bluuebunny/crossref_metadata_2025_split huggingface dataset:
Hemipelagites: processes, facies and model
Authors: Stow Dorrik A. V. , Tabrez Ali R. 
Published: April 2008

Abstract

Detailed sedimentological studies have been carried out on Pleistocene to Recent sediments from the Makran margin and on Miocene to Recent sediments from the Oman margin of the NW Indian Ocean. In both area, hemipelagites make up
c.
50% of the succession, being closely interbedded with turbidites (Makran) and pelagites (Oman). Their sedimentary features (structure, texture, fabric and composition), bedding style and rates of sedimentation are reported and compared with previous detailed studies of both modern and ancient hemipelagites. A composite facies model is presented. Hemipelagites are finegrained sediments typically occurring in deep-water settings. They generally comprise an admixture of >10% biogenic pelagic material and >10% terrigenous or volcanigenic material, in which >40% of the terrigenous (volcanigenic) fraction is silt size or greater (i.e. >4 µm). They are deposited by a combination of vertical settling and slow lateral advection. Hemipelagites are mostly thoroughly bioturbated with ichnofossils including
Zoophycos, Planolites, Chondrites, Phycosiphon
and others. They are generally devoid of primary sedimentary structures, except for an organic-rich facies deposited in anoxic conditions which has a distinctive fissile lamination. The submicroscopic fabric is closely packed with subparallel alignment of clays. The grain size is strongly influenced by the composition, although in general the deposits are fine grained (mean size 5–35 µm), and poorly sorted, in some cases with bimodal, trimodal or polymodal grain-size distributions. The composition includes a biogenic fraction that is calcareous and/or siliceous, and a terrigenous fraction dependent on the nature of the supply pathway, i.e. river plumes, aeolian dust, glacigenic input or volcaniclastic fallout. Chemogenic components (phosphorites, glauconite, ferromanganese nodules) are common in some settings, whereas high organic carbon contents characterize others. Cyclic bedding, typical of climatic forcing, is common although bed boundaries are generally very gradational. Sedimentation rates typically vary from <5 cm/ka to >20 cm/ka and greatly influence development of the different characteristics outlined above. Extremely high rates (>100 cm/ka) occur in zones of hemipelagic sediment focusing, such as slope canyon systems and sediment pathways.
Changes in groundwater chemistry due to rising groundwater levels in the London Basin between 1963 and 1994
Authors: Mühlherr I. H. , Hiscock K. M. , Dennis P. F. , Feast N. A. 
Published: April 2008

Abstract
Commencing in the last century, groundwater abstraction has modified groundwater flow and lowered groundwater levels in the Chalk aquifer of the London Basin. Since the 1960s, a decline in pumping has caused a constant rise in groundwater levels and groundwater is now re-entering strata which had formerly been dewatered. In order to assess the effects of the changed hydraulic conditions on the chemical composition of groundwater, Chalk groundwater samples along a transect from Watford to Croydon across the London Basin were analysed for several chemical parameters in 1994, and compared with results from a similar hydrochemical study reported in 1963.
A general deterioration in groundwater quality was identified. Much higher sulphate and alkali and alkaline earth metal concentrations were found in almost the entire study area and are associated with the reflushing of formerly dewatered strata. In the Chalk outcrop areas, high nitrate, sulphate, chloride and sodium concentrations indicate anthropogenic aquifer pollution. Several chemical and physical processes that control the groundwater chemistry down-gradient towards the centre of the Basin were identified. Finally, five groundwater zones established in 1963 on the basis of hydrochemistry were revised to produce three groundwater zones.
Aquifer susceptibility to side-effects of groundwater exploitation
Authors: Adams B. , Macdonald A. M. 
Published: April 2008

Abstract
Aquifer vulnerability has become a ‘cornerstone’ of groundwater protection policies, taking into account, as it does, aspects of pollution resulting from applied contaminant load. However, this is not the whole story and groundwater resource degradation can also result from other impacts of development such as piezometric level decline, saline intrusion and subsidence. It is generally the case that such impacts are normally diagnosed after they have occurred. Nevertheless, it is evident that certain aquifers are more susceptible to these impacts (often referred to as ‘over-exploitation’) than others.
As a first step towards a method to determine aquifer susceptibility to such impacts, a diagnostic method is introduced as a means to provide managers and planners with an additional method of evaluating potential aquifer degradation as a result of development.
The method, as presented, is at an early stage of development and cannot yet be used for inter- regional comparisons. Used critically, however, the authors believe it provides a useful tool for giving a first estimate of aquifer susceptibility. Planned future developments of the methodology are also presented.
A geophysical investigation of saline intrusion and geological structure beneath areas of tidal coastal wetland at Langstone Harbour, Hampshire, UK
Authors: McDonald Robert J. , Russill Nicholas R.W. , Miliorizos Marios , Thomas Jonathan W. 
Published: April 2008

Abstract
Experiments are described that test how tide and geological structure affect saline intrusion beneath an area of coastal wetland in Hampshire, southern England. Resistivity tomography and time-dependent ground conductivity surveys were carried out at two closely located survey sites. Resultant maps and sections show clear geophysical anomalies that can be attributed to tidal saline intrusion and to the position of geological structures. Results suggest that migration of saline groundwater is, in places, out of phase with the tidal cycle, and is concentrated along sub-surface tidal channels, gravel aquifers and depressions in Chalk bedrock topography. We conclude that the seawater-fresh-water boundary in the studied wetland environment is complicated by shallow-level saline flow along geologically definable routes within the Quaternary deposits above the less permeable Chalk bedrock.
Evidence for rapid groundwater flow and karst-type behaviour in the Chalk of southern England
Authors: MacDonald A. M. , Brewerton L. J. , Allen D. J. 
Published: April 2008

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.
Water transport through the unsaturated zone of the Middle Chalk: a case study from Fleam Dyke lysimeter
Authors: Jones H. K. , Cooper J. D. 
Published: April 2008

Abstract
The role of fractures in transporting water in the unsaturated zone of the Chalk has been a source of controversy for many years. There is clearly a potential for fractures to transport water and contaminants rapidly to the saturated zone, although direct observations are rare. The evidence from well hydrographs, solute profiles and soil water measurements is ambiguous concerning the importance of by-pass flow.

This paper examines drainage data from several years’ operation of a 5 m cube undisturbed monolith lysimeter at Fleam Dyke, Cambridgeshire, for evidence of by-pass flow and investigates the factors responsible for its onset. Evidence from measurements of unsaturated hydraulic conductivity of the Chalk at the site and
in situ
measurements of soil water are also presented. Both the lysimeter and soil water observations agree that flow through fractures accounts for approximately 30% of the total drainage from this site, significantly higher than has been inferred at other Chalk sites. By-pass flow was observed to occur above a threshold of approximately −50 cm H
2
O matric potential, or above a flow rate of approximately 1 mm day
−1
. During periods of no soil water deficit, about 50% of drainage occurred via fractures. In general, there was a delay of three or four days between rainfally and drainage at the base of the lysimeter. On once occasion, intense summer rainfall satisfied the existing soil moisture deficit and initiated fracture flow, the peak in drainage through the lysimeter occurring within 24 hours. Drainage from the lysimeter was consistently higher than MORECS estimates of hydrologically effective precipitation during the period after the onset of soil water deficits.
Recharge through the drift: a study of contrasting Chalk catchments near Redgrave Fen, UK
Authors: Soley R. W. N. , Heathcote J. A. 
Published: April 2008

Abstract
Redgrave and Lopham Fens lie at the headwaters of the Rivers Waveney and Little Ouse in East Anglia, UK. The Fens are supported by groundwater discharge from the drift-covered Chalk underlying the area. This discharge and the ecological value of the Fens have been derogated by public water supply abstractions from a nearby borehole.
Aspects of the investigations undertaken to construct a groundwater model to assess options for relocating the borehole away from the Fens are described. Particular attention is given to the value of using basic river flow data analysis to identify contrasts in the hydrological characteristics of the two catchments and constrain estimates of recharge.
The Waveney catchment is flashy and dominated by run-off whereas the Little Ouse receives a more significant groundwater baseflow contribution. These catchment contrasts reflect subtle variations in boulder clay thicknesses and topography. They also reflect the contrasting effectiveness of the two rivers as drains on the groundwater system.

Average recharge to the Chalk through the boulder clay-covered interfluves is estimated at around 36 mm a
−1
(which is similar to other values quoted in the literature). Recharge may locally fall to less than 10 mm a
−1
where the boulder clay is thick and the Chalk is strongly confined.
Assessment of groundwater vulnerability in England and Wales
Authors: Palmer R. C. , Lewis M. A. 
Published: April 2008

Abstract
Groundwater represents around 35% of water distributed for potable supply in England and Wales and is also utilized by industry and for private supplies. It is of high quality and requires little treatment before use. It is important, therefore, that a robust policy of groundwater protection is available to protect the resource from pollution, as rehabilitation of polluted groundwater is expensive, and often may not be technically feasible. One way to protect aquifers is to control activities which pose a threat to groundwater quality, especially in areas where they are naturally poorly protected. Areas where groundwater is at risk of pollution from surface activities are indicated on maps by characterizing the properties of the underlying soils and rocks. The rocks are classified on the basis of their permeability, into Major, Minor and Non-Aquifers. The soils overlying aquifers are then categorized on their physical and chemical properties into leaching potential classes. The Groundwater Protection Policy of the Environment Agency, of which the groundwater vulnerability maps are part, provides guidance on those types of development which require controlling if groundwater quality is to be maintained in the long term. The relevance of these maps to other environmental issues is discussed.
Is all groundwater worth protecting? The example of the Kellaways Sand
Authors: Mather John , Halliday Dawn , Joseph Jeremy 
Published: April 2008

Abstract

Many formations treated as Minor Aquifers for the purposes of resource protection do not meet accepted definitions of aquifers. The Kellaways Sand is a typical example, and modern waste-disposal licences require the installation of an engineered containment system to protect groundwater within it. However, the formation is clay-rich and only 4 to 5 m in thickness, and has a transmissivity of around 1.3 × 10
−1
m
2
day
−1
. Contained water quality is poor with salinities well above maximum admissible concentrations for drinking water. As the Kellaways Sand does not yield water to pumping wells, the only rational reason for protection is that the formation might contribute to baseflow in the River Ouse to the northeast. However, hydraulic gradients are such that seepage rates to the river will not exceed more than a few cubic metres per year over the length of outcrop adjacent to the river. It is concluded that the installation of engineered mineral liners at the base of brickpits in the Lower Oxford Clay do nothing to reduce the risks associated with waste disposal. Resources spent on protecting the Kellaways Sand could be better used in other areas of environmental protection.
Giant fields, petroleum systems and exploration maturity of Algeria
Authors: Macgregor Duncan S. 
Published: April 2008

Abstract
The giant petroleum fields of eastern Algeria can be divided into six significant petroleum systems. More than 60% of the giant fields and 80% of the reserves within them lie in a specific setting in reservoirs immediately below or above the Hercynian unconformity, and below the Liassic salt seal. The second main reservoirs are Early Devonian sandstones. Timing of generation and preservation potential, as controlled by tectonic history, seal type and source rock burial, are key controls on the distribution of petroleum in these giant fields, particularly for oil. Field size distributions within the six petroleum systems can be related to structural style and to differing levels of exploration maturity within each system. Comparisons can be made between the productive trap types and petroleum systems of Algeria and analogous but more heavily drilled basins elsewhere in the world. Such comparisons suggest that further giant fields lie undiscovered in Algeria, particularly in subtle forms of traps, and in the 250–500 MMBOE (million barrels oil equivalent) reserves range. The Ghadames petroleum system may be the system which carries the highest yet-to-find, as predicted from a range of criteria, including discovery trends, drilling density, field size distribution and source rock supply analysis. The challenge to the oil industry in Algeria is to identify the locations of the missing giant fields suggested by this analysis.
Controls on hydrocarbon occurrence and productivity in the F6 reservoir, Tin Fouyé-Tabankort area, NW Illizi Basin
Authors: Alem N. , Assassi S. , Benhebouche S. , Kadi B. 
Published: April 2008

Abstract
Several oil accumulations of various sizes have been found within the F6 reservoir of Upper Silurian-Lower Devonian age in the Tin Fouyé region of the Illizi Basin, about 1500 km from Algiers. Lithologically, the F6 consists of interbedded sandstones and shales and is subdivided into units: M1, M2, A, B1, B2, C1, C2 and C3, the upper units often being missing through erosion. These reservoirs, which were deposited in offshore coastal bars and tidal channels, show a high degree of lateral and vertical facies variation. Reservoir quality is maximized in medium- to coarse-grained clay-poor bar sands within units C1 and B2, which contain significant secondary porosity. Hydrodynamic activity is very important in controlling the distribution of hydrocarbons. Study of the hydrology of the F6 reservoir shows that connate water and oil have been frequently flushed by fresh water in the most permeable units, particularly C1 and B2. This finding is supported by hydrochemical data, by the inclination of oil-water contacts and by a high observed hydraulic gradient. This mechanism has led to the flushing of some oil from the Tin Fouyé Est (TFE) structural trap and a concentration of oil in an accumulation with a tilted contact on the Tin Fouyé Nord structural nose. Many kinds of traps have been recognized in this region. These include a simple structural trap at Tin Fouyé, a combination hydrodynamic-stratigraphical-structural trap at Tin Fouyé Nord, a hydrodynamic-structural trap at TFT-2 and a pure hydrodynamic trap at TFT 8. Hydrodynamic activity is not favourable for the accumulation of hydrocarbons in low-amplitude structures but can lead to significant accumulations in favourably orientated structural noses, even when these lack closure.
Sedimentological evolution of the Givetian-Eifelian (F3) sand bar of the West Alrar field, Illizi Basin, Algeria
Authors: Chaouchi Rabah , Malla M. S. , Kechou F. 
Published: April 2008

Abstract
Commercial accumulations of hydrocarbons depend on several factors, among which sedimentological conditions are of prime importance. Such sedimentological factors are of particular importance in controlling both entrapment and reservoir quality in the western part of the Alrar field, within the F3 sandstone reservoir of Givetian-Eifelian age. Recent well results have shown facies and reservoir quality distribution to be more complex and generally more favourable than previously predicted. A series of tidally influenced littoral bars are now believed to have existed in the Alrar area at the time of deposition of the reservoir, these being elongated in a NW-SE direction in the West Alrar area, with an interbar area in the vicinity of well DZ-1. The highest reservoir quality is largely confined to this bar facies and understanding its distribution is therefore vital to locating development wells. Four possible sedimentological interpretations of the extension of the F3 reservoir to the south and west have been developed. In most areas, the pinchout seems to be gradual, but in other areas where the transition to lagoonal shale is sharp, fault control may apply. Significant uncertainties however remain in predicting this shale-out in undrilled areas.
Mesozoic and Cenozoic petroleum systems of North Africa
Authors: Macgregor Duncan S. , Moody Richard T. J. 
Published: April 2008

Abstract
The Mesozoic and Tertiary petroleum systems of North Africa have formed in a suite of extensional basins developed along the Atlantic and Tethyan margin. Petroleum is unevenly spread across the region of Mesozoic basin development, with one world-class petroleum province developed (the Sirt Basin of Libya), and several more modest developments of reserves in Western Egypt and offshore Libya-Tunisia. A set of these extensional basins were later subject to compression on the Atlas and Syrian trends, with generally unfavourable effects for the preservation of the petroleum systems affected. The Moroccan passive margin is also a region of currently low exploration success. Productive reservoirs are spread throughout the Mesozoic section, though, with the exception of the prolific Early Cretaceous clastic deposits and the Paleogene carbonates, are generally not seen to correlate between basins. Effective source rock development is heavily concentrated in the Late Cretaceous and Eocene. Recent evidence also suggests an additional contribution from lacustrine source rocks of Early Cretaceous and older age in the southeastern Sirt Basin. Patterns of maturity on these main source levels, and timing of that maturity relative to trap formation, seem to be the main controls on basin success and failure, with preservation an additional key factor in inverted basins. The Mesozoic basins of North Africa are generally lightly explored compared with basins of similar petroleum productivity elsewhere in the world. Exploration to date in the Sirt Basin, for instance, has barely progressed beyond the structural trap stage, with application of analogues suggesting undrilled potential in various types of subtle trap. Further exploration remains necessary in the Atlas trend and Moroccan margin, focused on plays with more favourable timing relationships between generation and structural development than those drilled to date.
Late Permian to Recent magmatic activity on the African-Arabian margin of Tethys
Authors: Wilson M. , Guiraud R. , Moreau C. , Bellion Y. J.-C. 
Published: April 2008

AbstractMagmatic activity on the African-Arabian margin of Tethys has fluctuated significantly during the past 250 Ma in response to major phases of mantle plume activity and to extensional stresses within the African plate related to periods of continental break-up. A series of maps have been compiled, based upon stratigraphic constraints and K-Ar and40Ar-39Ar age determinations, to illustrate the changing patterns of igneous activity since the Late Permian. These are divided into five stages: I, Late Permian (256 Ma)-Latest Triassic (208 Ma); II, Early Jurassic (208 Ma, Liassic)-Mid Jurassic (157 Ma, Dogger); III, Late Jurassic (157 Ma)-Earliest Aptian (120 Ma); IV, Mid Aptian (119 Ma)-Mid Eocene (42 Ma); V, Late Eocene (42 Ma)-Recent. Extensive tholeiitic basaltic magmatism atc.200 Ma, preceding continental break-up in the Central Atlantic, is attributed to upwelling of the Cape Verde mantle plume beneath the West African craton. Contemporaneous magmatic activity in the Levant is related to the break-up of the Eastern Mediterranean margin. Early Cretaceous alkaline magmatism around the margins of the Equatorial Atlantic and within the Levant may also be plume related. During Neogene-Recent times there was a major change in the convection pattern within the upper mantle, generating extensive magmatism across the entire margin. Magmatism appears to be extremely long lived in a number of areas (e.g. Sudan, Air), with volcanic centres located along major basement fault zones, such as the Guinean-Nubian lineaments.