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High-Resolution Geological Maps of Central London, UK: Comparisons with the London Underground

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High-Resolution Geological Maps of Central London, UK: Comparisons with the London Underground Geoscience Frontiers 7 (2016) 273e286 HOSTED BY Contents lists available at ScienceDirect China University of Geosciences (Beijing) Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf Research paper High-resolution geological maps of central London, UK: Comparisons with the London Underground Jonathan D. Paul Department of Earth Sciences, Bullard Laboratories, University of Cambridge, Madingley Rise, Cambridge, CB3 0EZ, UK article info abstract Article history: This study presents new thickness maps of post-Cretaceous sedimentary strata beneath central London. Received 31 March 2015 >1100 borehole records were analysed. London Clay is thickest in the west; thicker deposits extend as a Received in revised form narrow finger along the axis of the London Basin. More minor variations are probably governed by 14 May 2015 periglacial erosion and faulting. A shallow anticline in the Chalk in north-central London has resulted in a Accepted 21 May 2015 pronounced thinning of succeeding strata. These results are compared to the position of London Available online 25 June 2015 Underground railway tunnels. Although tunnels have been bored through the upper levels of London Clay where thick, some tunnels and stations are positioned within the underlying, more lithologically Keywords: London variable, Lower London Tertiary deposits. Although less complex than other geological models of the London Underground London Basin, this technique is more objective and uses a higher density of borehole data. The high London Clay resolution of the resulting maps emphasises the power of modelling an expansive dataset in a rigorous Lambeth Group but simple fashion. Chalk Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Tunnelling Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ Boreholes licenses/by-nc-nd/4.0/). 1. Geological background “relatively simple” (Ellison et al., 2004). Only two faultsebetween Wimbledon and Streatham, and at Greenwicheare currently shown The centre of London, the largest city in the EU with a popula- on the largest-scale geological maps. However, minor faults and tion approaching 15 million, is located approximately on the axis of folds superimposed on this simple synformal structure have been an EeW-trending syncline that constitutes the greater part of the known for nearly a century (Wooldridge, 1923, 1926). Currently, London Basin. This structure formed during late-Oligocene to mid- there is a growing body of evidence for a considerably greater de- Miocene times in response to the Alpine orogeny (Ellison et al., gree of complexity in the structure of the Chalk and the succeeding 2004). Cretaceous chalk is the major aquifer: in central London, stratigraphy, localised swarms of sub-vertical faults, and the Chalk is covered by a thick (up to 70 m) blanket of Cenozoic Pleistocene-aged periglacial erosive features (Berry, 1979; sediments. These sediments include the Eocene-aged London Clay Newman, 2009; Newman et al., 2010; Royse, 2010; Royse et al., Formation, though which much of the London Underground 2012). network was bored, and the more varied underlying strata collec- Such discoveries have largely arisen due to a major leap forward tively termed the Lower London Tertiary deposits (Sumbler, 1996; in our collective computational ability to model the sub-surface in Ellison et al., 2004). three dimensions. 3D block models have typically been applied to Attempts to model or map the sub-surface geology of the Lon- the entire London Basin (Royse, 2010; Mathers et al., 2014), or to don Basin initially focused on a limited number of heavily simpli- discrete localities with implications for major civil engineering fied cross-sections through the entire basin (Whittaker, 1872, 1889; projects (e.g. Aldiss et al., 2012). This study focuses upon an inter- Dewey and Bromehead, 1921). Indeed, the geological structure of mediate scale: central London, as approximately delineated by the the Cretaceous and Palaeogene sediments that overlie the Palae- central Travel Zone 1 of Transport for London (Fig. 1). Surfaces and ozoic basement (the London Platform) has been described as isopachs of the Palaeogene sedimentary succession are computed and discussed with reference to the position of deep London Un- derground railway tunnels.1122 records from the British Geological E-mail address: [email protected]. Survey (BGS) borehole scan archive were extracted and processed Peer-review under responsibility of China University of Geosciences (Beijing). (available in the Supplementary Materials attached to this article). http://dx.doi.org/10.1016/j.gsf.2015.05.004 1674-9871/Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 274 J.D. Paul / Geoscience Frontiers 7 (2016) 273 e 286 Figure 1. (a) Topographic map of central London, based on borehole elevation data. Open circles ¼ 1122 boreholes used in this study. GP ¼ Green Park; SP ¼ St James’s Park; Ba ¼ Bank of England; Br ¼ British Museum; Bu ¼ Buckingham Palace; C ¼ Piccadilly Circus; E ¼ Elephant and Castle; K ¼ King’s Cross railway station; N ¼ Natural History Museum; P ¼ Paddington railway station; T ¼ Tower of London. (b) Topographic map based on OS Terrain 5, a 5 m-resolution Digital Elevation Model (DEM). Anomalous linear features in NE ¼ railway cuttings. Blue lines ¼ subterranean rivers (Barton, 1992; Paul and Blunt, 2012). (c) Landsat satellite image. Lettered black circles ¼ location of photos in Fig. 3. (d) Superficial and bedrock geology, digitised from the BGS DiGMapGB geological dataset. J.D. Paul / Geoscience Frontiers 7 (2016) 273e286 275 Fig. 1a shows their distribution within central London, together 1.1. Stratigraphy with an elevation model calculated from the surface elevation of each borehole record. This model compares favourably with a 5 m- The depositional environments and variation in facies of the resolution Digital Elevation Model (DEM) derived from satellite Palaeogene succession have been well documented (e.g. Sumbler, data (Fig. 1b), emphasising the virtue in generating objective, full- 1996; Ellison et al., 2004; Royse et al., 2012), the following sec- dataset topography using borehole records, and the potential to tion is therefore a brief summary. Fig. 2 is a generalised strati- map the sub-surface topography of geological interfaces. graphic column for the study area. Figure 2. Generalised central London stratigraphic column. Thickness values on left ¼ range of borehole data for 6 units (made ground, drift/alluvium; London Clay; Lambeth Group; Thanet Sand; Chalk). Thickness values on right ¼ typical strata thickness in the London Basin (Ellison et al., 2004; Royse et al., 2012). 276 J.D. Paul / Geoscience Frontiers 7 (2016) 273e286 1.1.1. Chalk Whittaker, 1872). King (1981) used a combination of lithological Formerly divided at the Formation level into Upper, Medium, variation, marine flooding events, and biostratigraphy, to define 5 and Lower divisions, new divisions have recently been defined on laterally and vertically consistent divisions throughout the London the distribution of hardgrounds, chert horizons, and other marker Basin (Fig. 2). This scheme was later refined by Ellison et al. (2004). beds (Royse et al., 2012). Only 6 of the 1122 borehole records The London Clay Formation is underlain by estuarine sands and penetrated through the entire Chalk succession, revealing a range glauconitic pebble-gravels of the Harwich Formation, which rests of thicknesses between 162 and 192 m beneath central London (c.f. unconformably upon the Lambeth Group, filling deeply incised “155e265 m” and “w200 m” for the London Basin; Ellison et al., channels. The youngest Eocene sediments of the London Basin, the 2004; Royse et al., 2012). The Chalk was deposited during mid- to red sands of the Bagshot Formation, rest conformably on the Lon- late-Cretaceous times in open marine conditions, presently forming don Clay. These sediments cap Hampstead Heath and Primrose Hill prominent topographic escarpments on the N and S of the basin in north London, but were largely removed by subsequent uplift (the Chilterns and North Downs, respectively). As the region’s and erosion elsewhere (Sumbler, 1996; Royse et al., 2012). Histori- principal aquifer, the Chalk is famous historically for natural arte- cally, engineers have planned London Underground railway tunnels sian flow, which once provided water for the fountains in Trafalgar to scrupulously remain within the London Clay; Fig. 3d shows the Square (Marsh and Davies, 1983; Paul and Blunt, 2012). Beneath the “box” bored under Westminster Tube Station as part of the Jubilee Chalk, Upper Greensand deposits locally overlie stiff grey Gault Line Extension in the 1990s, the base of which was designed to clays, which are present across the entire basin. correlate with the base of the London Clay (Standing and Burland, 2006; Paul, 2009). However, the nature and engineering proper- 1.1.2. Thanet Sand Formation ties (such as permeability, natural moisture content, and plasticity) Resting unconformably but in hydraulic continuity with the of lower London Clay can differ considerably from those encoun- eroded upper Chalk surface, the Thanet Sands are generally tered higher in the series (Gourvenec et al., 2005). Other engi- disposed as a coarsening-upward sequence of very dense, medium- neering problems, including large and unpredictable volumetric fine, glauconitic silty sands, thickening to a maximum of w30 m in changes, lack of lateral continuity, and intrusion of incompetent, the SE. The unit contains a basal layer, up to 0.5 m thick, of cobble- water-filled sand and gravel lenses, are discussed in Section 2. sized chalk-derived flints known as the Bullhead beds (Sumbler, 1996; Ellison et al., 2004). Dewatering is often required prior to 1.1.5. Alluvium tunnelling work (Aldiss et al., 2012). The post-Anglian evolution of the present Thames drainage planform has been dominated by the cyclic development of a 1.1.3.
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