Lithostratigraphy and biostratigraphy of the lewes and seaford chalks: A link across the Anglo-Paris basin at the Turonian-Senonian boundary

The positioning of the Turonian/Senonian boundary has been highly discussed since the introduction of the Senonian stage by d'Orbigny. In England the Early Senonian is conventionally represented by white chalks at the base of the Micraster cortestudinarium zone which to some has meant the top surface on the Top Rock or Navigation Hardgrounds. New macro- and microfaunal data on the Sussex White Chalk of Southern England, while providing a suitable framework for the comparison between the microfaunal zonation of the Paris basin chalks and macro- and microfaunal zonations commonly used in England, emphasize the positioning of the Turonian/Senonian boundary in the Anglo-Paris basin. This boundary falls within the interval Chalk Rock/Top Rock and is outlined by distinct changes in Micraster lineage (appearance of M. normanniae) and in benthic foraminiferal associations.     

Stratal geometries, facies and sea-floor erosion in Upper Cretaceous Chalk, Normandy, France

Unusual lenticular stratal geometries and facies of the Upper Cretaceous Chalk of coastal Haute Normandie, France, are described and interpreted. Thirteen facies within these chalks are described and illustrated on the basis of field, thin-section and SEM investigations: nannofossil mudstones, nannofossil hardgrounds, echinoderm wackestones and packstones, echinoderm hardgrounds, bryozoan mudstones and wackestones, bryozoan hardgrounds, bryozoan packstones and wackestones, inoceramid wackestones, inoceramid hardgrounds, sponge hardgrounds, marly chalks, conglomeratic chalks and debris flow chalks. These facies occur within lenticular bedded structures with both concave-up and concave-down geometries which have been previously interpreted as megaripples, mud mounds or tectonic structures. Detailed examination of the structures and the associated facies indicates that the concave-up geometries were formed from submarine erosion, and redeposition in NW-SE longitudinal channels. The concave-down geometries developed between adjacent channels. Assessment of the regional and temporal setting indicates that the erosion occurred in the Armorican-Cornubian straits of the Anglo-Paris Basin during sea-level lowstands. Within these straits channelling is preferentially developed on the positive, south-western block to the Lillebonne-Fécamp-Cotentin Fault.     

New evidence of the Cretaceous overstep of the Mendip Hills,Somerset, UK

The Mendip Hills, located on the north-western margin of the Wessex Basin, clearly show the onlap of Upper Triassic to Middle Jurassic sediments onto folded Palaeozoic strata. Recent field mapping on the crest of the Beacon Hill pericline at Tadhill, near Frome, augmented by a suite of shallow boreholes, proved up to 6.2 m of glauconitic grey and green silty sand. These glauconitic sands rest unconformably on Silurian volcanic rocks and Devonian sandstone. Lithological and ipalaeontological analyses of these glauconitic sands indicate that they are part of the Lower Cretaceous Upper Greensand Formation. This provides the first evidence for the Albian transgression across the Mendip Hills. The implications for the Cretaceous overstep on the margins of the Wessex Basin, and the analogies with the Upper Greensand succession in Devon are discussed.     

Integrating geophysical and hydrochemical borehole-log measurements to characterize the Chalk aquifer, Berkshire, United Kingdom

Geophysical and hydrochemical borehole-logging techniques were integrated to characterize hydraulic and hydrogeochemical properties of the Chalk aquifer at boreholes in Berkshire, UK. The down-hole measurements were made to locate fissures in the chalk, their spatial extent between boreholes, and to determine the groundwater chemical quality of the water-bearing layers. The geophysical borehole logging methods used were caliper, focused resistivity, induction resistivity, gamma ray, fluid temperature, fluid electrical conductivity, impeller and heat-pulse flowmeter, together with borehole wall optical-imaging. A multiparameter data transmitter was used to measure groundwater temperature, electrical conductivity, dissolved oxygen, pH, and redox potential of the borehole fluid down-hole. High permeability developed at the Chalk Rock by groundwater circulation provides the major flow horizon at the Banterwick Barn study site and represents a conduit system that serves as an effective local hydraulic connection between the boreholes. The Chalk Rock includes several lithified solution-ridden layers, hardgrounds, which imply a gap in sedimentation possibly representing an unconformity. Lower groundwater temperature, high dissolved-oxygen content, and flowmeter evidence of preferential groundwater flow in the Chalk Rock indicated rapid groundwater circulation along this horizon. By repeating the logging at different times of the year under changing hydraulic conditions, other water-inflow horizons within the Chalk aquifer were recognized. Electronic Publication     

A chalk revolution: what have we done to the Chalk of England?

Inversion tectonic episodes are identified in the Upper Turonian - Lower Coniacian, Santonian - Lower Campanian and later Lower Campanian Chalk. It is suggested that episodic tectonism created the seabed topography on which sea levels and erosional currents acted. Marked differentiation into linear belts of local basins and swells with a greater variety of sediments is present in the Santonian and Lower Campanian. During this same period the locus of sedimentation shifts westwards from the southern margin of the Weald to Wessex as Weald Basin inversion increases. Tectonic episodes also produced synsedimenary fracturing of the Chalk and evolution of vein networks and stylolytes. Upper Cretaceous tectonic and sea-level events also affected the platform of Europe, the Carpathians and the Syrian Arc where sedimento-tectonic scenarios provide analogues for the Chalk. Linking sea-level oscillations and tectonic episodes with microtectonic studies suggests a frequency of events within the range of 0.35-1.5 Ma.     

Revealing deep structural influences on the Upper Cretaceous Chalk of East Anglia (UK) through inter-regional geophysical log correlations

New borehole geophysical log interpretations between Wiltshire and north Norfolk show detailed lateral changes in the spatial relationships of Chalk Group marker beds. They show how marker beds in the Turonian and Coniacian Chalk Group in East Anglia pass laterally into their correlatives further west, and reveal unusual lateral thickness changes affecting stratigraphical intervals in the East Anglian succession. Newly enhanced regional gravity and magnetic data indicate that these thickness changes are probably related to WNW to ESE trending structural lineaments in the Palaeozoic basement rocks of the buried Anglo-Brabant Massif.The later part of the Mid Turonian and early part of the Late Turonian succession across East Anglia is greatly thickened, and shows almost no lateral variability. These relatively soft, smooth-textured chalks equate with thinner, hard, nodular beds formed in both shallow marine and deeper basinal settings elsewhere in southern England. Since it seems unlikely that there was greater sediment accommodation space across East Anglia at this time compared to basinal areas, this thickening may reflect a localised coccoliths productivity pulse, or perhaps a sheltered palaeogeographical position that protected the area from sediment-winnowing marine currents.A residual gravity low across north Norfolk, previously interpreted as a granite pluton, may instead represent two elongated (fault-bounded) sedimentary basins.     

Morphology and genesis of nodular chalks and hardgrounds in the Upper Cretaceous of southern England

The Upper Cretaceous chalks of southern England are a thick sequence of rhythmically bedded, bioturbated coccolith micrites, deposited in an outer shelf environment in water depths which varied between 50 and 200–300 m. The products of sea floor cementation are widely represented in the sequence, and a series of stages of progressive lithification can be recognized. These began with a pause in sedimentation and the formation of an omission surface, followed by (a) growth of discrete nodules below the sediment-water interface to form a nodular chalk, erosion of which produced intraformational conglomerates. (b) Further growth and fusion of nodules into continuous or semicontinuous layers: incipient hardgrounds. (c) Scour, which exposed the layer as a true hardground. At this stage, the exposed lithified chalk bottom was subject to boring and encrustation by a variety of organisms, whilst calcium carbonate was frequently replaced by glauconite and phosphate to produce superficial mineralized zones. In many cases, the processes of sedimentation, cementation, exposure and mineralization were repeated several times, producing composite hardgrounds built up of a series of layers of cemented and mineralized chalk, indicating a long and complex diagenetic history. Petrographic study of early cemented chalks indicates lithification was the result of the precipitation of small crystals on and between coccoliths and coccolith fragments. By analogy with known occurrences of early lithification in Recent deeper water carbonates, the cement is believed to have been either high magnesian calcite or aragonite, and more probably the former. The vast scale of operations involved in the cementation process precludes carbonate in expelled pore fluids as the source of cement, whilst quantities of aragonite incorporated in sediment are also inadequate. This, plus the observed association of horizons of early lithification with pauses in sedimentation associated with omission surfaces suggests seawater as a source of cementing materials. Stratigraphic studies indicate that processes of early lithification leading to hardground formation proceeded to completion in intervals to be measured in tens or hundreds of years. Regional studies suggest that early lithification characterized relatively shallow water phases associated with regional regression over the whole of the area, whilst in detail, the distribution of mature mineralized hardground complexes is strongly correlated with sedimentary thinning and condensation over small areas and the buried flanks of massifs. Early cementation in more basinal areas is typically in the form of nodular developments and incipient hardgrounds, whilst day contents in excess of a few percent appear to have inhibited early lithification. The striking rhythmicity of hardgrounds and nodular chalks is no more than a particular expression of the overall rhythmicity of chalk sequences. The stage of early lithification reached in any instance is dependent on sediment type, the time interval represented by the associated omission surface and the degree of associated scour and erosion (if any). Chalk hardgrounds differ from most others described in the geological literature in their widespread distribution (individual hardgrounds may cover up to 1500 km²), the presence of striking glauconite and phosphate replacements of lithified carbonate matrices, their frequently sparse epifaunas, and boring infaunas dominated by clionid sponges. These differences reflect the deeper water shelf setting of the chalk, and the more open marine, oceanic circulatory system, both strikingly different from the setting of other, shallower water hardgrounds. Litho- and biostratigraphic variation in the chalk sequences of the area studied are summarized in an appendix.     

A relative water-depth model for the Normandy Chalk (Cenomanian–Middle Coniacian,Paris Basin,France) based on facies patterns of metre-scale cycles

A relative water-depth model for the Chalk of the Paris Basin is proposed, based on the lateral variations of the high-frequency metre-scale cycles, which are characteristic features easily identified in the field. The studied outcrops are the Cenomanian–Middle Coniacian cliffs of Normandy. The main result of this study is to highlight the importance of storm activity in the deposition of the Chalk. The relative water-depth model is based on storm-induced shell concentrations observed within the two components of the metre-thick cycles: the depositional interval itself and the top hiatal surface.Six types of shell concentrations are defined, along with seven types of depositional facies making up the depositional units, as well as eight types of hiatal surface. Three cycle associations, differing in their thickness and the amount and type of non-carbonate constituents, can be identified in the Lower to Upper Cenomanian, the Upper Cenomanian to Lower Turonian and the Middle Turonian to Middle Coniacian.A relative water-depth profile model for all these cycles is based on the shell concentrations and a “water-depth equivalence” is proposed between the three cycle associations (lateral “facies” substitution diagram). This model is tested using palaeocological data (irregular echinoids) and by correlating field sections in terms of stacking patterns. Most of the studied deposits accumulated above the storm wave base (upper offshore zone or mid ramp).     

The late cretaceous transgression in the SWAnglo-Paris Basin: Stratigraphy of the Craie de Villedieu Formation

The litho- and biostratigraphy of the Craie de Villedieu Formation (Coniacian-Santonian)of western France are described in detail. The formation is subdivided into three members each containing a number of lithologically distinct named hardgrounds and marker beds. These constitute an onlapping sequence that thins from > 15 m in the NE around Cangey and Villedieu-le-Château, to < 2 m in the SW around St Michel-sur-Loire, a distance of 70 km. Thickness variation is related to the interaction of differential subsidence with eustatic transgression. Comparison with the Chalk Rock Formation of southern England indicates that transgressive and regressive hardground suites may be differentiated on bed geometry and hardground surface characteristics. The Craie de Villedieu rests everywhere on a regional hardground that coincides with the Turonian/Coniacian boundary in expanded successions, but probably marks a significant hiatus. South-west of Tours, onlap results in Santonian strata resting disconformably on strata of Turonian age. The basal Craie de Villedieu contains a succession of three Coniacian ammonite faunas characterized by Peroniceras and Forresteria (Harleites) (oldest), Gauthiericeras margae (Schlüter), and Protexanites (youngest). Volviceramus ex gr. involutus (J. de C. Sowerby) occurs with the two uppermost ammonite assemblages. A Santonian ammonite fauna dominated by Placenticeras polyopsis (Dujardin) occurs with Texanites gallicus Collignon and common Cladoceramus in the middle of the formation. Cordiceramus ex gr. cordiformis (J. de C. Sowerby) is recorded with Santonian ammonites in the upper part of the formation. A correlation with the Micraster zones of chalk facies is suggested, based on the inoceramid stratigraphy. The record of T. gallicus in association with Cladoceramus affords the first direct evidence for the position of the base of the Santonian in the Anglo-Paris Basin.     

Chalk thickness trends and the role of tectonic processes in the Upper Cretaceous of southern England

A series of six thickness maps created at a formation scale for the Chalk of the Southern and Transitional Chalk provinces of SE England reinforce the difficulty in determining the controls on Chalk deposition. However, at the broad scale, they do appear to show that thickness patterns in the Cenomanian to Turonian chalks of the West Melbury Marly Chalk, the Zig Zag Chalk and the Holywell Nodular Chalk show correspondence with the underlying Mesozoic extensional basin structure. The major exception to this is the south Dorset area which was uplifted in the Early Cretaceous as an eastern extension to the Cornubian Ridge. The younger New Pit Chalk and Lewes Nodular Chalk show a switch toward thicker successions on the London Platform and thinner, more uniform successions across the Mesozoic basins to the south. This change may indicate some initial basin inversion starting in the mid Turonian which caused a shift in the main locus of Chalk deposition toward East Anglia. The work potentially suggests multiple control-modes shaping the geometry of Chalk deposits, involving an interplay of: 1) long-lived basin-defining faults and structural blocks acting to shape large-scale thickness trends through differential compaction and interaction with relative sea level change; 2) smaller scale structures that may function to more effectively dissipate stress created by intra-Cretaceous tectonic events, producing more localised/sub-regional thickness and facies variations; 3) early basin inversion reflecting the broader basin-scale response to intra-Cretaceous tectonics, potentially responsible for regional shifts in patterns of sedimentation.     

A note on the physical properties of the chalk

The Chalk is one of the most extensively distributed series in England. It is essentially a soft limestone principally consisting of the remains of marine organisms, deposited in shallow water.The Upper Chalk of Kent, in particular, is characterized by a high porosity and relatively low dry density. The porosity and dry density of the Lower Chalk of Yorkshire and the Middle Chalk of Norfolk are lower and higher respectively, because of the higher content of interstitial secondary calcite. Porosity is not a significant factor as far as the gross permeability of the Chalk is concerned.The Upper Chalk of Kent is moderately weak, when tested in unconfined compression, whilst the Lower and Middle Chalk are moderately strong. All three groups of Chalk suffer a substantial reduction in strength when saturated, in the case of the Upper Chalk the loss in strength is dramatic. The indirect tensile strength is usually less than one twentieth that of the unconfined compressive strength. When subjected to undrained triaxial tests the Upper Chalk first underwent brittle failure at lower confining pressures but above 4.9 MN/m² significant plastic deformation occurred leading to barrel-shaped failures.Young's modulus is not a simple constant but varies with stress, increasing somewhat with increasing stress in the Chalk from Yorkshire and Norfolk. This did not happen in the Upper Chalk since plastic deformation began much earlier.     

Carbonate banks and slump beds in the Upper Cretaceous (Upper Turonian-Santonian) of Haute Normandie, France

Large scale sedimentary structures present in the Upper Turonian to Santonian chalks of Haute Normandie (northern France) represent the remains of a carbonate bank complex which formerly extended over an area of at least 1500 km². Cliff exposures along the Channel coast from St Valéry-en-Caux to Cauville and along the Seine from Sandouville to Lillebonne show sections of banks up to 50 m high and 1500 m across, their internal structures picked out by hardgrounds, nodular chalks and horizons of burrow flint. Associated with banks are slump sheets up to 20 m thick, slump scars, sedimentary breccias, injection phenomena and faults contemporaneous with sedimentation. Later diagenetic features include extensive dolomitization and silicification. These structures compare closely with the Waulsortian banks of the Palaeozoic, and bryozoan bioherms known from the Upper Cretaceous and Palaeocene of Denmark. Frame-building, sediment trapping and stabilizing organisms are absent, and bank development and stabilization was probably due to a plant covering, either algal or of marine angiosperms. Banks generated much of their own sediment, whilst a pelagic constituent (calcareous nannofossils and Foraminiferida) is also present. The distribution of the bank complex is related to a basement controlled swell area, whilst the life of the complex was limited to a relatively shallow water, regressive episode in the predominantly transgressive Upper Cretaceous history of the region. Les falaises littorales du Pays de Caux comprises entre Antifer et St Valèry-enCaux, et les affleurements de la basse vallée de la Seine permettent d'observer des formations du Turonien supérieur-Sénonien inférieur qui présentent des stratifications irrégulières soulignées par de nombreux hardgrounds, des horizons de craie noduleuse et des cordons de silex. Ces structures sont identifiées à des accumulations de calcilutite et calcarénite sous forme de bancs sous-marins dont la hauteur peut atteindre 50 m et qui couvrent une surface supérieure à 1500 km²; ils apparaissent au-dessus de hardgrounds subhorizontaux qui indiquent un haut-fond régional stable. Des glissements sous-marins sont associés à ces bancs et engendrent des niveaux avec des déformations souples atteignant 20 m d'épaisseur. Des brèches apparaissent localement et contiennent des blocs basculés de hardgrounds fragmentés lors du glissement; on y observe aussi de petites failles intrasédimentaires et des phénomènes d'injection. Aucun organisme constructeur ou capable de piéger et retenir le sédiment n'a été observé. La stabilisation de ces bancs serait due à une couverture végétale (algues ou angiospermes marines) dont on sait qu'elle peut disparâitre sans laisser de trace lors de la fossilisation. La croissance de ces bancs serait réalisée par un apport de sédiment comprenant une part de nourrissage autochtone comme cela existe pour les bancs récents en eau peu profonde, associée au dépôt d'une fraction pélagique.     

Erratum to “The Albian-Cenomanian boundary at Eggardon Hill, Dorset (England): An anomaly resolved?” [Proc. Geol. Assoc. 120 (2009) 108-120]

Re-examination of the classic exposures of the Eggardon Grit (topmost Upper Greensand Formation) at Eggardon Hill, Dorset shows that the upper part of this unit has a more complex stratigraphy than has been previously recognised. The Eggardon Grit Member, as described herein, is capped by a hardground and associated conglomerate, and is entirely of Late Albian age. The hardground is probably the lateral equivalent of the Small Cove Hardground, which marks the top of the Upper Greensand succession in southeast Devon. The conglomerate is overlain by a thin sandy limestone containing Early Cenomanian ammonites. This limestone is almost certainly the horizon of the Early Cenomanian ammonite fauna that has previously been attributed to the top of the Eggardon Grit. The limestone is regarded as a thin lateral equivalent of the Beer Head Limestone Formation (formerly Cenomanian Limestone) exposed on the southeast Devon coast. The fauna of the limestone at Eggardon suggests that it is probably the age equivalent to the two lowest subdivisions of the Beer Head Limestone in southeast Devon, with a remanié fauna of the Pounds Pool Sandy Limestone Member combined with indigenous macrofossils of the Hooken Nodular Limestone Member. The next highest subdivision of the Beer Head Limestone in southeast Devon (Little Beach Bioclastic Limestone Member), equates with the ammonite-rich phosphatic conglomerate of the ‘Chalk Basement Bed’, which caps the Beer Head Limestone at Eggardon, and which was previously regarded as the base of the Chalk Group on Eggardon Hill.Petrographic analysis of the Eggardon Grit shows that lithologically it should more correctly be described as a sandy limestone rather than sandstone. The original stratigraphical definition of the unit should probably be modified to exclude the softer, nodular calcareous sandstones that have traditionally been included in the lower part of the member.Without the apparently clear evidence of unbroken sedimentation across the Albian-Cenomanian boundary, suggested by the previous interpretation of the Eggardon succession, it is harder to argue for this being a prevalent feature of Upper Greensand stratigraphy in southwest England. Correlation of the Eggardon succession with successions in Dorset and southeast Devon reveals a widespread regional break in sedimentation at the Albian-Cenomanian boundary. The sand-rich facies above this unconformity represent the true base of the Chalk Group, rather than the ‘Chalk Basement Bed’ of previous interpretations.Selected elements of regionally important Upper Greensand ammonite faunas previously reported from Shapwick Quarry, near Lyme Regis, and Babcombe Copse, near Newton Abbot, are newly figured herein.     

The Albian–Cenomanian boundary at Eggardon Hill,Dorset (England): an anomaly resolved?

Re-examination of the classic exposures of the Eggardon Grit (topmost Upper Greensand Formation) at Eggardon Hill, Dorset shows that the upper part of this unit has a more complex stratigraphy than has been previously recognised. The Eggardon Grit Member, as described herein, is capped by a hardground and associated conglomerate, and is entirely of Late Albian age. The hardground is probably the lateral equivalent of the Small Cove Hardground, which marks the top of the Upper Greensand succession in southeast Devon. The conglomerate is overlain by a thin sandy limestone containing Early Cenomanian ammonites. This limestone is almost certainly the horizon of the Early Cenomanian ammonite fauna that has previously been attributed to the top of the Eggardon Grit. The limestone is regarded as a thin lateral equivalent of the Beer Head Limestone Formation (formerly Cenomanian Limestone) exposed on the southeast Devon coast. The fauna of the limestone at Eggardon suggests that it is probably the age equivalent to the two lowest subdivisions of the Beer Head Limestone in southeast Devon, with a remanié fauna of the Pounds Pool Sandy Limestone Member combined with indigenous macrofossils of the Hooken Nodular Limestone Member. The next highest subdivision of the Beer Head Limestone in southeast Devon (Little Beach Bioclastic Limestone Member), equates with the ammonite-rich phosphatic conglomerate of the ‘Chalk Basement Bed’, which caps the Beer Head Limestone at Eggardon, and which was previously regarded as the base of the Chalk Group on Eggardon Hill.Petrographic analysis of the Eggardon Grit shows that lithologically it should more correctly be described as a sandy limestone rather than sandstone. The original stratigraphical definition of the unit should probably be modified to exclude the softer, nodular calcareous sandstones that have traditionally been included in the lower part of the member.Without the apparently clear evidence of unbroken sedimentation across the Albian–Cenomanian boundary, suggested by the previous interpretation of the Eggardon succession, it is harder to argue for this being a prevalent feature of Upper Greensand stratigraphy in southwest England. Correlation of the Eggardon succession with successions in Dorset and southeast Devon reveals a widespread regional break in sedimentation at the Albian–Cenomanian boundary. The sand-rich facies above this unconformity represent the true base of the Chalk Group, rather than the ‘Chalk Basement Bed’ of previous interpretations.Selected elements of regionally important Upper Greensand ammonite faunas previously reported from Shapwick Quarry, near Lyme Regis, and Babcombe Copse, near Newton Abbot, are newly figured herein.     

New data on speleothem deposition and palaeoclimate in Britain over the last forty thousand years

²³⁰Th/²³⁴U dates are presented for 26 speleothems from three areas in Britain: Assynt, N.W. Scotland; N.W. Yorkshire; Mendip Hills, Somerset. These dates suggest that speleothem growth was widespread but rare in the period from 40 to 26 Ka, absent from 26 to 15 Ka, and abundant from 15 Ka to the present. The absence of speleothems between 26 and 15 Ka is attributed to glaciation in Yorkshire and Assynt and, tentatively, to continuous permafrost development in the Mendip Hills. The occurrence of speleothems before 26 Ka places restrictions on the extent of any ice sheet which may have been present in Scotland, and indicates that N.W. Scotland was not subjected to continuous permafrost at that time.     

A radiometric airborne geophysical survey of the Isle of Wight

A high resolution airborne geophysical survey across the Isle of Wight and Lymington area conducted in 2008 provided the first modern radiometric survey across the geological formations that characterise much of southern England. The basic radiometric data are presented and it is evident that bedrock geology exerts a controlling influence on the broad response characteristics of the naturally occurring radioelements. A GIS-based geological classification of the data provides a quantitative assessment and reveals that a relatively high percentage of the variability of the data is explained by the Cretaceous bedrock geology but this is much reduced in the Palaeogene. The three traditional Chalk units (Lower, Middle and Upper Chalk depicted on the currently available Geological Map) provide the lowest and most distinct behaviour within the Cretaceous sequence. Mineral content within the Chalk appears to increase with increasing age. A new method of representing the baseline radiometric information from the survey in terms of the mean values of the geological classification is presented. The variation of radioelement geochemistry within individual formations is examined in two case studies from the Cretaceous Lower Greensand Group and the Palaeogene Hamstead Member (Bouldnor Formation). The Cretaceous sequences provide the higher levels of discrimination of localised variations in radioelement distributions. A more detailed case study examines the potential influences from the degree of water saturation in the soil and superficial deposits.     

A geological and hydrogeological assessment of the electrical conductivity information from the HiRES airborne geophysical survey of the Isle of Wight

A recent high resolution airborne geophysical survey across the Isle of Wight (IoW) and Lymington area has provided the first electromagnetic data across the relatively young geological formations characterising much of southern England. The multi-frequency data provide information on bulk electrical conductivity to depths of the order of 100 m. A GIS-based assessment of the electrical conductivity information in relation to bedrock geological classification has been conducted for the first time. The analysis uses over 104,000 measurements across onshore IoW and has established average and statistical properties as a function of bedrock geology. The average values are used to provide baseline maps of apparent electrical conductivity and the variation with depth (measured as a function of frequency). The average conductivity as a function of depth within the main aquifer units is summarised. The data indicate that the majority of the Palaeogene is characterised by values consistently in excess of 100 mS/m and with a surprisingly high degree of spatial heterogeneity. The youngest (Oligocene) Hamstead Member displays some strong edge effects and the largest localized values in conductivity. The central Upper Chalk is associated with the lowest observed conductivity values and mineral content and/or porosity appears to increase with increasing age. The large central outcrop of the Lower Greensand Group, Ferruginous Sand Formation provides persistently low (<30 mS/m) conductivity values which imply a relatively uniform distribution of clean sand content. Non-geological (e.g. environmental) responses are contained within the data set and examples of these in relation to a closed municipal landfill and an area of potential coastal saline intrusion are discussed. In the south, the Gault clay/mudstone of the Early Cretaceous appears as a distinctive conductive unit. Cross sectional modelling of the data has been undertaken across the aquifer units of the Southern Downs. The results indicate that the Gault Formation, acting as an aquitard, can be traced as a distinct unit under the more resistive Early Cretaceous Upper Greensand and Late Cretaceous Chalk formations. The conductivity modelling should therefore allow an estimation of the subsurface configuration of the aquifer and aquitard units.     

The implications of laboratoryRn flux measurements to the radioactivity in groundwaters: the case of a karstic limestone aquifer

Laboratory time-scale experiments were conducted on Carboniferous Limestone gravels from the Mendip Hills area, England, with the purpose of evaluating the release of²²²Rn to the water phase. The specific surface areas of the samples were 4.14 and 1.69 cm² g⁻¹ , which provided, respectively, values of 50.6 and 12.7 pCi for the released Rn. These results allowed the calculation of the emanation coefficient of this rock matrix with respect to the release of Rn, where completely different values corresponding to 23% and 6% were found, suggesting that the extent to which grain boundaries or imperfections in aggregates of micro-crystals of calcite intersect the particle surface certainly affects the Rn release. They also permitted the evaluation of models for the generation of Rn in rocks and transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the karstic aquifer of the Mendip Hills area, where the calculated activities in groundwater based on the values of 23% and 6% for the emanation coefficient were about 51 and 15 times higher than actually measured in groundwater. Therefore, the emanation coefficient in nature is considerably smaller than in the lab experiment, and another factork (0 < k < 1) may be introduced into the equations related to the modelling, with the aim of adjusting the theoretical-practical results.     

Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels,slumps, faults,folds and glacitectonics

Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.     

The stratigraphy of the Chalk Group (Cretaceous) of the Devon coast,UK

An almost continuous layer of Upper Cretaceous deposits up to 1000 m thick was probably deposited across much of SW England. Phases of uplift in the late Cretaceous and early Cenozoic, each of which was followed by extensive erosion and dissolution, resulted in the removal of all except a few outliers of Chalk Group that crop out in east Devon and south Somerset. Those on the Devon coast between Sidmouth and Lyme Regis are some of the best exposed Cenomanian to early Coniacian successions in NW Europe and include the most westerly chalks preserved onshore in England. They form an integral part of the Dorset and East Devon World Heritage Site. In contrast to the Chalk of much of southern England, the older formations in Devon, the Beer Head Limestone, Holywell Nodular Chalk and New Pit Chalk, show marked lateral lithological variations that result from a combination of penecontemporaneous movements on local faults and relatively shallow-water environments close to the western edge of the Chalk depositional basin. The younger parts of the succession, the Lewes Nodular Chalk and Seaford Chalk Formations, comprise chalks that do not appear to have been greatly affected by penecontemporaneous fault movements. These formations include lithological marker beds that have been correlated with marker beds in the Sussex type area. The principal sedimentary breaks in the Devon succession cannot be correlated with confidence with eustatic changes in sea level.