OR/17/006 Superficial deposits

Monaghan, A A, Dochartaigh, B O, Fordyce, F, Loveless, S, Entwisle, D, Quinn, M, Smith, K, Ellen, R, Arkley, S, Kearsey, T, Campbell, S D G, Fellgett, M, and Mosca, I. 2017. UKGEOS - Glasgow geothermal Energy Research Field Site (GGERFS): initial summary of the geological platform. British Geological Survey Open Report, OR/17/006.

Overview

A complex succession of superficial deposits covers the Clyde Gateway area, including widespread glacial till and marine, lacustrine and fluvio-glacial deposits, overlain by fluvial deposits, recent alluvium and anthropogenic deposits (Figure 30, Table 7; see also Monaghan et al., 2013). There is also widespread made, filled and landscaped ground (Figure 30).

Figure 30    Clyde Gateway area summary of superficial geology from BGS © NERC 2016 DigMap 1:10 000-scale data.

Table 7    Summary of lithostratigraphic units differentiated in the 3D superficial deposits geological model based on borehole and 1:10 000-scale map data. X indicates that each lithology is represented (e.g. XSV is a unit containing sand and gravel as opposed to SV which would be gravelly sand), where C = clay, Z = silt, S = sand, V = gravel, DMTN = diamicton, ARDP = artificial deposits, P = peat.

Code	Geological Unit	Equivalent description on 1:10 000 scale published map
Water	Water	Unattributed polygons or underlying sediments described
MGR-ARTDP	Made Ground (made and worked ground undifferentiated)	Made Ground (MGR), Made Ground and Worked Ground (WMGR), Infilled Ground (WMGR)
PEAT-P	Peat	Peat – blanket or basin peat, Flandrian (PEAT)
LAWSG-XCZSVP	Law Sand and Gravel Member	Alluvium – modern river floodplains — located along the upper reaches and tributaries to the River Clyde, Flandrian (ALV) Also includes some Alluvial Fan Deposits, Flandrian (ALF) and some River Terrace Deposits, Flandrian (RTD1 and RTD2)
GOSA-XCZSV	Gourock Sand Member	Marine Deposits – located along the lower reaches of the River Clyde, Flandrian (MDU) and Alluvium – modern river floodplains – along the upper reaches of the River Clyde, Flandrian (ALV)
KARN-XSV	Killearn Sand and Gravel Member	Generally Raised Marine Deposits, Devensian (RMDV), Raised Marine Deltaic Deposits, Devensian (RMDDD) or Raised Marine Intertidal and Subtidal Deposits, Devensian (RMIS)
PAIS-XCZS	Paisley Clay Member	Generally Raised Marine Deposits, Devensian (RMDV) or Raised Marine Intertidal and Subtidal Deposits, Devensian (RMIS)
BRON-XSVZ	Bridgeton Sand Member	Largely concealed beneath younger deposits, where present, exposures usually represented as Raised Marine Deposits, Devensian (RMDV)
RSSA-XSV	Ross Sand Member	Glaciolacustrine Deposits, Devensian (GLLDD), Glaciolacustrine Deltaic Deposits, Devensian (GLDDD) or Glaciofluvial Deposits, Devensian (GFDUD)
RSSA-XSZ	Ross Sand Member (silt, sand)	Largely concealed beneath younger deposits, identified at depth from borehole data, rare exposures represented as Glaciolacustrine Deposits, Devensian (GLLDD) or Glaciolacustrine Deltaic Deposits, Devensian (GLDDD)
BHSE-XSV	Broomhouse Sand and Gravel Formation (sand and gravel)	Largely concealed beneath younger deposits, where present, exposures usually represented as Glaciofluvial Deposits, Devensian (GFDUD), but also as Glaciofluvial Ice-Contact Deposits, Devensian (GFICD)
BHSE-S	Broomhouse Sand and Gravel Formation (sand)	Not recorded on the maps in the Clyde Gateway area (concealed beneath younger deposits), identified at depth from borehole data
WITI-DMTN	Wilderness Till Formation	Till - Devensian (TILLD)
CADR-XSV	Cadder Sand and Gravel Formation	Generally concealed beneath younger deposits, identified at depth from borehole data, rare exposures represented as Glaciofluvial Deposits, Devensian (GFDUD)
SUPD-XSV	Sand and gravel	Not recorded on the maps in the Clyde Gateway area (concealed beneath younger deposits), identified at depth from borehole data

Boreholes, map data

The majority of the 3400 borehole records BGS holds (includes site investigations and trial pits) within the Clyde Gateway area penetrate to depths of less than 30 metres and define the complex artificial and superficial deposit succession (see Figure 30).

BGS-published superficial deposits map data include the NS66SW 1:10 000-scale geological map (2007 — revised after initial modelling), the NS56SE 1:10 000-scale geological map (2007 — not including revisions from subsequent modelling). BGS 1:50 000 scale map sheets 30E Glasgow (1994 superficial deposits) and 31W Airdrie (1992 superficial deposits). BGS Memoirs form a definitive reference source for the geology of this area (Hall et al., 1998, Forsyth et al., 1996). Map data is available as GIS shapefiles (DigMapGB) or as scans (Mapviewer).

Key publications on the superficial deposits geology of the region include Browne and McMillan (1989), descriptions in Hall et al., (1998), Forsyth et al. (1996) and more recently by Finlayson et al. (2010).

GSI3D (deterministic) modelling

The Clyde Gateway area falls within the previously published ‘Central Glasgow’ model (Merritt et al., 2009; Monaghan et al., 2013). This model has been updated between the coordinates SW corner 258000, 660850, NE corner 265000, 665000 (an area approximately 7 km x 4 km) to include all boreholes drilled since 2005 (Figure 31).

The model was completed using GSI3D^© software (Kessler et al., 2008). This involves correlating the various geological units through borehole logs in 2D cross-sections; the cross-sections intersect to form a connected network across the model area. The correlated sections and geological map data provide information on the lateral distribution of each geological unit which is saved as an ‘envelope’ for each geological unit. Together, the envelopes and sections are used to model each of the geological units in 3D.

Updating the superficial deposits model

The approach taken to update the model in 2016 was to focus on updating existing section line interpretations and adding new section lines in parts of the area with new borehole data. This improved the detail and certainty in parts of the model.

Figure 31    Existing GSI3D section lines (green) from the previous Central Glasgow model, location of boreholes entered into the BGS databases post-2005 (purple) and areas of the model updated (red circles).

The post-2005 boreholes were used to update the model in the following ways:

• Added into existing sections – existing sections were re-routed to include additional boreholes where possible.
• Used to create a new section – new sections were created or existing sections extended if there was not an existing section close to the additional boreholes. New sections were always extended to cross existing sections so the geological units could be tied-in with the existing network of sections.
• Projected into existing sections – where an existing section passed through a large number of additional boreholes spread along its length, the most appropriate approach was to project boreholes that were close into the existing section and adjust the geological linework as necessary. This approach quickly improved existing sections without the need to re-draw the section.
• Informed the presence or absence of a unit across the model – away from the lines of section the additional boreholes provide a constraint when drawing the geological unit ‘envelopes’ which show the lateral extent of a unit.

Changes were also made to any crossing sections where necessary and edits were made to the associated geological envelopes, where required. A metadata report (Arkley, 2017) provides more information on the superficial deposits model.

Following the editing of sections and envelopes across the Clyde Gateway area, the model was calculated (Figure 32). Grids for each geological unit were exported (25 m cell size) to GIS along with information such as boreholes used and lines of section. A rockhead grid was also calculated and exported for use in the bedrock model (Figure 33).

Due to improvements in software, availability of additional data and external requirements the superficial deposits model has been altered and updated several times since its original development. As a result the model may have legacy inconsistencies (Arkley, 2017 gives more detail).

Figure 32    Overview of the superficial deposits model across the Clyde Gateway area, looking north-east, three times vertical exaggeration.

The 3D superficial deposits model contains 15 subdivisions and is of use in borehole prognosis, site planning and in a hydrogeological conceptual model (Figure 46 below). For example, the modelled rockhead surface shows significant variability across the area from - 40 m to +36 m relative to Ordnance Datum (O.D., Figure 33). Artificial (made) ground is not subdivided.

Figure 33    Rockhead surface for the Clyde Gateway area generated by combining the bases of the modelled superficial deposits. Includes mapping data licensed from Ordnance Survey. © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

Stochastic modelling

Automated stochastic modelling techniques have been used to develop predictive lithology and property models for glacial and fluvial deposits in Glasgow (Kearsey et al., 2015). A moderate improvement in the prediction of lithology when using a lithologically-derived stochastic model compared with a conventionally interpolated lithostratigraphic model is documented (Kearsey et al., 2015). In addition, simulated lithofacies distributions were also used as input in a flow model for numerical simulation of hydraulic head and groundwater flux using the Glasgow area (Bianchi et al., 2015). Future work could further utilise and develop these techniques to inform borehole prognoses and hydrogeological characteristics for GGERFS.

Figure 34    Example of a cross-section showing the sequential indicator simulation model most frequently occurring lithology and the probability of the presence of separate lithologies from the stochastic model, reproduced with permission from Kearsey et al. (2015). The black lines mark the positions of the geotechnical boreholes used to create the model.