OR/13/053 Description of data themes

Woods, M A, Haslam, R B, Mee, K, Newell, A J, and Terrington, R L. 2013. A guide to a new Geographical Information System for the Chalk of the Thames Basin: The Thames Chalk Information System (TCIS). (FutureThames cross-cutting Programme). British Geological Survey Internal Report, OR/13/053.

Chalk group

The following themes appear in the Table of Contents of the TCIS:

• Top Chalk: Displays extent of Chalk below younger, rockhead-forming strata (predominantly Palaeogene in age).
• Base Chalk: Displays total extent of Chalk, concealed and unconcealed, within the project area.
• Boreholes: Displays 3 subsets of boreholes: (i) those proving the contact of Chalk with younger rockhead formations (‘Top Chalk (CK) surface picks (V5)’); (ii) all boreholes that prove the local top of the Chalk Group (‘Top Chalk (CK) all boreholes (V5ii)’; (iii) all boreholes proving the base of the Chalk Group (‘Base Chalk (CK) boreholes (V5)’).
• Group Surfaces: Shows the top and base of the Chalk group as greyscale surfaces, shaded according to their OD value (pale colour = greater elevation above OD), with maximum (above OD) and minimum (below OD — negative) values indicated. Information about how the Group and the Formational surfaces were created is given below (5).
• Surface Control: Top Chalk and Base Chalk. This appears under both the ‘Chalk Group’ and the ‘Subgroup and formational data’ tabs in the GIS Table of Contents. It is a colour coded map of the density of boreholes used to control the calculation of Group, Subgroup and formational surfaces. Areas with a higher density are coloured green, those with lower density in red. The creation of these layers is described in greater detail below (5.4).
• Thickness (in metres) of the Chalk Group: Displays the thickness of the Chalk Group as a colour-shaded surface (dark colour = greater thickness), with maximum and minimum values indicated. The data are extrapolated for whole project area based on known values from boreholes. The negative thickness values are a spurious feature caused by different levels of data resolution, particularly contrasts between surveyed outcrop margin and modelled formational surfaces (see 10 below).

Subgroup and formational data

• Outcrop: Displays the concealed and unconcealed (outcrop) extent of each of the named subgroups and formations for which data could be obtained.
• Borehole distribution: Displays geographical distribution of boreholes that intersect the tops/bases of the named subgroups/formations.
• Borehole thickness (in metres): Displays geographical distribution of boreholes containing named formations, with thickness ranges in these boreholes indicated by variable symbology (4 below).
• Formational surfaces: Displays the bases of named formations as greyscale surfaces shaded according to their OD value (in metres; pale colour = greater elevation above OD), with maximum (metres above OD) and minimum (metres below OD – negative) values indicated. Information about how the Formational surfaces were created is given below (5 below).
• Surface control: As for Group level data above, but shown for each named formation (5.4 below).
• Formational bases: Displays bases of named formations as surveyed at outcrop within the project area (7 below).
• Calculated thickness of Chalk formations (in metres): As for Group level data above, displayed as colour-shaded surfaces extrapolated for whole project area based on known values from boreholes. Includes thicknesses of traditional Lower, Middle and Upper Chalk units (6 below).

Fault and fold lineaments

• Two layers are displayed in the GIS. The first (‘Filtered structures (V1)’), shows all documented records of significant fold and fault lineaments in the project area. The second layer (‘Filtered structures (V10)’) shows only the larger fault lineaments, which in some cases have been linked together on the basis of more detailed structural analysis of marker-horizons and gravity data. See 8 (below) for further details.

Chalk stratigraphy interpretations

Displays four layers, summarising details of: geophysical logs and cored boreholes with interpretations listed in BoGe; sections and boreholes illustrated in published media and BGS Technical Reports; chalk localities with stratigraphical details described in BGS Technical Reports; distribution of named hardgrounds forming the Chalk Rock Member in the lower part of the Lewes Nodular Chalk Formation. See 9 below for more details.