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1.
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.  相似文献   

2.
《Applied Geochemistry》1998,13(2):143-153
The Cretaceous Chalk [Ulster White Limestone Formation (UWLF)] in Northern Ireland has been diagenetically altered to a hard, low porosity, low matrix-permeability rock. It subcrops extensive beneath thick horizontal Tertiary Basalt lavas, but has a restricted strip of outcrop (approximately 80 km2) around the periphery of the overlying igneous rock. Despite its nature and location, numerous springs issue from the Chalk and are used for public and local supply, although little is known about the origin of the groundwater or the type of flow within the Chalk. We have addressed these unknowns using hydrogeochemical data from both rock types and surface stream waters, and fluorescent dye tracing. We have demonstrated that leakage recharge from the basalt to the Chalk accounts for less than 20% of the total water issuing from the Chalk springs using geochemical mass balance of conservative species and discharge data. The majority of the discharge derives from streams coming off the basalt and entering sink holes in the Chalk aquifer. Field and water tracing evidence shows that groundwater flow in the Chalk is dominated by conduit and fissure flow near to the outcrop areas. Karstification has been widespread near outcrop but has not ocurred in the sub-basalt region. Beneath the basalts, the Chalk exhibits different hydraulic properties due to the reduced recharge and dissolution potential that has prevented extensive development of fissure permeability in the UWLF. The Chalk aquifer thus appears to be strongly zoned in terms of the source of water, flow regime and recharge rate.  相似文献   

3.
High-concentration saltwaters occur in many places in the regional Chalk aquifers of North-West Europe; to investigate deep occurrences, profiles of interstitial porewater chemistry have been studied from three 250–450m deep cores drilled in the eastern parts of Zealand, Denmark. At the studied location, saline water in the Chalk resides at depths from 40 to 80m and salinity increases with depth. Concentrations of chloride up to ca. 30,000ppm have been observed at depths of 400m. Measured vertical hydraulic heads in open boreholes suggest that advective groundwater flow is now restricted in deeper parts of the Chalk formation and diffusive transport is thus the predominant transport mechanism. Laboratory-measured porosity and effective diffusion coefficients were used as input to a numerical 1D diffusion model of the interface between freshwater in an upper, fractured aquifer and modified connate formation water below. The model satisfactorily simulated the observed chloride and δ18O profiles. The diffusive refreshening of the Chalk formation has been going on for about 0.9 million years. The connate water in the Chalk of parts of the sedimentary basin seems to have been modified by transport of saltwater from underlying Mesozoic and Paleozoic sediments during compaction, which presumably ceased around 4 million years ago.  相似文献   

4.
The chemical and isotopic composition of groundwater from 52 sites in the London (U.K.) area was determined as part of a project aimed at assessing the spatial variation in the age of Chalk groundwater, and in determining the relationship between fracture and matrix groundwater in this dual porosity system.Systematic changes in groundwater chemistry take place in the downgradient direction in response to several chemical processes. These processes include early concentration by evaporation and congruent dissolution of calcite followed by widespread incongruent dissolution and ion exchange in addition to local oxidation-reduction reactions, gypsum dissolution and saline intrusion. As a result of the above processes, Chalk groundwater follows an evolutionary path from Ca bicarbonate type to Na bicarbonate type.The age of Chalk groundwater was modelled using14C, δ13C,3H, δ2H and δ180. There is a general increase in the groundwater age in a downgradient direction with the oldest water found in N central areas of the basin. Groundwater in the unconfined zones and in areas S of the Greenwich fault is almost entirely of unevolved, modem composition. Carbon-14 modelling suggests that Chalk groundwater in the S basin is generally less than 10000 a old while that in the north is generally between 10000 and 25000 a old. The presence of3H in concentrations of up to 7 TU in groundwater which yields ages of several 1000 a, however, indicates that mechanisms exist for the rapid introduction of recent groundwater to the confined aquifer. Results of palaeorecharge temperature determinations using δ2H, δ180 and noble gas analytical results suggest that significant Devensian recharge did indeed occur in the aquifer.A model of the development of the Chalk recognizes that it is a classic dual porosity aquifer in which groundwater flow occurs predominantly in the fracture system. The upper 50 m of the aquifer was flushed with fresh water during the 2–3 × 106 a of the Quaternary and therefore meteoric water largely replaced the Tertiary and Cretaceous marine water that previously saturated the system. Most processes which control the chemistry of the groundwater occur in the matrix where the surface area is exceptionally high. Although fracture flow dominates the flow regime, diffusion from the matrix into the fracture porosity controls the chemistry of Chalk groundwater.  相似文献   

5.
Chalk is a variable material, the properties of which are dependent upon its composition, textural features and diagenetic history. With the exception of certain horizons in the Lower Chalk that contain appreciable amounts of clayey material, the English Chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions. The latter comprises 70–80% of chalk. Cementation took place more or less contemporaneously with deposition so that the sediment was able to support relatively high overburden pressures. Hence, high values of porosity were retained. Chalk varies appreciably in density and hardness. The harder chalks are the result of diagenetic processes and bioturbation that brought about densification. In soft chalks the grains are only bound together at the points of contact by thin films of calcite.

The latest classification of chalk is based on an assessment of intact dry density, discontinuity aperture and discontinuity spacing. Chalk tends to vary from moderately weak to moderately strong and its strength is significantly reduced on saturation. Under triaxial loading conditions diagonal shear failure tends to occur at lower confining pressures but at higher confining pressures barrel-shaped failure occurs indicating plastic deformation and textural disaggregation. Similarly, at low loading, chalk exhibits low volume compressibility but much more significant consolidation occurs if the yield stress is exceeded.

Chalk undergoes dissolution and so solution features are found throughout its outcrop.

Mineworkings in the Chalk extend back into the distant past, the most ancient being those excavated in the Neolithic Age for flint. Several types of workings exist. Collapse of old mineworkings, most of which are unrecorded, is difficult to predict. The potential for subsidence, caused by the collapse of both mineworkings and dissolution features, affects development and its occurrence can lead to the abandonment of property or, worse, the loss of lives.  相似文献   


6.
The Upper Turonian Chalk Rock occurs within a nodular unit within the otherwise generally soft, white chalk that dominates the English Upper Cretaceous. The nodular unit is condensed, and contains a number of hardgrounds that are designated here as the Chalk Rock Formation. The Chalk Rock contains some seven or eight hardgrounds, most of which are lithologically distinctive and can be traced over distances of up to 250 km. Nine beds within the Chalk Rock are named, comprising six hardgrounds and three marl seams. The lowermost widespread hardground appears to be more or less equivalent to the “Spurious Chalk Rock” of the south coast of England. In two areas the thickness of the Chalk Rock is greatly diminished. The most marked area, in west Wiltshire, is located close to the Palaeozoic Mendip Hills and indicates that the Mendip structure has influenced Turonian sedimentation. The other region of thinning is a platform-like area in the eastern Chiltern Hills WNW of London.  相似文献   

7.
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.  相似文献   

8.
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9.
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.  相似文献   

10.
Remapping the Chalk of the Central Chalk Mass of the Isle of Wight between Carisbrooke (Newport), Calbourne and Shalcombe, including the Bowcombe Valley, has identified a complex series of tectonic ‘rolls’ and ‘flats’ in a region that has been interpreted to be a relay ramp between the Needles and the Sandown faults. A major new WNW trending fault at Cheverton throws the Chalk down by >50 m to the SW in a 80-100 m wide zone of faulting within which some chalk blocks have near vertical dips. The fault location and trend closely follows the edge of the Cranbourne-Fordingbridge High and could also reflect, for the first time, the surface expression of part of the Needles Fault, a major inversion reverse fault. Located along this fault zone deep Quaternary weathering of the Chalk and Quaternary gravel deposits are present. The trend of the Cheverton Fault brings it towards Gotten Leaze where a groundwater pumping station is located and groundwater springs regularly cause flooding on the Brighstone-Calbourne Road. Analyses of the jointing in the Chalk show that stratabound fracture patterns typical of the Chalk formations elsewhere in Southern England are present in the Central Mass. In addition, there are numerous small faults along which valleys have formed. Tectonic structure and lithology have had a profound influence on the geomorphology and groundwater flow in the Chalk in the Central Mass.  相似文献   

11.
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.  相似文献   

12.
The Chalk is an important water supply aquifer, yet ecosystems within it remain poorly understood. Boreholes (198) in seven areas of England (UK) were sampled to determine the importance of the Chalk aquifer as a habitat, and to improve understanding of how species are distributed. Stygobitic macro-invertebrates were remarkably common, and were recorded in 67 % of boreholes in unconcealed Chalk, although they were not recorded in Chalk that is concealed by low-permeability strata and thus likely to be confined. Most species were found in shallow boreholes (<21 m) and boreholes with deep (>50 m) water tables, indicating that the habitat is vertically extensive. Stygobites were present in more boreholes in southern England than northern England (77 % compared to 38 %). Only two species were found in northern England compared to six in southern England, but overall seven of the eight stygobitic macro-invertebrate species found in England were detected in the Chalk. Two species are common in southern England, but absent from northern England despite the presence of a continuous habitat prior to the Devensian glaciation. This suggests that either they did not survive glaciations in the north where glaciers were more extensive, or dispersal rates are slow and they have never colonised northern England. Subsurface ecosystems comprising aquatic macro-invertebrates and meiofauna, as well as the microbial organisms they interact with, are likely to be widespread in the Chalk aquifer. They represent an important contribution to biodiversity, and may influence biogeochemical cycles and provide other ecosystem services.  相似文献   

13.
The Austin Chalk and Eagle Ford Shale are Upper Cretaceous deposits that extend across Texas from the northeast to southwest. These formations contain organic carbon enriched mudstones and chalks that were deposited during transgressions of the Cretaceous epeiric sea in North America. Recent workers in petroleum geochemistry have demonstrated that these organic enriched rocks possessed attributes common to oil source rocks. The present study of these Austin Chalk and Eagle Ford Shale rocks is from the perspective of organic petrology, and it augments the earlier geochemical work that documented source variability within units of these formations. As with the earlier work, the results of this study show that both formations contain intervals that are, when mature, capable of generating commercial quantities of liquid hydrocarbons. However, this work further revealed that Eagle Ford rocks not only exhibit greater or ganic carbon contents, but also have greater quantities of oil-prone kerogen (fluorescent amorphinite and exinite) when compared with rocks from the Austin Chalk. These source rock differences relate to levels or degrees of organic preservation. Dysaerobic to oxic depositional settings seem to be more characteristic of the Austin Chalk than of the Eagle Ford Shale. Such oxic environments do not consistently favor the preservation of organic matter. Usually, well-preserved kerogen forms under more anoxic conditions, such as those that occurred during deposition of some Eagle Ford units. These anoxic conditions suggest that the geographically more extensive Eagle Ford Shale is a more important source for oil than is the Austin Chalk.  相似文献   

14.
Five pithonellid blooms recognised in the Chalk Group of the Isle of Wight are correlated via foraminiferal biostratigraphy to regional and global events. Blooms were recognised in the Holywell Nodular Chalk to basal New Pit Chalk formations (foraminiferal zones BGS7 to BGS9); M. guerangeri to Mytiloides standard (macrofaunal zones); middle Lewes Chalk (questionably foraminifera Zone BGS12; S. plana standard macrofaunal Zone); basal Seaford Chalk (BGS14; base M. coranguinum standard macrofaunal Zone); lower Newhaven Chalk (base BGS18; base U. socialis standard macrofaunal Zone); and uppermost Newhaven to basal Culver formations (BGS19-20; O. pilula to low G. quadrata standard macrofaunal zones). The blooms appear to be coeval with oceanographic change and the general trend towards an increase in the proportion of planktonic taxa may suggest upwelling and/or dysaerobic bottom waters.  相似文献   

15.
This paper presents results of hydrochemical and isotopic analyses of groundwater (fracture water) and porewater, and physical property and water content measurements of bedrock core at the Chalk River Laboratories (CRL) site in Ontario. Density and water contents were determined and water-loss porosity values were calculated for core samples. Average and standard deviations of density and water-loss porosity of 50 core samples from four boreholes are 2.73 ± 12 g/cc and 1.32 ± 1.24 percent. Respective median values are 2.68 and 0.83 indicating a positive skewness in the distributions. Groundwater samples from four deep boreholes were analyzed for strontium (87Sr/86Sr) and uranium (234U/238U) isotope ratios. Oxygen and hydrogen isotope analyses and selected solute concentrations determined by CRL are included for comparison. Groundwater from borehole CRG-1 in a zone between approximately +60 and −240 m elevation is relatively depleted in δ18O and δ2H perhaps reflecting a slug of water recharged during colder climatic conditions. Porewater was extracted from core samples by centrifugation and analyzed for major dissolved ions and for strontium and uranium isotopes. On average, the extracted water contains 15 times larger concentration of solutes than the groundwater. 234U/238U and correlation of 87Sr/86Sr with Rb/Sr values indicate that the porewater may be substantially older than the groundwater. Results of this study show that the Precambrian gneisses at Chalk River are similar in physical properties and hydrochemical aspects to crystalline rocks being considered for the construction of nuclear waste repositories in other regions.  相似文献   

16.
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  相似文献   

17.
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.  相似文献   

18.
The Chalk aquifer of Champagne (France) baseline geochemistry has been determined using a solid solution approach for the modelling of calcite dissolution. The water–rock interactions are modelled by the speciation code CHESS from field data and Ca, Mg and Sr aqueous concentrations in groundwater. The stoichiometries of solid solutions are defined in each stratigraphic unit of the Chalk aquifer from bulk geochemistry and Chalk mineralogy of samples taken from boreholes. The initial mineralisation of water at the bottom of the unsaturated zone and the characterisation of the theoretical evolution of groundwater chemistry along the flow lines associated with incongruent calcite dissolution are calculated from this approach.  相似文献   

19.
Extraordinary, long-distance litho-marker beds such as the Lewes and Shoreham Tubular Flints and associated marl seams and fossils, recognised in cliff exposures and cliff-fall boulders, are keys to unlocking the stratigraphy and tectonic structures in the Late Cretaceous of the Dorset and East Devon Coast World Heritage Site (Jurassic Coast). Durdle Cove is a special gem exposing the Lewes and Seaford Chalk stratigraphy where new marker beds are identified and sediments and tectonic structures provide clues to timing of movements that produced a Late Cretaceous pericline which grew into a Miocene monocline along the line of the underlying Purbeck Reverse Fault. During ‘inversion’ along this fault some Late Cretaceous Chalk formations were in part or completely ‘lost’ (e.g. Middle Turonian New Pit Chalk Formation) and others were condensed (e.g. Late Santonian and Early Campanian Newhaven Chalk Formation). Excavation of the A354 road cutting at the Lower Bincombe Farm, has greatly added to the stratigraphical records of Late Cretaceous fossils in South Dorset, especially Coniacian and Early Campanian inoceramid bivalves and the various stratigraphically specific forms of the Late Santonian to Early Campanian echinoid fossil Echinocorys scutata spp. not recorded before in this coastline. The very large bivalve fossil Platyceramus sp. provides clues to chalk sea-floor environments.  相似文献   

20.
The thermal diffusivity of Upper Chalk from the Micraster coranguinum zone had been calculated from 8 year temperature data obtained from depths between 30 cm and 21.34 m at Harestock, near Winchester, Hampshire.Amplitudes and phase lags were calculated from these data using Fourier analysis.Values of thermal diffusivity were calculated from the variation of amplitude with depth and phase lag with depth for the first harmonic using the Fourier heat flow equation.Agreement between the two methods was very good between 30 cm and 914 cm depth. An overall mean value is 0.004489 ± 0.00052cm?2s?1.Using appropriate values of porosity (47%) and density the mean thermal conductivity is 1.519W m?1°C?1± 0.326, for the water saturated state.  相似文献   

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