OR/16/034 Modelled surfaces/volumes

Barron, A J M. 2016. Metadata report for GSI3D cross sections along the HS2 route in Area 9 (Birmingham spur). British Geological Survey Open Report, OR/16/034.

The natural geology of Area 9 sections of the route comprises bedrock units of the Triassic Mercia Mudstone and Sherwood Sandstone groups, together with superficial deposits of glacigenic and fluvial origin. Artificial ground was modelled according to the corresponding 1:50 000 scale geological maps. The level of detail and extent of these units in the model may differ from that shown in other BGS datasets, including 1:50 000 scale geological maps. Table 1 lists the units modelled in the cross-sections and should be used as the legend for viewing images of the cross-sections in this report.

Table 1    Units present in the vicinity of the Birmingham Spur, down to 30 m below OD.

Name in model (LEX-RCS)	Description	Comments
WMGR-ARTDP	Worked and Made Ground — Artificial Deposits
MGR-ARTDP	Made Ground — Artificial Deposits
ALV-XCZSV	Alluvium — clay, silt, sand and gravel
HEAD-XCZSV	Head — clay, silt, sand and gravel
RTD1-XSV	River Terrace Deposits, 1 — sand and gravel
RTD2-XSV	River Terrace Deposits, 2 — sand and gravel
GFDMP-XSV	Glaciofluvial Deposits, Mid Pleistocene — sand and gravel
TILMP1-DMTN	Glaciolacustrine Deposits, Mid Pleistocene, lower — clay and silt
TSSM-MDSS	Tarporley Siltstone Formation and Sidmouth Mudstone Formation, undifferentiated — mudstone, siltstone and sandstone
MMG-DLST*	Mercia Mudstone Group — dolomitic siltstone	Also known as ‘skerry’. Several thin beds locally mapped in Branscombe Mudstone and Sidmouth Mudstone formations
MMG-MDST	Mercia Mudstone Group — mudstone
BMS-SDST	Bromsgrove Sandstone Formation — sandstone	Now named Helsby Sandstone Formation
WRS-SDST	Wildmoor Sandstone Formation — sandstone	Now named Wildmoor Sandstone Member of Wilmslow Sandstone Formation
KDM-PESST	Kidderminster Formation — pebbly sandstone	Now named Chester Formation
HPBR-BRSS	Hopwas Breccia Formation — breccia and sandstone, interbedded
EN-SCSM	Enville Member — sandstone with subordinate conglomerate, siltstone and mudstone

Units shown in the geological map in Figure 2 but are not represented in the cross-sections are marked *

Bedrock geology

Mercia Mudstone Group (MMG) lies at geological rockhead beneath the Birmingham Spur throughout its length apart from the extreme west end (Figure 2). It is subdivided into five formations in the West Midlands, in descending order: the Blue Anchor (about 8 m of greenish grey dolomitic mudstone and siltstone), Branscombe Mudstone (20 to 30 m of reddish brown mudstone and siltstone), Arden Sandstone (4 to 10 m of greenish grey and brown mudstone, siltstone and sandstone), Sidmouth Mudstone (SIM, about 250 m of reddish brown mudstone and siltstone) and Tarporley Siltstone (20 to 40 m of reddish brown and greenish grey siltstone, mudstone and sandstone).

The lowest four of these formations are distinguished and modelled in the adjoining Area 7, and three are seen in the extreme southern end of the southern branch of the spur. The two lowest units, Sidmouth Mudstone and Tarporley Siltstone formations, are generally thought to be present in the Birmingham Spur area but were not separately mapped on the Birmingham 1:50 000 scale map sheet and cannot be meaningfully and consistently differentiated in the borehole logs. Thus throughout the Area 9 cross sections, the interval is shown as Tarporley Siltstone Formation and Sidmouth Mudstone Formation undifferentiated (TSSM-MDSS), although it is thought that the Sidmouth Mudstone Formation forms much the greater part and is the unit at rockhead. This is consistent to MMG-MDST and SIM-MDST in the Area 7 cross sections.

The Sherwood Sandstone Group underlies the Mercia Mudstone Group, and is at rockhead west of the Birmingham Fault. It is subdivided into formations on the Birmingham sheet, of which the upper two, the Bromsgrove Sandstone and Wildmoor Sandstone formations are represented in the Area 9 cross sections. However, they are difficult to distinguish in borehole logs (the Bromsgrove Sandstone is pebbly in parts, the Wildmoor Sandstone is fine-grained) and the boundary between them is uncertain.

File:OR16034fig2.jpg

Figure 2    DiGMapGB-50 bedrock geology in Area 9.

Black dashed lines are mapped faults; solid black line is the HS2 Birmingham Spur route section; red lines are cross-sections constructed to inform the route section. Key to geology as per Table 1. Contains Ordnance Survey data © Crown Copyright and database rights 2016.

Superficial deposits

Superficial deposits are extensive in the western part of the Birmingham Spur area (Figure 3). Fluvial deposits associated with the River Tame and River Lea comprise alluvium and river terrace gravels. Glacigenic deposits in the area are dominated by sand and gravel with patches of till. In DiGMapGB-50 v7, most/all of the glacigenic deposits in the district are attributed as Devensian (Late Pleistocene; e.g. GFDUD rather than GFDMP), presumably as a result of recent research, whereas in the memoir (Powell et al., 2000, p.81), they are inferred to be Anglian/Wolstonian (Middle Pleistocene) in age. However, this re-interpretation is known to be controversial/not yet well substantiated. In the memoir, deep superficial deposit-filled hollows that impinge on the Birmingham Spur are identified, delineated and named in figures 24 and 27 as the Proto-Tame and Gilson ‘palaeovalleys’.

File:OR16034fig3.jpg

Figure 3    DiGMapGB-50 superficial deposits in Area 9.

Black line is the HS2 Birmingham Spur route section; red lines are cross-sections constructed to inform the route section. Blue ticked lines in the west represent the margins of ‘palaeovalleys’ (buried channels). Key to superficial deposits as per Table 1. Contains Ordnance Survey data © Crown Copyright and database rights 2016.

Artificially modified ground

Extensive areas of artificially modified ground are mapped along the River Tame and River Lea floodplains (Figure 4), which are represented in the cross-sections using available borehole information to inform the thickness/base. This artificially modified ground layer is subdivided into classes, with made ground (areas where the land surface has been artificially raised) and made and worked ground (such as backfilled shallow mineral working) present in the area. Only mapped artificial ground is represented in the cross-sections, although some of the boreholes used record artificial ground where none is mapped.

File:OR16034fig4.jpg

Figure 4    DiGMapGB-50 artificially modified ground in Area 9.

Black line is the HS2 Birmingham Spur route section; red lines are cross-sections constructed to inform the route section. Artificially modified ground is shown as hatched areas: made ground is coloured blue and worked and made ground is coloured green. Contains Ordnance Survey data © Crown Copyright and database rights 2016.