OR/15/044 Modelled surfaces/volumes

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Wakefield,O J W, and Barron, A J M. 2015. GSI3D Model metadata report for HS2 Area 6 (Cubbington to Hampton-in-Arden). British Geological Survey Internal Report, OR/15/044.

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    List of geological units modelled.
Name (LEX-RCS) Lex Description Lithology Comment
WMGR-ARTDP Worked and Made Ground Artificial Deposits
MGR-ARTDP Made Ground Artificial Deposits
WGR-VOID Worked Ground Void
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
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
TILMP1-DMTN Till Mid Pleistocene Diamicton Added for outlier on GFDMP at Balsall Common
GFTMP-XSV Glaciofluvial Terrace deposits Mid Pleistocene Sand and gravel In north only
GFDMP-XSV Glaciofluvial deposits Mid Pleistocene Sand and gravel
DMG-XSV Dunsmore Gravel Member, Wolston Glacigenic Formation Sand and gravel
GLLMP1-XCZ Glaciolacustrine deposits Mid Pleistocene Clay and silt [UPPER UNIT ADDED BY ILC]
TILMP-DMTN Till Mid Pleistocene Diamicton Includes much ODT
GFDMP0-XSV Glaciofluvial deposits Mid Pleistocene Sand and gravel Unit beneath TILMP added by ILC, restricted to north of area
GLLMP-XCZ Glaciolacustrine deposits Mid Pleistocene Clay and silt Very small outcrops only
ODT-DMTN Oadby Member, Wolston Glacigenic Formation Diamicton Now restricted to small outcrops in extreme south of area. Remainder to TILMP
WOC1-XCZ Wolston Clay Member, Wolston Glacigenic Formation Clay and silt Upper unit added by ILC, restricted to south-east of area
WOSG-XSV Wolston Sand and Gravel Sand and gravel restricted to south of area, now Wigston Sand and Gravel Member, Wolston Glacigenic Formation
WOC-XCZ Wolston Clay Member, Wolston Glacigenic Formation Clay and silt Restricted to south of area
THT-DMTN Thrussington Member, Wolston Glacigenic Formation Diamicton Now restricted to south of area. Remainder to TILMP
BGSG-XSV Baginton Sand and Gravel Formation Sand and gravel Restricted to south of area, one outcrop to GDU
GDU-XCZS Glacial deposits, undifferentiated Clay, silt and sand Includes indivisible BGSG, GLLMP and GFDMP beneath TILMP in west central part of Area 6 added by ILC
SASH-MDST Saltford Shale Member Mudstone Only present in down-faulted block in NW
WCT-MDLM Wilmcote Limestone Member Mudstone and limestone, interbedded Only present in down-faulted block in NW
CTM-MDST Cotham Formation Mudstone Only present in down-faulted block in NW
WBY-SDST Westbury Formation Sandstone Only present in down-faulted block in NW
BAN-MDST Blue Anchor Formation Mudstone Only present in down-faulted block in NW
BCMU-MDST Branscombe Mudstone Formation Mudstone See below
AS-SDSM Arden Sandstone Formation Sandstone, siltstone and mudstone See below. Important to note that in Area 6 this was originally modelled as AS-SISM, due to differences in lithology being mapped in Area 7. This was subsequently changed to AS-SDMS to match Area 7 (in Nov 2017)
SIM-MDST Sidmouth Mudstone Formation Mudstone See below
TPSF-MDSA Tarporley Siltstone Formation Mudstone and sandstone, interbedded See below
BMS-SDST Bromsgrove Sandstone Formation Sandstone Modelled unit includes locally mapped lenticular mudstone beds
WRS-SDST Wildmoor Sandstone Formation Sandstone Only present in faulted block in NE
AW-MDSD Ashow Formation Mudstone and sandstone Modelled unit includes locally mapped lenticular sandstone beds
KHS-SDST Kenilworth Sandstone Formation Sandstone Modelled unit includes locally mapped lenticular mudstone and conglomerate beds
TLM-ARSD Tile Hill Mudstone Formation Argillaceous rocks and [subequal/subordinate] sandstone, interbedded Modelled unit includes locally mapped lenticular sandstone beds
ASY-ARSC Allesley Member, Salop Formation Argillaceous rocks and [subequal/subordinate] sandstone and conglomerate, interbedded Modelled unit includes locally mapped lenticular sandstone beds
KRS-ARSC Keresley Member, Salop Formation Argillaceous rocks and [subequal/subordinate] sandstone and conglomerate, interbedded Modelled unit includes extensively mapped sandstone beds
Figure 2    DiGMapGB-50 bedrock geology for HS2 Area 6. Proposed route shown in blue. Key to geology as Table 1.

The strata of the Warwickshire Coalfield (central portion of model area: Figure 2) at rockhead and at depth to -30 m OD in the model all belong to the upper Carboniferous to Permian Warwickshire Group. Mapping and borehole data has enabled these to be fully subdivided in this model into four formations: in descending order the Ashow, Kenilworth Sandstone, Tile Hill Mudstone and Salop formations, the last divided into the Allesley and Keresley members (Table 1). However, within all these units beds of subordinate lithologies are widely mapped, but they are not distinguished within the model because of their significant lenticularity, the lack of information in boreholes, and the considerable additional work modelling these would impose. It is also likely that they would compromise successful and reasonably fast calculation of the model. See also Model limitations.

In the digital geological map data incorporated in the model (DiGMapGB-50; Figure 2) the Mercia Mudstone Group (MMG-MDST) was not subdivided, with the exception of the Arden Sandstone Formation (AS-SDSM), and locally thin siltstone beds or ‘skerries’ are not distinguished in the model. The presence of the Arden Sandstone where mapped at surface enables the distinction of the Branscombe Mudstone and Sidmouth Mudstone formations above and below, respectively. Inferred thicknesses of the constituent formations of the Mercia Mudstone Group, borehole data and field observations enabled the separation of the otherwise unmapped Tarporley Siltstone Formation at the base in the south-east of the model area (Figure 3). In addition, a fault-bounded outcrop of Bromsgrove Sandstone Formation in the north was reattributed as Tarporley Siltstone Formation. These changes were fed into the corporate 1:10 000 scale digital map data.

Figure 3     Bedrock envelope coverages with DiGMapGB-50 faults in Area 6 model. Proposed route shown in blue.

The area includes part of a highly complex Anglian glacigenic succession, which in places may be inadequately mapped for the purpose of constraining a 3D model (e.g. unclear superposition relationships, unlikely pinch-outs). Lenticular beds and repeated similar lithologies within this present further difficulties in modelling true/viable stratigraphical units. In addition, the correct identification of the Thrussington Member (THT) till and Oadby Member (ODT) till were doubtful in places and caused misfits. To address this, Oliver Wakefield reattributed some outcrops as Till, Mid Pleistocene (TILMP), although all Mid Pleistocene units are part of the Wolston Glacigenic Formation. After the model was submitted for technical check and QA, it was decided that in part of the area (west centre) it was necessary to combine the glaciofluvial (GFDMP-XSV) and glaciolacustrine (GLLMP-XCZ) deposits, plus a single outcrop of (dubious) Baginton Sand and Gravel Formation (BGSG-XSV), beneath the main till unit (TILMP) into one undifferentiated unit named Glacial deposits undifferentiated unit (GDU-XCZS, yellow in Figure 6 below) — see Model limitations.

Figure 4    DiGMapGB-50 superficial deposits for HS2 Area 6. Legend as Table 1.
Figure 5    DiGMapGB-50 superficial deposits for west central area. Legend and LEX-RCS codes as Table 1.
Figure 6    Superficial deposits envelope coverages in west central part of Area 6 model. Legend and LEX-RCS codes as Table 1.

Sections were named according to a wider HS2 project convention, with six longitudinal sections running southeast to northwest named HS2_Area6_NWSE_X_OLIVERW as appropriate including one to coincide with the proposed HS2 route (HS2_Area6_NWSE_line_OLIVERW), and thirteen northeast to southwest ‘rung’ sections named HS2_Area6_SWNE_X_OLIVERW. Eighteen additional sections were constructed to help constrain faulted areas and where issues were encountered with the model calculation.

Figure 7    Complete cross-section set for Area 6 model — looking towards the north-east. Legend as Table 1. Vertical exaggeration x5.