: Abstract Since the last Chalk symposium in 1989 in Brighton, our understanding of the UK Chalk has undergone a revolution, not just in terms of the stratigraphical and engineering properties of this enigmatic material, but also in its spatial distribution and variability in 2D and 3D space from geological mapping. The old tripartite Chalk stratigraphy has been replaced with a more detailed stratigraphical scheme with up to nine Formations. The increased stratigraphical precision this brings has revealed far more geological structure and facies variability across the outcrop than previously recognized. Most of the major basin boundary faults, including the Hog’s Back, Mere and Pewsey Faults are now known to cut the Chalk sequence. Geological mapping has also provided a wealth of information on associated superficial and mass movement deposits, karst features and hydrogeology. The role of the engineering geology community in providing data has been a key part of this revolution. High precision site-specific information from ground investigations including the identification of key stratigraphical marker beds in boreholes, quarry and coastal sections is critical in characterizing the Chalk succession. Such point-specific data combined with spatially extensive data derived from geological mapping data enables thickness and facies trends to be identified and allows the prediction of ground conditions over wide areas. This paper outlines how our understanding of the facies variability and structure of the Chalk can be improved using high resolution geological mapping, and how this can benefit the engineering community using specific examples. Advances in technology means that the Chalk is increasingly being visualized in three dimensions through the production of 3D geological models that can be updated dynamically as new site investigation data comes in. These models can be used as a geotechnical risk management tool, promoting a cycle of risk reduction. The accuracy and resolution of these models depends on the quantity, quality and spatial distribution of 3D data, particularly the availability of accessible, high quality, well described borehole logs identifying key stratigraphical markers.
The venue for the 2016 British Cave Research Association Cave Science Symposium Field Trip
is the underground ‘Bath Stone’ quarries around Box, near Corsham, Wiltshire (Figure 1). The
aim of the field trip is to examine the cambering and gulling, gull caves and karstic features
observed in the quarries. The underground quarries at Box lie at the southern end of the Cotswold
Hills, on the southern side of the By Brook valley, a tributary if the River Avon. This valley has
incised though the Great Oolite Group, a sequence of Middle Jurassic (Bathonian) limestones and
mudstones. The underground quarries are developed within the Chalfield Oolite Formation. This
is an excellent building stone, as it can be sawn by hand in any direction as a ‘freestone’ which
then hardens on exposure to air, rather than having a distinct cleavage like slate.
Abstract The Chalk is an unusual karst aquifer with limited cave development, but extensive networks of smaller solutional conduits and fissures enabling rapid groundwater flow. Small-scale karst features (stream sinks, dolines, dissolution pipes, and springs) are common, with hundreds of stream sinks recorded. Tracer velocities from 27 connections between stream sinks and springs have median and mean velocities of 4700 and 4600 m d −1 . Tests to abstraction boreholes also demonstrate very rapid velocities of thousands of metres per day. Natural gradient tests from observation boreholes have rapid velocities of hundreds of metres per day. There is strong geological control on karst with dissolution focused on stratigraphical inception horizons. Surface karst features are concentrated near the Paleogene boundary, or where thin superficial cover occurs, but rapid groundwater flow is also common in other areas. The Chalk has higher storage and contaminant attenuation than classical karst, but recharge, storage and flow are influenced by karst. Point recharge through stream sinks, dolines, losing rivers, vertical solutional fissures, and soakaways enables rapid unsaturated zone flow. Saturated zone networks of solutional fissures and conduits create vulnerability to subsurface activities, and enable long distance transport of point source and diffuse pollutants, which may be derived from outside modelled catchment areas and source protection zones.
Table S2. This table documents the U-Pb geochemical and isotope ratios (to 1 sigma % error), and calculated ages (to 2 sigma absolute error) for the detrital zircon dating analyses.
Pen Park Hole is currently the UK's only known hydrothermal cave system. Situated under a large housing estate in north Bristol, it contains one of Britain's largest natural underground cavities, the roof of which is only a few metres below the surface. Discovered in 1669, the cave has had a long history of investigation, including being the object of the world's first published cave survey in 1683. In 2006, a telecommunication company submitted a planning proposal to erect a transmitter close to the site. Given the sensitive nature of the cave and potential uncertainties in the accuracy of a more recent cave survey, Bristol City Council planners required a better fix on the location of the cave. The urban surroundings prevented traditional geophysical techniques being used, and a ground penetrating radar survey failed because of thick residual clay soils over the site. Instead, radio-location was employed to check on the accuracy of the existing cave survey, allowing it to be geo-rectified in relation to the surface. The confirmed reliability of the survey, coupled with an assessment of the cave geomorphology, has allowed the planners to assess the potential for the proposed mast works to intersect the cave or any associated undiscovered voids.
After an introduction and a brief outline of the main sources of geological information on the area, this report describes the early Palaeogene deposits of North Kent (the area between the River Medway and the River Great Stour). These are the Thanet Formation, the Upnor Formation, the Woolwich Formation and the Harwich Formation. The main types of overlying superficial deposits are also described briefly.
Although these Palaeogene formations are predominantly silty, sandy or pebbly, clay-rich facies also occur, notably in the lower part of the Thanet Formation. This suggests that there is potential for an aquifer within the early Palaeogene sequence to be hydraulically separated from the Chalk aquifer in at least part of the area.
It is probable that the delineation of certain Palaeogene formation boundaries could be significantly improved by a new, detailed, geological survey. It seems unlikely that the individual formations could be subdivided in any significant or useful way by new geological surveys. Even if such improvements in the geological information were achieved, there is no guarantee that they would significantly advance the hydrogeological understanding of the North Kent marshes.
Acquisition and interpretation of natural gamma logs from currently available boreholes is most likely to produce useful new information about lithological variation in the Palaeogene sequence, relatively quickly and cheaply, if further opportunities for this exist.
This report is part of a wider project co-funded by the Environment Agency to investigate the geology of the North Downs aquifer.
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