OR/13/053 Chalk stratigraphy information
|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 stratigraphy information in the TCIS comprises: boreholes with interpretations of Chalk Group stratigraphy held in the Borehole Geology (BoGE) database; distribution and bibliographic reference source of illustrated outcrop sections and boreholes; Chalk localities that have descriptions of detailed lithostratigraphy and biostratigraphy in publications and reports; distribution of key hardground units described by Bromley and Gale (1982) in the Lewes Nodular Chalk Formation.
Borehole stratigraphy information is contained in the ‘Geophysical logs and cored boreholes layer’, and comprises 333 data points. The layer shows the distribution of boreholes for which reliable Chalk stratigraphical interpretations exist in the BGS Borehole Geology (BoGe) database. Details of the SOBI index number for the relevant borehole are accessed by interrogation of data points with the Arc information tool. The SOBI index number can be used to search and view interpretations in BoGe. Access to BoGe requires an Oracle ID and interpreter code arranged through SNS and Ken Lawrie (Murchison House, BGS Edinburgh). Where interpretations are based on geophysical logs, access to the primary log data is either through RECALL (corporate archive for digital log data; information from Sandy Henderson, Murchison House, BGS Edinburgh) or the Intranet Data Access (IDA) at http://bgsintranet/scripts/ida/menus/Geophys_data_menu.cfm for non-digital data. In both cases, the SOBI number is the search key for accessing data for individual boreholes. These borehole interpretations provide primary control points for many of the 3D models of the Chalk in the Thames Basin. Assigned numerical values for each formation-base in BoGe indicate relative interpretation confidence.
Outcrop sections and boreholes
A wide range of publications, particularly BGS technical reports, illustrate outcrop sections and cored boreholes in the Chalk Group of the London Basin. This information is contained in the ‘Illustrated sections and boreholes’ layer, comprising 217 data points. These sections contain a wealth of data that is not captured by the geological map, such as the relative development of lithological features, biozonal data, and the presence of named marker beds that are of value for long-range correlation. Interrogation of individual data points with the Arc information tool reveals the source publication for accessing the illustrated information at the data-point location. Cited non-BGS publications are available through the BGS library. Access to BGS technical reports is through paper archives held in NGRC (National Geoscience Records Centre) or scanned archives. BGS library and NGRC staff can provide information about access to digital and hardcopy report collections. Digital availability of reports is incomplete because many were originally classified as restricted, and only title pages were available for archive by the BGS library.
Chalk sample localities
In the course of geological mapping, BGS has amassed vast biostratigraphical collections for the Chalk Group. The purpose of the collections is to provide control and evidence for the geological boundaries depicted on published maps. In many cases biostratigraphical data allows classification of the Chalk at a level of greater refinement than shown on geological maps, and can also provide information about the likely development of marker-beds that are important for long-range correlation. Data for macro-fossil sample sites in the London Basin is given in the ‘Chalk localities (biostratigraphy/lithostratigraphy)’ layer, and comprises 775 data points. BGS also has data for micro-fossil collections from the Chalk, but time constraints prevented inclusion of these in the data compilation.
Interrogation of individual data points using the Arc information tool reveals the source reference providing detailed information about Chalk stratigraphy at the data point location. Many of these reports are BGS technical reports spanning the last 20 years. These reports include re-evaluations of older, historical collections of macrofossils as well as material collected since the 1990s. Access to BGS technical reports is through paper archives held in NGRC or scanned archives. BGS library and NGRC staff can provide information about access to digital and hardcopy report collections. Digital availability of reports is incomplete because many were originally classified as restricted, and only title pages were available for archive by the BGS library.
Lewes nodular chalk formation hardgrounds
Work by Bromley and Gale (1982) described the stratigraphical and spatial development of hardgrounds in strata equivalent to the lower and middle parts of the Lewes Nodular Chalk Formation. Traditionally, these hardgrounds have been grouped into those forming the ‘Chalk Rock’ and those forming the ‘Top Rock’. Bromley and Gale (1982) showed that three suites of hardgrounds (Lower, Middle and Upper Suite) comprise the ‘Chalk Rock’ (= Chalk Rock Member of current BGS terminology; Hopson, 2005), and that the development of these suites of hardgrounds is laterally variable. Stratigraphical details of the hardgrounds and their correlation are provided by Bromley and Gale (1982). The significance of these hardgrounds is that they affect the continuity of the preserved stratigraphy, so lateral variability in the development of hardgrounds occupying different stratigraphical horizons results in significant lateral changes in the character of the preserved Lewes Nodular Chalk succession. The spatial extent of the individual suites of hardgrounds is shown in three layers: ‘Lower Hardground Suite’, ‘Middle Hardground Suite’, and ‘Upper Hardground Suite’. Sites where Bromley and Gale (1982) identified hardgrounds comprising the ‘Top Rock’ (= Top Rock Bed of current BGS terminology; Hopson, 2005) are shown by the ‘Top Rock’ layer. The Top Rock Bed appears to have a more restricted spatial development than the Chalk Rock Member, and this may be significantly related to features of basin architecture.
3D geological models
The TCIS includes a layer showing the extent of 3D geological models covering parts of the Thames Basin. The key models are as follows:
- Farringdon Station
Author: Aldiss, D T
Key Reference: Aldiss et al. (2012)
Limitation: Covers a small geographical area (4 km2).
- Lower Lea Valley Model
Author: Royse, K
Key Reference: Royse et al. (2006)
Limitation: Focussed primarily on the Quaternary and Lambeth Group strata. Still to be approved.
- London Lithoframe 50k areas 1 to 14
Authors: Mathers, S J, Burke, H, Thorpe, S, Ford, J, Cripps, C, Terrington, R
Date: 2005 to 2013
Reference: Ford et al. (2008)
Limitation: Modelled at 1:50 000 scale. Many boreholes considered but not all used for the modelling so may miss some key boreholes that prove the top of the Chalk. Some areas, lacking borehole coverage, infer top Chalk using thickness and local knowledge provided by memoirs and map sheets.
- Southern East Anglia (Colchester and Ipswich models)
Author: Mathers, S J
Date: 2004 to 2008
References: Mathers, S J (2012a, b)
Limitations: Modelled at 1:50 000 scale. Many boreholes considered but not all used for the modelling so may miss some key boreholes that prove the top of the Chalk. Some areas, lacking borehole coverage, infer top Chalk using thickness and local knowledge provided by memoirs and map sheets.
- Thames Gateway Area 5
Author: Royse, K
Reference: Royse et al. (2008)
Limitations: Modelled at 1:50 000 scale. Many boreholes considered but not all used for the modelling so may miss some key boreholes that prove the top of the Chalk. Some areas, lacking borehole coverage, infer top Chalk using thickness and local knowledge provided by memoirs and map sheets. Will soon to be superceded by the London LithoFrame 50k modelling.
- BROMLEY, R G, and GALE, A S. 1982. The Lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273–306.
- HOPSON, P M. 2005. A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland with statements on the Chalk of Northern Ireland and the UK Offshore Sector. British Geological Survey Research Report RR/05/01.
- ALDISS, D T, BLACK, M G, ENTWISLE, D C, PAGE, D P, AND TERRINGTON, R L. 2012 Benefits of a 3D geological model for major tunnelling works: an example from Farringdon, east-central London, UK. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 45 (4). 405–414.
- ROYSE, K, ENTWISLE, D, PRICE, S, TERRINGTON, R, and VENUS, J. 2006. Gateway to Olympic success. Geoscientist, Vol. 16, 4–10.
- FORD, J, BURKE, H, ROYSE, K, and MATHERS, S J. 2008. The 3D geology of London and the Thames Gateway : a modern approach to geological surveying and its relevance in the urban environment. Cities and their underground environment : 2nd European conference of International Association of engineering geology: Euroengeo 2008. Madrid, Spain.
- MATHERS, S J. 2012a Model metadata summary report for the Ipswich-Sudbury LithoFrame 10–50 model. British Geological Survey Open Report, OR/12/080.
- MATHERS, S J. 2012b Model metadata summary report for the Colchester LithoFrame 10–50 model. British Geological Survey Open Report, OR/13/001.
- ROYSE, K, REEVES, H, and GIBSON, A. 2008. The modelling and visualisation of digital geoscientific data as an aid to land-use planning in the urban environment, an example from the Thames Gateway. 89–106 in Communicating environmental geoscience. LIVERMAN, D G E, PEREIRA, C, and MARKER, B (editors). Special Publication of the Geological Society, London, Vol. 305.