OR/18/030 Modelled surfaces/volumes

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Burke, H, Martin, C, and Terrington, R. 2018. Metadata report for the City of London 3D geological model. British Geological Survey Internal Report, OR/18/030.

Table 1 lists the modelled geological units in the relative stratigraphic order used in the model. Brief descriptions are provided below, with more detail available in the (www.bgs.ac.uk/lexicon/home.cfm/). The geological units in Table 1 are coloured as they appear in the model and serve as the legend when viewing images of the model later in this report.

Table 1    List of modelled units.
Geological unit Age Model name Comments
Artificial Ground Quaternary MGR-ARTDP All subtypes of artificial ground are modelled as MGR-ARTDP (made ground, worked ground, infilled ground etc.). Variable composition and thickness.
Alluvium Quaternary ALV-XCZ Mainly consists of silty clay and clayey silt.
Langley Silt Quaternary LASI-Z Mainly composed of silt with minor amounts of gravel. Forms a veneer on the terrace gravels.
Shepperton Gravel Quaternary SHGR-XSV 1st Thames terrace (sub-alluvial gravel), composed of gravel and sand.
Kempton Park Gravel Quaternary KPGR-XSV 2nd Thames terrace, composed of gravel and sand.
Taplow Gravel Quaternary TPGR-XSV 3rd Thames terrace, composed of gravel and sand.
Hackney Gravel Quaternary HAGR-XSV 4th Thames terrace, composed of gravel and sand.
Lynch Hill Gravel Quaternary LHGR-XSV 5th Thames terrace, composed of gravel and sand.
London Clay Formation Palaeogene LC-CLAY Mapped/modelled at rockhead throughout the model area. Composed of silty clay with occasional claystone bends.
Lambeth Group Palaeogene LMBE-CLSSG Lambeth Group (undivided). Variable composition, including clay, sand and gravel.
Thanet Sand Formation Palaeogene TAB-SANDU Primarily composed of grey to green sand with a distinctive basal bed of green coated flints.
Chalk Group Late Cretaceous CK-CHLK Chalk Group (undivided). Composed of white chalk with flinty layers.

Artifical ground

No artificial ground is mapped within the model area. However, artificial ground is known to be present through the vast majority of the model area because of its urbanised nature. Artificial ground is modelled based on its presence and thickness recorded in borehole logs. No distinction has been made on the type of artificial ground present or its composition; all types of artificial ground are modelled as the single unit MGR-ARTDP. The maximum thickness of made ground in the model is 12 m (Figure 2).

Figure 2    Artificial ground thickness grid derived from the model (50 m cell size), shown with a blue to red colour ramp. Areas of thickest artificial ground are coloured red, thinnest in blue, white areas show where artificial ground is absent. Note that the model area does not cover the entire City of London area (outlined in black).

Superficial deposits

A total of seven superficial units are mapped/modelled. These comprise alluvium associated with the River Thames and its tributaries, four Thames terrace gravels and Langley Silt (brickearth), which forms a thin veneer (up to 3.6 m thick) over the terrace gravels. The mapped distribution of Langley Silt is much wider than in the model because it is only modelled where proven in boreholes. Borehole evidence indicates that much of the Langley Silt has been removed through the emplacement of artificial ground.

Bedrock

Four bedrock units are modelled. From youngest to oldest, these are: London Clay Formation, Lambeth Group, Thanet Sand Formation and Chalk Group. The London Clay Formation is composed of grey to bluish grey silty clay with a basal unit of sand, silty clay, sandy silt and silty sand (Ellison, 2004[1]). The top of the London Clay Formation weathers to a brownish colour.

Lambeth Group (formerly named Woolwich and Reading Beds) underlies the London Clay Formation. Three subdivisions within the Lambeth Group are recognised in the literature (Upnor Formation, Woolwich Formation and Reading Formation), but are not separated out in the model. The Lambeth Group is lithologically variable, both vertically and laterally, consisting of mottled and laminated clays, sand, pebble beds, shell beds and limestone bands (Ellison, 2004[1]).

The oldest Paleogene unit in the London area is the Thanet Sand Formation, which underlies the Lambeth Group and rests on the chalk aquifer. The base of the Thanet Sand Formation is defined by a distinctive layer of green coated flints (the Bullhead Bed), which is approximately 0.5 m thick (Ellison, 2004[1]). The Thanet Sand Formation mainly consists of pale to medium grey to brown grey coloured fine grained sand, which can be clayey and silty towards the base (Ellison, 2004[1]).

The basal unit in the model is the Upper Cretaceous Chalk Group. This is mapped as Lewes Nodular Chalk Formation, Seaford Chalk Formation and Newhaven Chalk Formation (undivided), previously the Upper Chalk. This is composed of fine grained white limestone with layers of flint.

References

  1. 1.0 1.1 1.2 1.3 Ellison, R A. 2004. Geology of London. Special memoir for 1:50 000 Geological sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford). British Geological Survey publication.