bluuebunny/crossref_metadata_2025_split huggingface dataset:
An introduction to the crystalline basement of Africa
Authors: Key R. M. 
Published: April 2008

Abstract
The crystalline basement of Africa comprises three major suites of rocks: the granite-gneiss-greenstone association of Archaean cratonic nuclei; strongly deformed metamorphic suites in mobile belts, mainly of Proterozoic age; and anorogenic intrusions which include Phanerozoic intrusive magmatic rocks related to rifting. This heterogeneous basement is extensively concealed beneath a variable thickness of diverse, unmetamorphosed sedimentary and extrusive volcanic rocks and weathering products.
The Archaean cratons have similar geological histories, which generally culminate with anorogenic potassic granites emplaced at about 2500 Ma. The history of the Kaapval Craton is prematurely curtailed at about 3050 Ma. The Limpopo Mobile Belt is uniquely Archaean: its development is linked to differential movement between the oldest cratonic elements of southern Africa. Elsewhere in Africa, the oldest Archaean record suggests an absence of thick continental crust: early greenstone belts formed above mantle plumes on mobile, thin crust.
The development of the Proterozoic mobile belts is related both to collision of older cratons (Wilson cycle orogenesis) and to ensialic disruption of single cratons. Early extensional orogenic phases produced both active (new oceanic crust development) and failed rifts. The earliest Proterozoic mobile belts cut across Archaean cratonic domains and later belts may be superposed. Older fractures are commonly reactivated by new stress systems. Much of the Phanerozoic anorogenic magmatism is related to Mesozoic and Cenozoic continental fragmentation and rifting, possibly associated with hot spots (White & McKenzie 1989).
Notwithstanding the relative antiquity of most of the basement, it is the effects of high-level brittle fracturing and weathering which largely control groundwater storage. The fracturing varies in age from Archaean, within the greenstone belts, to Phanerozoic, for faults related to movement of the African Plate, with ongoing Quaternary faulting in the major tensional rifts such as the East African Rift System. The thickest weathering profiles occur above the oldest erosion surfaces; regional thickness variations are controlled by past and present climatic differences.
The use of ground electrical survey methods for siting water-supply boreholes in shallow crystalline basement terrains
Authors: Carruthers R. M. , Smith I. F. 
Published: April 2008

Abstract
Resistivity methods are widely used as an aid to siting water-supply boreholes in basement terrain. Adverse conditions due to shallow bedrock can be identified readily but specific targets such as narrow conductive zones may be missed; quantitative interpretation may be less reliable than is often assumed. Electromagnetic (EM) profiling will detect localized zones of deeper weathering and the effects of fracturing at shallow depths, and it is used for the precise location of anomalous ground indicated by lineations on aerial photography. Routine EM surveys give better lateral resolution than resistivity for the conductive targets of interest. The 2-D and 3-D EM modelling techniques needed for interpreting the conductor geometry realistically are still under development.
Geophysical methods must be applied with due regard for the targets being sought and the local geological setting: they will not always be appropriate and resources should be targeted on districts where problems can be solved. Qualitative interpretations are adequate in some situations but more rigorous field procedures and analytical techniques are needed to ensure that useful hydrogeological information is obtained.
New approaches to pumping test interpretation for dug wells constructed on hard rock aquifers
Authors: Herbert R. , Barker J. A. , Kitching R. 
Published: April 2008

Abstract
Large-diameter hand-dug wells are used to exploit the weathered zone aquifer of hard rock areas. A pumping test is described which is suitable for such dug wells. This test can be diagnostic of aquifer type and hydraulic properties.
Four techniques are described which can be used to interpret the pumping test. The first technique uses a digital model to simulate aquifer conditions and to reproduce changes in well water level observed during the test. Multi-layered conditions and partial penetration of the dug well can be simulated providing radial symmetry pertains. Two techniques are then described which require only manual calculation. In both these cases the aquifer is assumed to be single-layered but in one the aquifer is assumed to be confined and in the second it is unconfined. The fourth technique uses software developed for use on a personal computer. The software simulates a leaky aquifer and automatically determines the hydraulic parameters of the two layers of the leaky system using a least-squares technique. Finally, a week-long test is described which can be used to predict empirically the long-term behaviour of both dug and collector wells.
Bathonian to mid-Oxfordian sequence stratigraphy of the South Viking Graben, North Sea
Authors: Cockings James H. , Kessler L. Gifford , Mazza Thomas A. , Riley Leslie A. 
Published: April 2008

Abstract
Application of sequence stratigraphy to the analysis of the Middle Jurassic in the South Viking Graben provides the framework for a consistent, basinwide understanding of sedimentary facies distribution. The inherited lithostratigraphical nomenclature is found to be ambiguous and sequence stratigraphical principles have been useful in clarifying this problem. Biostratigraphical data aid in the delineation of five units, designated as sequences, which broadly correspond to Bathonian, early, mid- and late Callovian and early to mid- Oxfordian. These sequences are defined on the basis of the contemporaneous depositional systems which they each contain and the horizontal bounding surfaces which separate them from other sequences. The sequences represent transgressive-regressive episodes and the surfaces which separate them record maximum marine flooding.
Three structural styles are recognized to have controlled basin development during the late Bathonian to Oxfordian syn-rift extension phase. Extension was accommodated by terrace block faulting, half-graben development, or movement over ramps and flats. Different rates of extension between these diverse structural zones were transferred by complex, strike-slip fault systems. The three structural styles gave rise to differential subsidence laterally in the hangingwall and there is an observed relationship between structural development and sedimentary facies distribution. The basinwide recognition of sequences, the analysis of their internal architecture and the change in this through time, is related to an interplay of changes in sea level, sedimentation rates and extensional tectonics. Sequence mapping shows that tectonically controlled subsidence was the major control on sedimentary facies distribution during the middle Jurassic in the South Viking Graben.
The North Sea Chalk Play: examples from the Danish Central Graben
Authors: Megson J. B. 
Published: April 2008

Abstract
Currently, all hydrocarbon production in Denmark is from Upper Cretaceous and Danian chalk fields in the Central Graben. Seven fields are producing, and several more are under development. The field structures were formed by a variety of mechanisms: uplift on salt pillars (Skjold, Nils, Dagmar), doming over salt swells (Dan, Kraka), and end-Cretaceous and Miocene inversion tectonics (Tyra, Roar). The combination of structural styles, varying times of migration of hydrocarbons into the structures, and differences in chalk sedimentology have resulted in a wide range of reservoir porosities (20% to over 50%), and permeabilities (1 mD to 1000 mD). Recent advances in horizontal well technology have considerably increased predicted recovery factors in low permeability fields, such as Dan, and in fields where a thin oil leg is present beneath gas, such as Tyra.
Exploration for hydrocarbons in the chalk is now entering a new phase, as all structural closures have been drilled in the A. P. Møller concession area. There is evidence from existing chalk fields and discoveries of non-structural trapping mechanisms, such as: oil trapped within the chalk by an intra-chalk topseal, hydrodynamic trapping of oil, and diagenetic trapping. These concepts are being combined with seismic interpretation of high-resolution seismic data, targeted at the chalk, in order to develop new plays and prospects.

In addition it is shown that seismic data may be used as a direct hydrocarbon indicator where gas is present at the top chalk level. In 1991 an appraisal well, E-5X, was drilled
c
. 8 km from the main part of the Tyra Field, and encountered gas as predicted from a seismic phase reversal at top chalk.
Permeability images: a new technique for enhanced reservoir characterization
Authors: Bourke Lawrence T. , Corbin N. , Buck Stuart G. , Hudson Gavin 
Published: April 2008

Abstract

The small probe tip size of the nitrogen probe permeameter (typically 4 mm internal diameter) facilitates the investigation of small volumes of reservoir rock in core samples. The nitrogen probe permeameter measures gas flow rate at a known delivery pressure. This gas flow rate can in turn be calibrated to permeability. A closely spaced grid of such permeability measurements sampled every 4 mm in the
x
and
y
directions provides a basis for detailed sandstone permeability characterization. This paper presents permeability images, generated from such datasets, that reveal primary bedding fabric even where such features were not visually apparent in rock samples. A strong correlation is reported herein between images generated by probe permeametry from core samples and microresistivity images acquired from the borehole wall for the same reservoir interval. The similarity of features revealed by permeability and microresistivity images suggests a strong correspondence to rock properties measured by both techniques. Permeability images therefore offer considerable potential for the geological verification of microresistivity images where their direct interpretation is ambiguous. Furthermore, permeability and microresistivity images provide a mechanism for locating petrographical sampling points from which the controls on the permeability heterogeneities can be established.
High resolution molecular stratigraphy: analytical methodology
Authors: Smith Matthew B. , Poynter Jon G. , Bradshaw Stuart A. , Eglinton Geoffrey 
Published: April 2008

Abstract

In recent years Quaternary stratigraphy has become analytically intensive, given the realisation that the sedimentary record at specific locations is sufficiently continuous and complete for examination at 100 year resolution. This paper describes methods which enable high resolution molecular stratigraphy to be carried out quickly and efficiently. The primary technique for data generation is gas chromatography (GC), validated for certain samples by gas chromatography mass spectrometry (GC-MS). GC data are reduced using a suite of programs to provide compound abundances for entry in Lotus 1-2-3 spreadsheets.
xy
plots of compound abundance versus sub-bottom depth are produced, prior to the use of numerically intensive methods such as Fourier transform spectral analysis and principal components analysis. The paper is illustrated with time series data for the Uk
37
parameter from ODP Leg 108, Sites 658 A and B.
Lithostratigraphic applications for magnetic susceptibility logging of deep-sea sediment cores: examples from ODP Leg 115
Authors: Robinson Simon G. 
Published: April 2008

Abstract
Whole-core (WC) logging of low-field magnetic susceptibility (MS) is an extremely simple, rapid and non-destructive method of acquiring a quantifiable, lithologically related signal which is ideally suited to the task of correlating between deep-sea sediment sequences recovered from related holes at ODP sites. This is particularly useful for reconstructing uninterrupted composite sequences by splicing together stratigraphically continuous subsections of offset cores from adjacent holes. Continuity of sequence is established by pattern-matching the WCMS profiles of each hole, which are based on measurements made at intervals of only a few centimetres. It is essential that interhole correlation is effected with the highest possible lithostratigraphic resolution if composite sequences are to be used, for example, in studies of Milankovitch-band cyclicity in palaeoceanographic records, involving spectral analysis of down-core data.
In addition to identifying coring irregularities such as unrecorded gaps in the recovered sequence between cores, or repetition of sequence due to repenetration of the corer, WCMS profiling can be used to identify turbidite horizons and also intervals contaminated by ferrous metal artifacts from drilling (e.g. pipe rust). The presence of contaminants often indicates that the sequence contains horizons of slumped, or mechanically reworked sediment washed-in from uphole. Thus identified, turbidites and contaminated intervals can be avoided during sampling, and eliminated from stratigraphic reconstructions or accumulation rate estimates. The only category of deep-sea sediment lithology which exhibits WCMS values similar to those of contaminated intervals, is that of basic volcanic ash-rich horizons. Accordingly, this makes WCMS logging a potentially valuable tool in studies of marine tephrostratigraphy.
Inter-regional correlation between the WCMS profiles for holes drilled at different ODP sites is also possible in many instances, especially when lithological variations at each site are controlled by large scale palaeoceanographic, or global (i.e. orbitally forced) palaeoclimatic changes. In such circumstances, WCMS may also be used as a proxy palaeoclimatic indicator, duly assisting more conventional microfossil and isotopic methods of climato-stratigraphical zonation of the sequences recovered.
Magnetostratigraphic calibration of early Eocene depositional sequences in the southern North Sea Basin
Authors: Ali Jason R. , King Christopher , Hailwood Ernest A. 
Published: April 2008

Abstract
We present the results of a comprehensive magnetostratigraphic study of early Eocene (Ypresian) sedimentary sequences in the London Basin, Belgium, and the Varengeville outlier of northern France. A re-investigation of part of the lower Eocene section at Whitecliff Bay (Isle of Wight), in the Hampshire Basin, has also been carried out. The palaeomagnetic results from each area are used to refine existing litho- and biostratigraphic correlations. The magnetostratigraphy of the sedimentary sequence in each area is linked to the geomagnetic polarity time-scale using biostratigraphic data from calcareous nannoplankton and dinoflagellates.
The Ypresian deposits were laid down during the interval spanning geomagnetic Chrons C24BR to C22R. In the Hampshire Basin, deposition of Lutetian (middle Eocene) sediments extended into Chron C21R, but the apparent absence of a record of Chron C22N suggests an unconformity separating the lower and middle Eocene of between 0.6 and 2.0 Ma in duration, at the basin margin.
In this paper, recently acquired Ypresian stratigraphic data are integrated with previously published data, and a high-resolution ‘even-stratigraphy’ is developed for the Ypresian deposits of the southern margin of the North Sea Basin. A total of ten depositional sequences (‘cycles’), probably reflecting eustatic sea-level changes, are calibrated against this framework. An attempt is made to match these sequences, which represent a combination of third and fourth order cycles in the Exxon Model, with published sea-level cycle charts for the Early Eocene.
High resolution stratigraphy of marine Quaternary sequences
Authors: Weaver P. P. E. 
Published: April 2008

Abstract
The highest resolution stratigraphy of marine sequences can be obtained in the Quaternary. Oxygen isotope stratigraphy can be used to divide the Quaternary (0–1.66 Ma) into 63 isotope stages representing alternating glacials and interglacials. These climate changes are controlled by astronomical forcing involving eccentricity of the earth’s orbit (100 000 year periodicity); obliquity of the earth’s axis (41 000 year periodicity); and precession of the equinoxes (23 000 year periodicity). These factors can be combined to produce a time scale which has been used to tune the isotope records, providing ages for all the isotope stage boundaries. The percentage of calcium carbonate also varies in many marine sediments with similar frequencies to the isotope record and has been used in similar ways. Both these methods are unreliable in areas with reworking or sediment disturbances. They work best in combination with independent stratigraphic zonations such as those derived from calcareous nannofossils. Planktonic foraminifera provide palaeoclimate data but they are less well suited to detailed biostratigraphy in the Quaternary. Other microfossil groups including diatoms and radiolaria provide useful biostratigraphic information in some areas but they are often poorly preserved due to dissolution and when present they have rarely been used to provide high resolution stratigraphy.
Tephra layers as precise chronostratigraphical markers
Authors: Knox R. W. O’B. 
Published: April 2008

Abstract
Tephra layers provide the stratigrapher with very precise, tangible chronostratigraphical marker horizons. These can be used not only for correlation between specific sequences, but also as a means of assessing the accuracy and resolution of correlations based on more conventional stratigraphical criteria. Such tephrostratigraphical applications are, however, dependent on the recognition of individual tephra layers, to which end a variety of mineralogical and chemical fingerprinting techniques have been developed.
Tephra layers also provide a unique opportunity for direct and precise dating of sedimentary sequences. Recent developments in grain-specific dating techniques have greatly increased the precision and applicability of tephra dating, which is now seen as playing an essential role in the refinement of the Phanerozoic time scale. Also, as the resolution of dating techniques approaches that of standard biozones, it is becoming apparent that tephra dates may in themselves provide a means of direct correlation on a global basis; such correlations should be of particular value in resolving problems associated with climatically or geographically controlled faunal provinciality.
Worldwide correlation of Telychian (Upper Llandovery) strata using graptolites
Authors: Loydell David K. 
Published: April 2008

Abstract

The formal boundaries to the Telychian Stage have been stated to correlate with the bases of the
Monograptus turriculatus
Biozone at the bottom and the
Cyrtograptus centrifugus
Biozone at the top. The evidence for this, however, is far from conclusive.

The Telychian has been divided, mainly using monograptids, into eight or more biozones, although fewer have been recognized in Britain due to the general paucity of recent studies of the graptolite faunas of this age and to the existence of ‘barren beds’ high within most British Telychian sections. Each biozone has a duration of only a few hundred thousand years, but most are, at present, difficult to recognize in the absence of the index species or where this species’ stratigraphical range extends beyond the limits of its biozone.
Intercontinental correlation of Telychian strata using graptolites is hampered to a limited extent by provincialism and water mass specificity, and considerably by taxonomic problems. A thorough taxonomic revision of published species would provide a firm foundation for future biostratigraphical subdivision and correlation of the Telychian Stage. Knowledge of Telychian graptolite biostratigraphy would also be enhanced by restudies of two classic sections, the Trannon river section in Wales, and the Bajiaokou section, Ziyang, China.
The lateral extent of sequence boundaries
Authors: Cartwright J. A. , Haddock R. C. , Pinheiro L. M. 
Published: April 2008

Abstract

Sequence boundaries, as defined by the Exxon group, are regionally extensive surfaces (basin-wide scales) that are characterized in part by stratal discontinuity surfaces in the form of onlap, downlap and toplap (Mitchum
et al.
1977). The idea that sequence boundaries are so areally extensive is closely connected to the ideology favoured by the Exxon Group that sequence boundaries are eustatic in origin. This concept of sequence boundaries was initially based on seismic stratigraphic observations of stratal reflection geometry, but has now been extended to rock stratigraphic relationships in general. One of the important issues in sequence stratigraphy is to decide whether concepts based on the low resolution seismic method are applicable at much higher orders of resolution, such as in outcrop. This paper examines the stratigraphic conditions necessary for the development of regionally extensive and discrete surfaces of onlap, downlap and toplap. The main conclusions are that in order for onlap, downlap and toplap surfaces to develop as discrete surfaces there must be no contemporaneous sediment accumulation beyond the lap-out position. If this condition is not met, then discrete lap-out surfaces do not develop. Instead, closely-related families of surfaces that individually are of limited areal extent are more likely to form.

It is argued that in most depositional environments some form of sedimentation is likely to occur beyond the lap-out position. This suggests that sequence boundaries should only develop over extremely limited areas, and should not be expected to form regionally correlatable stratigraphic surfaces. The apparent regional extent of some sequence boundaries and their genetic linkage with eustatic changes is in conflict with predictions based on simple stratigraphic principles. The regional correlatability of sequence boundaries probably owes more to the limited resolution of seismic data and the map-driven need to correlate over large areas than to any physical continuity of a single surface.
Rift-related linked depositional systems and their seismic expression
Authors: Prosser Sarah 
Published: April 2008

Abstract
The spatial distribution and temporal evolution of depositional systems in active-fault bounded basins are considered to be significantly influenced by tectonics. Fault movement and stage of basin development control the potential for erosion and the rate of sediment flux. Distinct stages of rift evolution can be recognized, each with characteristic linked depositional systems and distinctive expressions on seismic reflection profiles. A four-fold division relating to rift initiation, rift climax, immediate post-rift and late post-rift stages of basin evolution is proposed to characterize most basin infill stratigraphies. The three-dimensional linked depositional systems (systems tracts) associated with each of these stages will vary according to climate, source rock composition, position relative to sea-level, and eustatic fluctuations; however, a dominant tectonic signature can still be isolated if analysis is undertaken in an appropriate fashion. The general suitability of the application of this new method of describing rift basin stratigraphies in terminal half-graben type basins is demonstrated using case studies and published examples. The introduction of new terminology for use in tectonically active settings is justified on the basis that no other technique sufficiently emphasizes the fundamental link between tectonics and sedimentation.