bluuebunny/crossref_metadata_2025 huggingface dataset:
Basement tectonics and their relationship to Mesozoic megasequences in the Celtic Seas and Bristol Channel area
Authors: Ruffell A. H. , Coward M. P.
Published: April 2008

A synthesis of new and pre-existing data on the deep geological structure of the Celtic Sea and Bristol Channel basins shows similarities to published models of deep structure from the onshore Wessex Basin to the east. A series of Variscan thrust planes, reactivated in the Mesozoic as normal faults, underlie the study area. Beneath the Bristol Channel, one such southward-dipping thrust may be equated with the ‘Variscan Front’. This structure is offset by NW-SE orientated transcurrent faults, structures thought to be active from the late Palaeozoic to the present-day. Mesozoic stratigraphic successions from the North Celtic Sea Basin and South Celtic Sea; Bristol Channel, and northern Wessex Basins suggest uplift and erosion of the latter areas in the late Jurassic. This is contrasted with increased subsidence at the same time in the North Celtic Sea. This uplift may have been facilitated by reactivation of the ‘Variscan Front’ thrust as a thrust in the east of the area, and by similar thrusts beneath the Pembrokeshire Ridge to the west. This reactivation and uplift during North Atlantic opening is suggested to have controlled the preservation of Upper Jurassic-Lower Cretaceous sedimentary megasequences.
Application of mineral deposit models in exploration: a case study of sediment-hosted gold deposits, Great Basin, Western United States
Authors: Adams Samuel S. , Putnam Borden R.
Published: April 2008

This paper emphasizes the development of mineral deposit models for exploration using as a case study the sediment-hosted, structure-controlled, disseminated gold deposits in the Great Basin of the Western United States. More than 60 deposits have been found in this area since the discovery of the Carlin deposit in 1962. Most deposits were discovered by direct prospecting and sampling of outcropping silicified zones (jasperoids). Similar poorly exposed and unexposed deposits remain to be discovered, but future exploration will require improved models that indirectly predict locations of these deposits.

Many of these sediment-hosted gold deposits occur within silicified and structurally complex zones in calcareous rocks below regional east-directed thrusts. Geological characteristics that have been successfully employed in exploration include: erosional windows (fensters) through the thrusts; silty to arenaceous calcareous host rocks below the thrusts; silicified zones; anticlines and domes of the lower plate calcareous sediments; alignment of deposits along linear trends; and geochemical anomalies (Au, Ag, As, Sb, Tl, etc.). Although the use of these criteria will discover new deposits, exploration now requires additional criteria that are reliably, but not so directly, associated with the deposits. Identification of these criteria will require the development of genetic models that tie more regional geological characteristics to ore formation and localization.

Development of successful mineral deposit models first requires a working definition of the sediment-hosted gold deposit class and its geological characteristics. The characteristics of each deposit included in this class are then documented and validated. Next, the geological processes most likely responsible for the shared characteristics are identified. Finally, the most reliable, informative and accessible criteria, reflective of each ore-forming process, are selected. From these criteria are chosen the most efficient and effective geological features for continued exploration.

Processes currently suggested to be important in the formation of sediment-hosted gold deposits include: regional wrench fault tectonics; plutonism; hydrothermal fluid flow driven by intrusions and/or thrusting; fluid flow along regional unconformities; gold precipitation following carbonate dissolution, or fluid mixing; and dissolution of synsedimentary gold from the allochthonous western assemblage sediments and reprecipitation of that gold in the deposits. A general approach for identifying additional possible processes, testing alternative processes and finally selecting exploration criteria is presented.
Mineralogy in the evaluation of ore deposits
Authors: Kingston G. A.
Published: April 2008

Mineralogical characterization of an ore deposit is an essential on-going activity in a continuum of evaluation exercises and decisions which operate from the very first recognition of mineralization to the cessation of mining and beyond. The main input of the mineralogist to ore evaluation is through the provision of a detailed qualitative and quantitative mineralogical representation of an ore that is specifically targeted to the needs of the process engineer in determining metallurgical recovery, especially during the exploration and pre-production stage. This process mineralogical input is often related to fine-tuning in mining and processing resulting from unexpected variations in the mineralogy of the ore to the mill. Mineralogical characteristics of platinum ore, refractory gold ores, and complex Cu-Zn-Pb ores are used to illustrate aspects of process mineralogical input at various stages in the life cycle of a mine. It is concluded that it is difficult for the mineralogist to become established as an essential and ongoing participant of exploration and mine evaluation exercises. It is thus important that both process engineers and geologists are given a better training in process mineralogy than is presently the case. Most mineralogical training and research in geology departments is exclusive of industrial applications.
Geophysical logs and the search for opencast coal reserves
Authors: MacCallum R.
Published: April 2008

The application of down-hole wireline logging techniques in the exploration for, and evaluation of, opencast coal reserves is described. The paper concentrates on the nuclear geophysical suite, in particular gamma and gamma-gamma logging which, to date, provide the most accurate geophysical means of identifying coal seams and estimating their thickness. The use of such logs in the identification of partings in coal seams, abandoned underground workings, sandstones, marine bands, ironstones and water levels is also described. The way in which such logs can be used in the correlation of individual coal seams is demonstrated, together with their use in the interpretation of geological structures in complex structural terrains. The advantages gained by the geophysical logging of boreholes far outweigh the increased costs incurred during exploration.
Sampling, grade control and ore reserve estimation at Renco gold mine, Zimbabwe: past, present and future
Authors: Armitage M. G. , Robinson I. E.
Published: April 2008

Modern gold mining activity at Renco Mine in southern Zimbabwe started in 1936. Since then a series of different companies have held the option to mine. The current owners, Rio Tinto Zimbabwe, took it over in 1973.

Accurate grade control and ore reserve estimation has in the past been hampered by the complex geology. However, improved geological understanding since 1986 has led to changes in the sampling techniques, which in turn have brought about improvements in both grade and tonnage factors.

For ore reserve evaluation, blocks of reef are defined by raises and reef drives. For each block the average grades and reef widths, calculated from the sampling of the peripheral developments, are used to estimate tonnages and grades.

Since 1987, a geostatistical software package has been used to generate kriged plans of gold grade and reef thickness. It is now planned to incorporate the results of more recent geological research into this software package in the hope of further improving the accuracy of the orebody evaluation and production data.
Computerized grade control in complex porphyry copper deposits
Authors: Franklin P. J.
Published: April 2008

Grade distribution within porphyry copper deposits can sometimes be erratic in nature. Conventional, manual grade control techniques for open pit mining have proved to be inadequate for accurately predicting mining head grades and for delineating ore and waste contacts.

This paper describes a highly interactive computerized grade control system that uses a combination of geostatistical modelling and graphical techniques to perform the daily grade control operations. The software is used to improve the reliability of grade predictions and to perform reconciliation of mined materials with planned materials.

The paper is illustrated with examples from operating copper open pits located in western Canada.
Reconciliation studies and reserve estimation
Authors: Sides Edmund J.
Published: April 2008

Reconciliation studies involve the comparison of different sets of data, and the analysis of differences between them in terms of accuracy and precision. Such studies are important during reserve estimation as a means of validating input data and modelling techniques used, as well as providing a means of checking the accuracy and precision of the resultant models. Examples of such studies from work done at the Neves-Corvo copper-tin mine in southern Portugal are given. It is argued that reconciliation studies should form an integral part of any reserve estimation, as a means of controlling the quality of the estimation procedures used.
Geostatistical ore reserve estimation: a case study in a disseminated nickel deposit
Authors: Dowd P. A.
Published: April 2008

This paper describes the geostatistical evaluation of a disseminated nickel orebody which contains remnants of high grade vein mineralization. The problems of dealing with high grade outliers are discussed together with the difficulties of modelling the semivariograms. Methods for the validation of the variogram models are presented prior to their use in a geometrically and geologically controlled kriging procedure to estimate recoverable blocked ore reserves.
The application of geostatistically controlled elliptical weighting techniques at Boulby potash mine, Cleveland
Authors: Annels A. E. , Ingram S.
Published: April 2008

Statistical studies of a portion of the Boulby potash seam revealed that distinct subareas are present in which stationarity could be shown to exist and which are characterized by statistically distinct populations of seam thickness and % KCl. Semivariogram modelling in these sub-areas allowed the definition of the axes of anisotropism in each case and hence the dimensions and orientation of the search ellipses for ore reserve purposes. Inverse distance weighting was then applied to evaluate individual ore blocks and the efficiency of the method tested by cross-validation of actual borehole values. Considering the problems induced by the sampling method applied (low angle deflection of in-seam drill holes), the results were generally satisfactory and form the basis for a more detailed comparative study of inverse distance and kriging techniques at Boulby.
Organic carbon removal in the sea: the continental connection
Authors: Ittekkot V. , Haake B. , Bartsch M. , Nair R. R. , Ramaswamy V.
Published: April 2008

Time series sediment traps have been deployed since May 1986 in the Arabian Sea and since October 1987 in the Bay of Bengal. The results of one and a half years from the western, central, and eastern Arabian Sea and of one year from the northern Bay of Bengal show that particle flux patterns are related to the strong monsoon winds and heavy rains. Particle flux maxima in the Arabian Sea are mainly related to wind-induced deeper mixing and nutrient enrichment of surface waters during the SW and NE monsoons. Extremely high particle fluxes with high biogenic opal contents during the SW monsoon at the western location show that it is reached by nutrient-rich water from the near-shore upwelling centres. In the northern Bay of Bengal, particle flux maxima coincide with the period of maximum discharge of the Ganges and Brahmaputra rivers during which river plumes enriched in nutrients are advected into offshore areas. Additionally, the winds and river plumes supply enormous amounts of mineral matter during the periods of high productivity and high particle fluxes. The interaction between marine-biogenic material with eolian and fluviatile mineral particles plays a key role in sedimentation. The incorporation of mineral matter into biologically formed aggregates ensures their rapid sedimentation and thus enhances the removal of biologically fixed atmospheric carbon dioxide to the deep sea.
Water depth and diagenetic constraints on the use of barium as a palaeoproductivity indicator
Authors: Von Breymann Marta T. , Emeis Kay-Christian , Suess Erwin
Published: April 2008

Sediments underneath the high productivity areas of the coastal upwelling and equatorial divergence zones in the Pacific and Indian oceans are enriched in barium, and on this basis the barium content of the sediments has long been suggested as a potential palaeoproductivity indicator. Analyses of sediments from the Peru margin corroborate the pattern of enhanced barium accumulation in areas of high primary productivity in the deep ocean, but also show that this pattern cannot be extended to shallow-water deposits. Sediments from the Peru shelf lack any barium enrichment, whereas this element is significantly enriched in slope and basinal deposits in water columns deeper than 2000 m. This depth effect is further illustrated at Site 682 located in the rapidly subsiding Lima Basin. The Quaternary sequence deposited at water depths >3000 m is enriched in organic carbon, opal and biogenic barium. The Miocene sediments, although deposited under highly productive waters associated with a coastal upwelling centre, do not show an enrichment in the barium record corresponding to the very high organic carbon and opal levels; a pattern that is consistent with deposition in shallow basins.

In addition to the effect of water depth on the barium distribution, overprinting of primary signals can be observed in sediments from upwelling areas undergoing strong anoxic diagenesis. Microcrystalline barite is partly dissolved in intervals depleted in interstitial sulphate as shown for sediments from the Peru margin. Dissolution of barite in sulphate depleted intervals of deep water sections leads to high barium concentrations near the termination of the sulphate reducing zone, where downward sulphate and upward barium diffusion foster local barite precipitation in diagenetic fronts.

The barium distribution in sedimentary oxic and suboxic environments at deep water depositional sites has a high potential as a palaeoproductivity indicator; however, barium accumulation as a proxy for ocean fertility should be used with caution in areas of varying water depths and in anoxic diagenetic environments where sulphate depletion undersaturates the interstitial waters with respect to barite.
The Benguela Current and associated upwelling on the southwest African Margin: a synthesis of the Neogene-Quaternary sedimentary record at DSDP sites 362 and 532
Authors: Diester-Haass Liselotte , Meyers Philip A. , Rothe Peter
Published: April 2008

Sediments cored at DSDP Sites 362 and 532 on the Walvis Ridge provide a Neogene-Quaternary history of the development of the upwelling system on the southwest African margin. Upwelling occurs principally on the landward side of the Benguela Current. The upwelling centres have shifted northward since the Middle Miocene as the current has intensified and has flowed farther to the north. Changes in productivity are recorded in the types, proportions and preservation of foraminifera, radiolaria, diatoms, organic matter, and clay minerals in the sedimentary record. Prior to the Late Miocene (10 Ma), the Benguela Current did not reach the Walvis Ridge, and enhanced productivity is not evident in the sediments at this location. Between 10 to 5.2 Ma, upwelling was recorded in the DSDP sites in glacial periods, indicating that the Benguela Current intensified during glacial periods and transported evidence of upwelling to Sites 362/532 from near-coastal areas. During interglacial periods the current was not as strong and did not reach the Walvis Ridge, turning instead to the west within the Cape Basin. Strengthening of the current continued such that by the Pliocene and Quarternary the upwelling signal is contained in interglacial sediments. Sediments deposited in these more recent glacial times contain a weak or absent upwelling signal because glacial intensification shifts the Benguela Current system northward to reach the Angola Basin before it turns westward away from its coast-parallel direction.
Techniques and applications of petrophysical correlation in submarine fan environments, early Tertiary sequence, North Sea
Authors: Hatton I. R. , Reeder M. , Newman M. St. J. , Roberts D.
Published: April 2008

Accurate stratigraphic delineations and correlations are key factors in exploration and development of early Tertiary reservoirs of the North Sea. In the submarine fan environment, application of biostratigraphic data can be complicated by stacking of sand sequences. A sand indicator is derived from a wireline log computation. The sand intervals are then edited out to generate a log profile of the background sedimentary sequence. Additional refinement is obtained by use of cross plot analysis to define petrophysical attributes of selected stratigraphic units. An example is described where such analysis has been applied to the Balder Formation where a ‘tuff profile’ has been established. Further examples illustrate how the methods serve to extrapolate detailed biostratigraphic data of wells into adjacent wells with limited or no data.
Evaluating thinly laminated reservoirs using logs with different vertical resolution
Authors: Ruhovets N. , Rau R. , Samuel M. , Smith H. , Smith M.
Published: April 2008

Reservoirs with thin laminations can be more accurately evaluated by using logging tools with inherently better vertical resolution, by employing enhanced vertical resolution input processing methods, and by incorporating interpretation models that properly handle log inputs with different vertical resolutions and reconstruct all outputs with high vertical resolution. The paper discusses a specific high-resolution interpretation model and provides comparative analyses of how model outputs are affected by the vertical resolution of its input logs and by the reservoir type. Three field examples are provided. Increases in predicted hydrocarbons were noted in two of these examples when the input log resolution was increased. In these two examples, the observed increases were confined to a number of isolated thin beds. In the third field example, significant decreases in predicted hydrocarbons were observed when high-resolution input data were used; the reservoir in this case appears to illustrate thin sand-shale laminae extended over a 27 metre interval.
Borehole breakouts and stress analysis in the Timor Sea
Authors: Hillis R. R. , Williams A. F.
Published: April 2008

Boreholes drilled in the search for oil in the Vulcan Sub-basin (Timor Sea, Northwest Shelf, Australia) commonly exhibit an elliptical cross section believed to be the result of wellbore failure known as borehole breakout. The azimuths of the long axes of breakouts identified in 13 wells in the Vulcan Sub-basin show a reasonably consistent 130–170°N trend implying that maximum horizontal compressive stress (
SH
max
) is oriented 040–080°N.

This NE-ENE
SH
max
orientation in the Vulcan Sub-basin does not coincide with the direction of absolute plate velocity, nor is it consistent with compression transmitted from the nearby Australia/Banda Arc collision zone. However, it is in agreement with theoretical models of stress distribution based on the plate-driving forces throughout the Indo-Australian plate.

The breakout analyses, in conjunction with structural interpretations, suggest that the instabilities observed in vertical wells will be exacerbated by orienting horizontal wells 040–080°N, and that drilling in the 130–170°N direction may enhance both stability and production rates.
An integrated interpretation of fracture apertures computed from electrical borehole scans and reflected Stoneley waves
Authors: Hornby Brian E. , Luthi Stefan M.
Published: April 2008

Fractures crossing the borehole are probed using electrical currents generated by Formation MicroScanner* pads and Stoneley waves generated by an acoustic source. New techniques have been developed to invert these measurements for estimates of equivalent fracture aperture, a crucial parameter controlling the fluid flow in fractured reservoirs. The objective of this paper is twofold: first, to compare aperture estimates in a variety of rock types; and second to exploit the different physical measurements in order to improve our understanding of the fracture system. Single fractures in controlled borehole experiments are used to cross-validate the two techniques, and good comparisons were found in the aperature range from 10 μm to several millimetres. Core data, acoustic borehole televiewer images, and in one case results using the Dual Laterolog* measurement serve as supporting evidence. Three case studies are presented: one from crystalline rocks, another from a chalk and shale sequence, and a third one from a fractured dolomite reservoir. In these case studies we found that a combined analysis of the two methods yields valuable additional information concerning fracture extent, fracture connectivity and borehole effects such as fracture enlargement and borehole rugosity.
An introduction to the crystalline basement of Africa
Authors: Key R. M.
Published: April 2008

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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Sediment geometries and domino faulting
Authors: Waltham David , Hardy Stuart , Abousetta Abdulnaser
Published: April 2008

A general forward modelling scheme is used to produce a model of sediment geometry in half-grabens formed during extension of domino fault blocks. The model includes the effects of erosion, sediment transport, sedimentation and isostasy and simulates both syn- and post-rift sedimentary sequences. The geometries and relationships produced by the model are broadly similar to natural examples seen on North Sea seismic sections. This similarity is emphasized by the generation of synthetic seismic data using the numerically modelled geometry as a template. The unconformable nature of the boundary between syn- and post-rift seismic sequences is clearly visible on the synthetic data in the region of the fault block crest.