OR/14/029 Model purpose and resolution

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H Burke, S J Mathers, J P Williamson, S Thorpe, J Ford and R L Terrington. 2014. The London Basin superficial and bedrock LithoFrame 50 Model. Nottingham, UK, British geological Survey.

This model is intended for use at resolutions around 1:50 000, together with the corresponding DiGMapGB-50 geological map data. This model is not recommended for any site specific studies or more detailed uses.

Work began on this model in 2006, starting with tiles 1 to 6, all the individual tiles were completed by 2010. The model tiles were then amalgamated into a single GSI3D model in 2012. In total, 922 cross-sections were constructed, consulting 7174 borehole records. In all, 74 superficial and bedrock geological units have been modelled, along with landslide deposits and 5 categories of artificial ground. The bedrock units and faults that cut them were also correlated in the GSI3D cross-sections down to the base of the Palaeogene, but the corresponding stratigraphic surfaces were generated in GOCAD® as the GSI3D software is unable to calculate these faulted units. Four further deeper low-resolution bedrock geological units (Cretaceous and Jurassic) derived from a larger regional model were added to this model to ensure complete coverage to a minimum depth of several hundred metres, although the depth of the model is variable across the area due to the distribution of the geological units modelled.

The initial framework of cross-sections was constructed in GSI3D for each model tile, with docking sections added along the grid line boundaries of the individual tiles. Where appropriate these were iterated between the tiles on either side to form points of commonality thus linking the geological interpretation across the whole area. On completion of all twelve tiles, the calculated stratigraphic surfaces for the superficial deposits for the whole area were checked for discrepancies occurring at the tile boundaries. Where present these were smoothed out in the combined model.

In cross-section construction, discrepancies noted between boreholes and the geology indicated by the DiGMap dataset were mainly resolved in favour of the boreholes. The model therefore updates the DiGMapGB-50 version 6 dataset. The start heights of the boreholes were retained as accurate unless they exhibited serious deviations from the digital terrain model, some were rejected as erroneous others were adjusted as required. The model is intended to complement the corresponding 1:50 000 scale geological map sheets albeit with minor enhancements. These map sheets are listed in the bibliography together with the accompanying memoirs/sheet explanations and the London and Thames Valley BGS Regional Guide (Sumbler, 1996).

Additional 1:50 000 scale artificial ground map polygons were added to the combined model to address inconsistencies and omissions in the representation of artificial ground on the different 1:50 000 scale geological map sheets. These instances of artificial ground (Made, Worked, Landscaped, Disturbed and Worked & Made Ground) identified and captured in a 2D GIS before being added to the 3D model. These artificial deposits remain indicated in the model only by their 2D coverage polygons, from which 3D volumes cannot be calculated.