OR/15/074 Modelled surfaces/volumes

Ambrose, K. 2015. GSI3D model metadata report for HS2 Area 8 (Drayton Bassett to Rugeley). British Geological Survey. (OR/15/074).

The modelled bedrock, superficial and artificial deposits are listed in Table 1 in the relative stratigraphic order used in the model. Brief descriptions of the geological units are given here, but more detail can be found in the BGS Lexicon of Named Rock Units. The level of detail and extent of the natural geology in the model may differ from that shown in other BGS datasets. Artificial ground was modelled according to the corresponding 1:50 000 scale geological maps. Table 1 should be used as the legend for viewing images of the model in this report.

Table 1    Geological units modelled in Area 8.

LEX-RCS code	Lex_Description	Comments including included units in DiGMapGB-50
WMGR-ARTDP	WORKED AND MADE GROUND	Variable composition
MGR-ARTDP	MADE GROUND	Variable composition
WGR-VOID	WORKED GROUND
LSGR-UKNOWN	LANDSCAPED GROUND	Variable composition
ALV-XCZSV	ALLUVIUM	Clay, silt, sand and gravel
HEAD-XCZSV	HEAD	Clay, silt, sand and gravel
PEAT-P	PEAT	Peat
RTD1-XSV	RIVER TERRACE DEPOSITS, 1	Sand and gravel. Underlies alluvium very extensively.
RTD2-XSV	RIVER TERRACE DEPOSITS, 2	Sand and gravel
RTD3-XSV	RIVER TERRACE DEPOSITS, 3	Sand and gravel
RTD4-XSV	RIVER TERRACE DEPOSITS, 4	Sand and gravel
RTDU-XSV	RIVER TERRACE DEPOSITS, UNDIFFERENTIATED	Sand and gravel
GFDUD-XSV	GLACIOFLUVIAL DEPOSITS, DEVENSIAN	Sand and gravel. Limits of Devensian glacial deposits are uncertain.
TILLD-DMTN	TILL, DEVENSIAN	Till. Limits of Devensian glacial deposits are uncertain.
TILMP1-DMTN	TILL, MID PLEISTOCENE	Till. Limits of Mid Pleistocene glacial deposits are uncertain.
GFDMP-XSV	GLACIOFLUVIAL DEPOSITS, MID PLEISTOCENE	Sand and gravel. Limits of Mid Pleistocene glacial deposits are uncertain.
TILMP-DMTN	TILL, MID PLEISTOCENE	Till. Limits of Mid Pleistocene glacial deposits are uncertain.
SIM-MDST	SIDMOUTH MUDSTONE FORMATION	Mudstone. MMG-MDST in DiGMap-50.
TPSF-MDSA	TARPORLEY SILTSTONE FORMATION	Mudstone and sandstone. MMG-MDST in DiGMap-50.
BMS-SDST	BROMSGROVE SANDSTONE FORMATION	Sandstone. This unit has been renamed the Helsby Sandstone Formation. Mudstones have been mapped within this unit in the Lichfield area but have not been modelled.
WRS-SDST	WILDMOOR SANDSTONE FORMATION	Sandstone. This unit has been renamed the Wilmslow Sandstone Formation.
KDM-PESST	KIDDERMINSTER FORMATION	Pebbly sandstone. This unit has been renamed the Chester Formation. Limits may be imprecise where overlain by glaciofluvial gravels.
LTWB-SDST	LITTLEWORTH BEDS	Sandstone. Limits imprecise in places owing to inconsistent recording in boreholes.They have not been mapped.
HPBR-BRSS	HOPWAS BRECCIA FORMATION	Breccia and sandstone. Mapped limits and borehole coding may be imprecise because of difficulties in distinguishing from overlying conglomerates of Kidderminster Formation and glaciofluvial gravels.
ALY-MDSS	ALVELEY MUDSTONE MEMBER, SALOP FORMATION, WARWICKSHIRE GROUP	Mudstone, siltstone and sandstone. Separately mapped and modelled only in south east corner of Area 8.
HA-MDSS	HALESOWEN FORMATION	Mudstone, siltstone and sandstone. Separately mapped and modelled only in south east corner of Area 8.
WAWK-MDSS	WARWICKSHIRE GROUP	Mudstone, siltstone and sandstone. Only subdivided into component formation in the south east corner.
PMCM-MDSS	PENNINE MIDDLE COAL MEASURES FORMATION	Mudstone, siltstone and sandstone. Shown as a single unit but includes mapped coals on the surface outcrop in the north west corner of Area 8.
PLCM-MDSS	PENNINE LOWER COAL MEASURES FORMATION	Mudstone, siltstone and sandstone. Shown as a single unit; occurs in subsurface only so no mapped coals or sandstones.

Bedrock geology

Figure 2 is a geological map of the bedrock units modelled, showing the current DiGMapGB-50 dataset, which has not yet been updated to show revisions made to the bedrock mapping in the model area. The Hopwas Breccia Formation, Kidderminster Formation, Wildmoor Sandstone Formation and Bromsgrove Sandstone Formation are part of the Sherwood Sandstone Group. Recent work on the Sherwood Sandstone Group has led to the re-naming of units from the Kidderminster Formation upwards respectively as the Chester Formation (mudstones mapped within this unit around Lichfield are thin and have not been modelled), Wilmslow Sandstone Formation and Helsby Sandstone Formation (Ambrose et al, 2014).

The Triassic aged units in the model are Tarporley Siltstone and Sidmouth Mudstone formations, which are part of the Mercia Mudstone Group. The Littleworth Beds have been distinguished as a separate unit in boreholes drilled for coal exploration, but have not been mapped and the unit has not been formalised. There is inconsistency in the recording of the unit, with some boreholes not recording it. The logs are mostly from old boreholes, drilled before 1970. Two have geophysical logs but are inconclusive in defining the unit. Although modelled, there is some doubt about the authenticity of this unit and it may be part of the Chester Formation. The Alveley Member (ALY) is part of the Salop Formation of the Warwickshire Group. This group, of Carboniferous age, has not been subdivided into its component formations due to insufficient evidence in many boreholes, with the exception of the south east corner. Pennine Lower and Middle Coal Measures form small outcrops in the north-west and south-east of the model area.

File:OR15074fig2.jpg

Figure 2    DigMapGB-50 bedrock geology map of the Area 8 model. Key as Table 1.

Superficial geology

The superficial deposits in the model area comprise patches of till and glacial sand and gravel associated with the last two major glaciations (Figure 3). The oldest of these are Mid Pleistocene aged glacigenic deposits, which occur mainly in the north and south of the model area. Younger Devensian aged glacigenic deposits are mapped in the west of the model area. A wide tract of fluvial deposits runs roughly north-south through the eastern half of the model area. These comprise river terrace gravels and alluvium associated with the River Tame and River Blithe and their tributaries. Head is mapped in valley floors and patches of peat are located in the south-west of the model area. Figure 3 shows the current DiGMapGB-50 superficial geology mapping of the area, which has not yet been updated with the recent revisions made to the geological maps of the area. One major change, for example, is the re-classification of a large area of glacial sand and gravel north of Lichfield to a river terrace gravel.

File:OR15074fig3.jpg

Figure 3    DigMapGB-50 superficial geology map of the Area 8 model. Key as Table 1.

Artificial deposits

Artificial deposits are mapped and modelled throughout the area. These are subdivided into four categories: made ground is used for areas where the land surface has been artificially raised, such as road and railway embankments; worked ground is used for areas where the land surface has been lowered, such as quarries and road cuttings; landscaped ground is areas where engineered cut and fill has occurred, such as industrial estates; and worked and made ground is used for backfilled pits and quarries.

Very little artificial ground was mapped in the Area 8 model area before the corresponding 1:10 000 scale map sheets were revised. Figure 4 shows the newly mapped/modelled artificial ground within the Area 8 model area, which is not yet incorporated into the DiGMap-GB datasets.

File:OR15074fig4.jpg

Figure 4    Distribution of modelled artificial ground in Area 8.