OR/14/029 Modelled surfaces/volumes

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H Burke, S J Mathers, J P Williamson, S Thorpe, J Ford and R L Terrington. 2014. The London Basin superficial and bedrock LithoFrame 50 Model. Nottingham, UK, British geological Survey.

Geological units modelled

In total, 64 superficial and artificial geological units were modelled (including mass movement deposits). Table 1 lists the units in broad stratigraphic order together with the BGS stratigraphic lexicon code for each (see https://www.bgs.ac.uk/lexicon/) and lithology. Standard BGS map colours have been used for all superficial units in the model (Figure 2). These should be referred to when viewing images of the model in this report. Note that the Head and Clay-with-flints deposits are known to be polycyclic, and in the case of the latter, its formation is likely to have started as early as the Pliocene.

Table 1 Stratigraphic table of artificial and superficial geological units modelled.
Inferred age LEXICON CODE Full name Lithology
Holocene deposits and artificially modified ground slip Landslide deposits Mass movement deposits; variable composition, dependent on the nature of the upslope material
wgr Worked Ground Artificially lowered area, or void, through man-made excavation, e.g. a gravel pit
mgr Made Ground Artificially raised areas, variable composition
wmgr Worked & Made Ground Area of artificial cut and fill, e.g. a backfilled quarry, variable composition
ddgr Disturbed Ground Area of disturbance associated with surface or near-surface collapse
lsgr Landscaped Ground Extensively remodelled areas where it is difficult to delineate zones of Made, Worked or Disturbed Ground. Variable composition
peat Peat Humic deposits, consisting of wet dark brown partially decomposed vegetation
tufa Tufa Inorganic calcium carbonate or sinter deposited at or near springs and seepages
alv Alluvium Fluvial deposits of modern flood plains, consisting of clay, silt, sand and peat
Late Anglian – Devensian glacigenic deposits and river terraces, various catchments rtdu River Terrace Deposits (undifferentiated) Sand and gravel deposits directly beneath alluvium
head Head Solifluction or hillwash deposit, composition dependent on source material. Usually gravelly sandy clay
cwf Clay-with-flints Formation Residual deposit formed through weathering of a previous cover of Palaeogene deposits, and through dissolution of Chalk bedrock. Typically orange-brown and red-brown sandy clay with flint nodules and pebbles
rtdo Pleistocene River Terrace Deposits (unclassified) Exposed river terrace deposits (not below alluvium). Composed of sand and gravel
igd Interglacial Deposits Composed of silty clay
lasi Langley Silt Member Varies from silt to clay, usually yellow brown and massively bedded
shgr Shepperton Gravel Member Gravel with clay and sand
no1a Northmoor Sand and Gravel Member Sand and gravel
no1b
no
rosi Roding Silt Member Varies from silt to clay, usually yellow brown and massively bedded
esi Enfield Silt Member Varies from silt to clay, usually yellow brown and massively bedded
cfsi Crayford Silt Member Varies from silt to clay, usually yellow brown, often contains wind-blown sand
kpgr Kempton Park Gravel Member Sand and gravel, with local lenses of silt, clay or peat
bggr Beenham Grange Gravel Member Sandy clayey gravel
sura Summertown-Radley Sand and Gravel Member Sand and gravel
rtd2 2nd river terrace deposit Sand and gravel
ilsi Ilford Silt Member Sandy clay and silt
tpgr Taplow Gravel Member Sand and gravel, locally with lenses of silt, clay or peat
thgr Thatcham Gravel Member Sandy clayey gravel
wv Wolvercote Sand and Gravel Member Sand and gravel
rtd3 3rd river terrace Sand and gravel
hagr Hackney Gravel Member Sand and gravel, locally with lenses of silt, clay or peat
lhgr Lynch Hill Gravel Member Sand and gravel, locally with lenses of silt, clay or peat
han Hanborough Gravel Member Sand and gravel
rtd4 4th river terrace deposits Sand and gravel
dasi Dartford Silt Member Varies from silt to clay, usually yellow brown, often contains wind-blown sand
figr Finsbury Gravel Member Sand and gravel, locally with lenses of silt, clay or peat
bht Boyn Hill Gravel Member Sand and gravel with possible lenses of silt, clay or peat
rtd5 5th river terrace deposits Sand and gravel
bpgr Black Park Gravel Member Sand and gravel with possible lenses of silt, clay or peat
rtd6 6th river terrace deposit Sand and gravel
sigr Silchester Gravel Member Clayey, sandy gravel
Anglian glaciation loft Lowestoft Formation Till containing chalk and flint clasts
gfdu Glaciofluvial deposits Sand and gravel
gstc Glacial silts and clays Composed of silt and clay
Pre- and Early Anglian terraces, various catchments wihg Winter Hill Gravel Member Clayey, sandy gravel
dhgr Dollis Hill Gravel Member Sandy, clayey gravel, with some laminated silty beds and local silt, clay or peat lenses
wogr Woodford Gravel Member Sand and gravel, locally with lenses of silt, clay, or peat and organic material
rtd7 7th river terrace deposits Sand and gravel
bsgr Beenham Stocks Gravel Member Clayey, sandy gravel
wlgr Westmill Gravel Member Gravel and sand, with local lenses of silt, clay or peat and organic material
gcgr Gerrards Cross Gravel Member Gravel and sand, with local lenses of silt, clay or peat and organic material
bygr Bucklebury Common Gravel Member Clayey, sandy gravel
swgr Satwell Gravel Member Sand and gravel
rtd8 8th river terrace deposit Sand and gravel
stgr Stanmore Gravel Formation Flint-dominated gravel with a clay and sandy clay matrix
whgr Well Hill Gravel Formation Gravel and sandy gravel
cwgr Chorleywood Gravel Member Sand and gravel
cagr Cold Ash Gravel Member Sand and gravel
bdgr Beaconsfield Gravel Member Sand and gravel
suhg Surrey Hill Gravel Member Flint-dominated gravel
chgr Chelsfield Gravel Formation Sandy flint-dominated gravel
wggr Westland Green Gravel Member Sandy, clayey gravel
sgao Sand and gravel of uncertain age and origin Sand and gravel
Figure 2 Superficial geological units modelled in GSI3D.

In addition 12 bedrock units (Tables 2 and 3) were included in the cross-sections and their distributions (envelopes or coverages) modelled in GSI3D, some of these units are faulted and the faults were also defined in the cross-sections. These data were then exported to GOCAD® for calculation of full faulted surfaces to complete the bedrock part of the model. The list of modelled bedrock units is given at Table 2 whilst their relationships and stratigraphic hierarchy is at Table 3.

Table 2. Bedrock units modelled in GSI3D-GOCAD
Lexicon code Name
LNM Lenham Formation
CMBS Camberley Sand Formation
STHP Stanners Hill Pebble Bed
WIDS Windlesham Formation
SAHP St Ann's Hill Pebble Bed
SWCL Swinley Clay Member
BGS Bagshot Formation
CLGB Claygate Member
LC London Clay Formation
HWH Harwich Formation
LMBE Lambeth Group
TAB Thanet Formation


Table 3 Stratigraphy of the bedrock units; those modelled are shown in bold.
Formation Member
Bracklesham Group Camberley Sand Formation
Windlesham Formation Stanners Hill Pebble Bed
Swinley Clay Member
Bagshot Formation St Ann's Hill Pebble Bed
.
Thames Group London Clay Formation Claygate Member
.
Harwich Formation .
Lambeth Group Reading, Woolwich and Upnor Formations
Thanet Sand Formation

Further details of each of the superficial units are given in McMillan et al. (2011)[1] and for all units in the systematic descriptions in the BGS lexicon of named rock units at https://www.bgs.ac.uk/lexicon/ and the geology of each district in the London Basin is covered in the respective BGS geological memoirs and sheet explanations listed in the bibliography.

Finally deeper surfaces defining four further geological units were added to the base of the model to complete coverage to a depth of several hundred metres throughout. The base of the model does vary across the area rather than being terminated by a specified depth. These surfaces were imported from a lower resolution model of the whole London Basin developed in GOCAD® (Terrington et al. 2011[2]). The surfaces comprise the base of the Chalk Group, the Gault and Upper Greensand combined, the Lower Greensand and undivided Jurassic strata.

References

  1. MCMILLAN, A A, HAMBLIN, R J O and MERRITT, J. W. 2011. A lithostratigraphical framework for onshore Quaternary and Neogene (Tertiary) superficial deposits of Great Britain and the Isle of Man. British Geological Survey Research Report, RR/10/03. Available from: http://www.bgs.ac.uk/downloads
  2. TERRINGTON, R L, MEE, K, ALDISS, D T and FORD, J R. 2011 Development of unified geoplogical model surfaces from legacy 3D models in the Thames Basin catchment area In: Model Fusion Conference, London, UK, 28–29 Nov 2011. (Unpublished)