to greater structural London Basin lies immediately north of the Variscan complexity than has ever been represented on geo- Front, on the margin of the , logical maps. and is bounded by the hills of the Chilterns and Countless site investigations have revealed brittle the . It formed a subaerial high failures in Palaeogene sediments across London throughout much of the Mesozoic but underwent (Figure 2). A Neolithic trackway, dating from be- complex and rapid deformation during the late Palae- tween 1520 and 1100 BC, unearthed in 1993 in ogene (Knox, 1996), and since then it has remained Beckton [TQ 427 820] was found to be cut by a subaerial and quiescent (Gibbard and Lewin, 2003). which is infilled with clay (Greenwood and Maloney, 1994), implying a significant London earthquake within the last 3000 years. The Earth- quake of 1884, which occurred at the northern edge of the London Basin, was probably the most damag- ing in Britain in the last 400 years (Musson and Winter, 1996).

Figure 2. a) Low angle reverse faults (thrusts) in the sediments beneath the Canary Wharf Crossrail station, 27 m below ground level. White box indicates the position of the detail shown in b) faults associated with periglacial features; c) Uplift of Thames terrace deposits by more than 20 m at Sheppey; and d) persistent tectonic shears in (exposed in a landslip). Scale bar is 0.5 m in each case.

Figure 1. The Cretaceous/Tertiary transtensional basins and Var- Although (Berry, 1979; Hutchinson, iscan basement fractures of southern England. The region is histori- 1980) and Holocene deposits (Akeroyd, 1972) mask cally aseismic but nonetheless influenced by NW-directed Alpine the deeper tectonic features, a regional uplift of at stresses. (Developed from Musson 2007, with Geological Map Data -1 BGS © NERC 2013 and Ordnance Survey Data © Crown copy- least 0.07 mm a across the Thames valley in the last right/database right 2012. An Ordnance Survey/EDINA supplied 900 ka has been detected (Maddy et al., 2000). A service) higher rate in the last 400 ka has also been suggested,

despite recent deglaciation (Nunn, 1983). Blundell Although its structural complexity was recognised (2002) estimates that only 50-70% of the uplift of the nearly a century ago (Wooldridge, 1923). Faults have British Isles, Norway and France in the last 2.5 Ma rarely been recognised in London (Aldiss, 2013) and years can be explained by glacier retreat and denuda- evidence for recent fault displacement is rarer still. A tional offloading. very simple unfaulted synclinal model (Sherlock, 1947) persisted into the 1990s, when some faults Reactivation of basement transcurrent faults dur- 3 DRAINAGE ing the Palaeogene and Neogene has caused inver- sion of both Hampshire and Weald basins (Blundell, The drainage network and chalk aquifer in London 2002; Chadwick, 1993). In the case of the Weald reflect a complex pattern of faulting (de Freitas, more than 1500 m of uplift has occurred since the 2009) that appears to be controlled by the Variscan Cretaceous (Jones, 1999a). Apart from a few scat- basement fractures noted above. The Thames is tered outcrops of Oligocene and Pliocene sediments, commonly considered to be a meandering river but the end of the Palaeocene in the London area is closer inspection reveals a number of sharp bends marked by a gap in the stratigraphy of some 40 Ma; a and straight sections indicative of an active tectonic significant period of subaerial conditions under a environment (Figure 4) with fault-controlled mean- compressive regime between the Alpine and ders. North Atlantic spreading (Musson, 2007), although it is not generally accepted that these forces affected the London Basin. However, a reappraisal of new and existing data in light of these observations shows that inversion is now affecting the London Basin (Royse et al., 2012); an interpretation in agreement with earlier inferences (Blundell, 2002; Wood and Johnson, 1978). Evidence for fault reactivation and inversion can be found in the borehole records held at the British Geological Survey and available publically online. Approximately 2000 borehole records of more than 20 m depth were analysed for the London Basin Fo- rum Atlas Project through a number of student pro- jects. Borehole TQ38SE1565 in the Lower Lea Crossing [TQ 394 809] has a 10.3 m repeated se- quence of Kempton Park (described as Thames Ballast) and London Clay (Figure 3).

Figure 4. Drainage patterns across London; note the sharp bends and straight sections on the Thames and Lea rivers. 1. Pre-Anglian drainage consists of NE-directed tributaries to a proto-Thames across the north of this map. 2. Modern drainage is almost perpen- dicular and ignores the path of the modern Thames. Places referred to in the text are: L=London, R=Richmond, H=Hampton, and W=Walton.

Furthermore, many tributaries to the Thames join at a high angle, implying a regional slope independ- ent of the river floodplain. This relationship is a par- ticularly evident east of Richmond, including in the so-called lost rivers of London (Barton, 1992; de Figure 3. Cross-section in the Lea Valley/Thames confluence showing the repetition of Kempton Park Gravels and London Clay Freitas, 2009). Upstream of Richmond, the Thames encountered in two boreholes, which has been interpreted to have takes a long detour south through Hampton, only been caused by thrusting. turning north again west of Walton. These large-scale deviations have previously been interpreted to result Devensian, 30 to 140 ka ago (Maddy et al., 2001). from river capture induced by glaciation (Gibbard Averaged over that time, the 10.3 m displacement is and Lewin, 2003) but are consistent with a tighter consistent with fault reactivation by inversion and the structural control than that proposed by Gibbard et al. uplift rates inferred earlier. Its location close to the (1988), and are caused by the displacement of base- confluence of the Lea and the Thames, the alignment ment blocks during inversion. the lower Lea and sediment distribution of the upper Other than the Thames, the drainage pattern shows Lea are all consistent with recent, probably ongoing, a dominantly SW—NE orientation of rivers south dextral slip on a basement transcurrent fault under and north of London (indicated by dashed lines (1) in the lower Lea valley. Figure 4) but a SE—NW orientation within London itself (dashed line (2) in Figure 4). Pattern type (1) is also apparent in the Chalk dry valleys south of Lon- 4 PERIGLACIAL FEATURES don. These type (1) valleys predate the diversion of the Thames during the Anglian glacial and are in ex- While glaciation was clearly paramount in the di- actly the orientation expected of tributaries to the version of the Thames, features associated with peri- former course of the Thames. glaciation may have had a deeper tectonic control. So why are the modern tributaries almost perpen- Throughout most of the last 400 ka, London has been dicular to this orientation? Indeed, why do several under periglacial conditions, with the ground perma- change course from that former direction across inner nently frozen to a depth of perhaps tens of metres. London, most notably the river Lea? Prior to the An- The lower Lea, particularly at its confluence with the glian, the landscape had evolved over a very long pe- Thames, is known for its drift filled hollows: anoma- riod of stability (Gibbard and Lewin, 2003) into a lous depressions in the London Clay infilled with al- mature fluvially-controlled landscape centred on the luvium (Figure 5). These hollows are usually associ- Thames flowing through and . The ated with former flowing artesian areas close to the major change to this pattern was not simply the An- Thames and have been interpreted as fluvial scour glian ice advance 300–400 ka ago; if it were, the features (Berry, 1979) in formerly open-system pin- tributaries would retain their former orientation gos (Hutchinson, 1980). However, certain aspects of across London. Rather, it was the initiation of inver- these features, particularly the dyke-like diapiric in- sion some time later that caused this change in direc- jection of Lambeth Group sediments and even Chalk tion by block tilting. upwards into the London Clay, imply deeper driving Note that the distribution of past terrace sediments processes. lies almost exclusively to the north of the current riv- We propose that restraining bends on the transcur- er course; this is also apparent in the sediments of the rent basement faults are the driving mechanism for Lea, north of its abrupt change in course. Together producing drift filled hollows. Restraining (and re- with the change in tributary orientation, this distribu- leasing) bends are a well known feature of transcur- tion implies a gradual tilting of the London basin rent fault systems (Cunningham and Mann, 2007) upwards towards the NW, in contrast to the earlier and act to locally concentrate horizontal displace- tilt SE towards the Weald. Considerable uplift took ment on a transcurrent fault into vertical movement place across southern England, and the Weald in par- on steeply dipping faults in a flower structure. ticular, within the last 2 to 3 Ma (Jones, 1999a, b) but In addition to fracturing and displacing the core of apparently slowed or ceased about 400 ka ago the flower structure upwards, water is also forced up (Maddy et al., 2000). However, strike-slip displace- from depth. During glacial times this water would ment on basement faults appears to have continued have frozen in the near-surface permafrost, causing and driven uplift across London basin, probably as frost-heave that resembles ice wedge or pingo-type far north west as the Chilterns. features and further damaging the ground. During the The faulted river terrace gravels in borehole transition to interglacial times, flow rates in the TQ38SE1565 are inferred to belong to the Kempton Thames and its tributaries was much greater than, al- Park member, dating from the early to middle lowing the rapid scouring of the thawed, fractured core of the flower structure to leave a deep oval de- al. (2015) infer current displacement rates of pression at the surface, later infilled with alluvium of ~1 mm a-1 using satellite data. Devensian age (Hutchinson, 1980). Inversion offers a new insight into the origin of drift filled hollows, as scoured and periglacially al- tered restraining bend flower structures. The location of these features and mapping of the straights and sharp bends on the rivers in London constrains the location of active transcurrent faults and restraining bends (Figure 6) and so helps predict the likelihood of encountering hazardous ground conditions during tunnelling and ground engineering.

Figure 5. Perspective view of London Clay and Lambeth Group top surfaces inferred from ~2000 borehole records. Layer separa- tion is exaggerated for clarity. Note the SE opening depression, likely formed by tectonism exploiting earlier basement fractures, and the steep slope across the north which evidently dis- placed London Clay and may have been recently reactivated by inversion. Localised depressions (1), (2), and (3) may result from restraining or releasing bends on reactivated transcurrent faults, causing damaged ground that was exploited by periglaciation and end-glacial flood scour to form drift filled hollows, a significant engineering hazard. The drift filled hollow at (1) extends into the Lambeth Group below. The depression at (4) does not appear in the London Clay but is evident in the Chalk below; it may be a karst feature.

5 CONCLUSIONS Figure 6. Summary map showing schematically the location of basement dextral transcurrent faults and blocking normal faults. The intersection of these act as restraining bends that may be re- The straight sections and sharp bends on the sponsible for drift filled hollows. Notice also the alignment of trib- Thames and its tributaries, as well as their peculiar utaries across London parallel to transcurrent basement faults. Key orientation, imply and active tectonic environment features are: (1) normal faults may control straight segments on the that explains many of the unexpected features en- Thames; (2) sediments on the upper Lea valley all lie to the north of the river, indicating block tilting towards the NW; (3) the unu- countered during tunnelling and ground engineering sual southward loop of the Thames may also result from block tilt- in London. An understanding of the geological histo- ing. ry of the region shows that it is dominated by SE– NW oriented transcurrent basement faults. Dextral reactivation of these faults by the combined stresses ACKNOWLEDGEMENT of the and North Atlantic spreading has driven inversion on a number of basins in south- Figures include Geological Map and Borehole Data ern Britain such that it is reasonable to assume that BGS © NERC 2013 and Ordnance Survey Data the London Basin is simply the most recent example. © Crown copyright/database right 2012. An Ord- Inversion is continuing at the present day; Mason et nance Survey/EDINA supplied service. The Lambeth Group surface was generated in Move, supplied gra- Lenham, J., Meyer, V., Edmonds, H., Harris, D., Mortimore, R., tis by Midland Valley Exploration Ltd. Reynolds, J., Black, M., 2006. What lies beneath: surveying the Thames at Woolwich. Proceedings of the ICE - Civil Engineering 159, 32-41. Maddy, D., Bridgland, D., Westaway, R., 2001. 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