Chalk Of Southern England Research Articles

The development and seismic expression of synsedimentary features within the Chalk of southern England

Seismic reflection data acquired onshore to the north of Bournemouth, southern England, image clearly a series of prominent concave-up (troughs or depressions) and concave-down (mounded) structures within the White Chalk Subgroup. The Chalk lithostratigraphy in the area of study is established from borehole geophysical log correlations. These correlations then provide calibration of the seismic reflection data, indicating that the structures are concentrated at levels between the Lewes Nodular Chalk Formation and Tarrant Chalk Member, and towards the base of the Portsdown Chalk Formation. Onlap, downlap and truncation of reflectors are observed, with the most dramatic features forming a stacked series directly overlying faulting of Jurassic strata. Similar stratal geometries have been described from comparable levels in the Chalk of the North Sea, and in outcrop in Britain and France. To many they represent channels formed by submarine erosion and redeposition of the Chalk during relative sea-level falls, linked to tectonics. We also interpret a number of apparently incisive structures and slumped infill sequences described here as being of primary syndepositional origin. The slumping and the close association of these features with underlying faults, and proximity to nearby major crustal faults, may be indicative of submarine erosion of the Chalk caused by sea-level changes linked to syndepositional fault movements. However, evidence of toplap–downlap pairs is also suggestive of more aggradational mounding or laterally migrating surfaces or facies belts. The presence of any such structures implies rapid lateral variations in the Chalk that would have implications not only for aquifer management and future storage schemes but also during the processing of seismic reflection data and general studies and correlations of the Chalk across southern Britain.

The seismic expression of synsedimentary channel features within the Chalk of southern England

Onshore seismic reflection data to the north of Bournemouth, reveal a series of troughs or depressions (concave-up) and mounded (concave-down) structures within the Upper Chalk. Onlap, downlap and truncation of reflections is observed and the structures lie in close proximity to faults in the underlying Jurassic strata. It is probably the first time that such prominent and significant erosive channel structures have been imaged so clearly and described on seismic reflection data from onshore UK. Similar stratal geometries observed in the Chalk at crop are interpreted as channels formed by submarine erosion and redeposition, during relative sea-level falls. We interpret the structures described here as of primary syndepositional submarine erosion of the Chalk, producing channels, the infill to which may be slumped. Eustatic global sea-level changes are often cited as the cause of anomalous Chalk thicknesses and structures. Increasingly, however, such features are being linked to syndepositional tectonics which were clearly affecting Chalk deposition in surrounding basins. The structures described here may provide important information in the debate about whether late Cretaceous tectonics locally contributed to sea-level changes which affected Chalk sedimentation in the Anglo-Pads Basin, and that considering just one external control (such as eustasy) is oversimplified.

Evidence for rapid groundwater flow and karst-type behaviour in the Chalk of southern England

Abstract With the growing importance of groundwater protection, there is increasing concern about the possibility of rapid groundwater flow in the Chalk of southern England and therefore in the frequency and distribution of ‘karstic’ features. Pumping test data, although useful in quantifying groundwater resources and regional flow, give little information on groundwater flow at a local scale. Evidence for rapid groundwater flow is gathered from other, less quantifiable methods. Nine different strands of evidence are drawn together: tracer tests; observations from Chalk caves; Chalk boreholes that pump sand; descriptions of adits; the nature of water-level fluctuations; the Chichester flood; the nature of the surface drainage; geomorphological features; and the presence of indicator bacteria in Chalk boreholes. Although the evidence does not prove the widespread existence of karstic features, it does suggest that rapid groundwater flow should be considered seriously throughout the Chalk. Rapid groundwater flow is generally more frequent close to Palaeogene cover and may also be associated with other forms of cover and valley bottoms.

The effects on groundwater quality of the introduction of secondary sewage treatment to an effluent recharge site on the Chalk of southern England

Abstract Detailed investigations were carried out at a site on the Chalk of southern England between 1978 and 1982 to determine the effects of sewage effluent recharge on groundwater quality. Before 1981 screened crude domestic sewage was recharged to the Chalk via a series of lagoons. During 1981 a secondary-treatment works was commissioned on the site and the effluent recharged through a new subsurface drain network. Monitoring of the inorganic, organic and microbiological quality of effluent and groundwater continued through the change-over, enabling the effects of the new regime to be assessed. The results of the investigation have shown the recharge of sewage effluent to groundwater to be an effective way of ameliorating the effluent quality. Following the introduction of secondary treatment at the site there seems to have been little change in the degree of contaminant removal.

Measurement of fracture patterns in the chalk of Southern England : Cawsey, D C Engng Geol, V11, N3, Sept 1977, P201–215

Measurement of fracture patterns in the chalk of Southern England : Cawsey, D C Engng Geol, V11, N3, Sept 1977, P201–215

Mineralogy of the non-carbonate material from the Chalk of Berkshire and Oxfordshire, England

AbstractThis paper presents detailed mineralogical analyses of the non-carbonate material obtained from four cored boreholes in the Chalk of Berkshire and Oxfordshire, England. Consideration is given to the relationship between the non-carbonate material and the interstitial water chemistry.Sample preparation was by means of acid digestion followed by vacuum sedimentation. Analyses were carried out using X-ray diffraction techniques.Results are presented in a semi-quantitative form based upon integrated peak intensities and are compared to those of previous studies on the Chalk of southern England. It is concluded that the zones of occurrence of the major clay minerals as demonstrated in this study are in general agreement with those established by other authors. The non-carbonate content of the Chalk increases from the concealed beds at the Fair Cross site towards the outcrop Chalk of Lambourn and Harwell. The variation in mineralogy and purity is ascribed to changing conditions of deposition and to diagenesis related to groundwater circulation.

The measurement of fracture patterns in the chalk of Southern England

A field investigation made to ascertain if recognisable fracture patterns exist within the Chalk of southern England is described. The areas investigated included: the Chilterns the Isle of Thanet; the South Downs; and the Isle of Wight. Analysis of field data indicate that strike directions of fractures may be concentrated into sets. The strike direction of each set remains reasonably constant within a particular area and the pattern of fracturing appears to be related to the main fold axes in each area. Most fractures within the Chalk were found to be steeply inclined, although significant variations exist when results from the areas investigated are compared. Results indicate that definite fracture patterns are recognisable and that the likely fracture pattern for a specified area could be predicted. Although the accuracy of prediction is at present somewhat limited, determination of such fracture patterns is considered to be of significance.

Interstitial water chemistry and aquifer properties in the Upper and Middle Chalk of Berkshire, England

Interstitial water extracted by centrifuge from core material from the Chalk of Southern England was found to have a variable chemical composition and to be more mineralised by as much as a factor of 10, compared with the fissure-derived water found in the borehole column. Measurements of porosity and permeability of the Chalk in the same core material, show that there is a correlation between the physical and chemical parameters. The hydrogeochemical and the physical properties logs are presented and their significance in explaining the origins of groundwater mineralisation are discussed.

The Influence of Soil Conditions on the Course of Succession on the Chalk of Southern England

During both the First and Second World Wars as in other periods of agricultural prosperity many areas of chalk-down in south-east England were ploughed and used for growing cereal crops, only to be abandoned when peace was restored and demand less. The process of re-colonization by native plants of the abandoned arable land does not appear to have been described in detail, but now some 25 years after the Second World War and 50 years after the First, the vegetation still retains a distinctive character. Indeed there is evidence from sites in Kent and Surrey (Cornish 1954) that chalk-downs ploughed in the last century may still be recognized by the composition of the vegetation they now support. The outstanding feature of the vegetation which has developed on shallow chalk soils which were ploughed during the Second World War is the great scarcity of grasses and their failure to form a true turf. Patches of bare chalky soil persist even after the lapse of 25 years, and the vegetation is composed largely of dicotyledons including a wide variety of herbs and several species of shrub, among which Thelycrania sanguinea and Crataegus monogyna are particularly characteristic. On older sites scrub develops, frequently dominated by Thelycrania, and at no time is there a stage in the succession corresponding to typical chalk grassland, unless grazing is severe (Hope-Simpson 1940). From some preliminary experiments made in 1961 it seemed that an inadequate supply of nitrogen and phosphorus might account for the poor growth of grasses on these younger sites, and that it was possibly this scarcity of grasses which favoured the early invasion of shrubs. The experiments now described were designed to test this hypothesis. The site selected for the investigation appears to be typical of many in the Chilterns and on the North Downs and is described in the following section.

Marine Biological Research in Great Britain

THE coast-line of Great Britain offers an unrivalled diversity of habitats for the study of marine benthic communities. There is a considerable proportion of rocky shore, including the peculiar facies provided by the chalk of southern England; there are many stretches of sand and shingle and numerous inlets affording specialized habitats of diverse kinds. Can it be said that we are playing our part in the biological exploration of the numerous communities thus available to us? Speaking as a botanist, I would answer this question in the negative.

The Plough and the Origin of Strip-Lynchets

N their recent work, The Open Fields, C. S. and C. S. Orwin devote an appendix to the question of the origin of lynchets, in which they express their difficulties in accepting the current theory that narrow, step-like terraces like those at Bishopstone (Wilts.), for instance, could have been formed as a result of ploughing (PLATE I, facing p. 49). In private correspondence they assure me that these difficulties are widely shared by other agriculturalists, but at the same time they admit that they have no better explanation to offer. They point out that these terraces are found distributed mainly on the chalk of southern England, but also on various other geological formations in the Midlands, west and north, and also in southern Scotland; also that there seem to be many varieties of strip-lynchets, and that possibly one explanation may not apply to all; but while rightly distinguishing between these and the small square plots of the Celtic field-system, they are very much in error in implying that the latter are bounded only by field-banks and not by lynchets (p. 3 19).