bluuebunny/crossref_metadata_2025 huggingface dataset:
Foraminiferal production and monsoonal upwelling in the Arabian Sea: evidence from sediment traps
Authors: Curry W. B. , Ostermann D. R. , Guptha M. V. S. , Ittekkot V.
Published: April 2008

Planktonic foraminifera collected in sediment traps in the Arabian Sea during 1986 and 1987 responded to the southern Asian monsoon with changes in productivity, relative abundance of species and isotopic shell chemistry. Most species of foraminifera increased in flux shortly after the advent of the southwest monsoon.
G. bulloides
increased its production rate by three orders of magnitude. The isotopic chemistry of
G. ruber
recorded the increase in monsoon upwelling by increasing its δ
18
O values by about 1‰, accurately reflecting the average 4°C sea surface temperature decrease associated with the upwelling. The mean value of δ
18
O for
G. ruber
was greater in the western Arabian Sea than in the central or eastern basins because upwelling in that region cools surface water. The carbon isotopic composition of
G. ruber
does not have a clear temporal or geographical relationship to upwelling. While its δ
13
C values decreased in the western Arabian Sea during the upwelling event, the mean δ
13
C values remained higher in the western than in the eastern and central Arabian Sea. This longitudinal gradient is opposite to that expected from the geographical gradient of upwelling: the region with the most intense upwelling should have lower δ
13
C values in surface waters because of the upwelling of low-δ
13
C water to the surface.
Organic matter variations in sediments from DSDP sites 362 and 532: evidence of changes in the Benguela Current upwelling system
Authors: Meyers Philip A.
Published: April 2008

Diatom-rich nannofossil oozes from DSDP Sites 362 and 532 on the Walvis Ridge continental terrace contain light-dark cycles which are related to fluctuations in upwelling intensity over these sites from Middle Miocene to Holocene times. The dark intervals in the alternations result from dissolution of carbonates and addition of clastic components. Oxidation of marine-derived organic matter is important to dissolution of carbonates in these sediments. Concentrations of organic matter in the dark sections are enhanced by larger contributions of both marine and land-derived material than in light sections. The dark sections appear to correlate with periods of greater Antarctic ice volumes, suggesting that regional upwelling was intensified and that transport of organic matter from Africa by winds and possibly rivers was increased during these times.
Sedimentological borehole image analysis in clastic rocks: a systematic approach to interpretation
Authors: Bourke Lawrence T.
Published: April 2008

Detailed study over the four years since the introduction of the Formation MicroScanner Tool* (FMS) has resulted in the evolution of a systematic interpretation approach to the extraction of a formal sedimentological description of extensive reservoir intervals using borehole images in clastic sediments. This approach takes account of thorough log quality control and the recognition of non-geological artifacts and involves the integration of the available open hole logs to establish lithology, along with the incorporation of core or sidewall cores as ‘hard’ lithological and textural data to ensure interpretation confidence.

The approach is practical and straightforward. However, a workstation provides significant advantages to the skilled interpreter compared with a paper-based analysis. After quality control and recognition of artifact images, five interpretation steps are suggested. (i) Fine-calibrate open hole logs (conventional lithology indicators) from sidewall and conventional core samples. Enhance lithology recognition using dipmeter resistivity curves. (ii) Annotate the above lithological information onto FMS images. (iii) Constrain feature recognition from images using local geological knowledge. (iv) Pick and classify feature dips, then check dip category validity by generating dip and azimuth histograms. Reclassify as necessary. (v) Extract relevant sedimentary and lithological data from images to construct a conventional sedimentological facies description.

One of the key aspects of borehole image analysis is the development of an interpreter’s skill in recognizing geological features from microresistivity images. Finally, aspects of core reconciliation such as depth matching are considered.
Application of FMS images in poorly recovered coring intervals: examples from ODP Leg 129
Authors: Salimullah A. R. M. , Stow D. A. V.
Published: April 2008

A thick sedimentary section was penetrated at three sites in the West Central Pacific during the Ocean Driling Program (ODP) Leg 129. Although average core recovery ranged from 17.3 to 29.5%, recovery in some intervals falls as low as 0–5%. Furthermore, the recovered portion of the core is conventionally assigned to the topmost part of the cored interval, although this is now known to be incorrect in most instances in consolidated parts of the section. These drawbacks have led to incomplete and even erroneous sedimentological interpretations. High-resolution electrical (Formation MicroScanner) images of the borehole obtained by scanning the borehole wall with arrays of small electrodes pressed against the borehole surface can provide:

(1)

detailed sedimentary structure and texture of missing sections of each core, through calibration with visual data obtained from recovered cores;

(2)

the original sedimentary features, where these have been disturbed or brecciated during drilling, as well as drilling artifacts on the borehole wall;

(3)

recognition of sedimentary facies (e.g. slump units in hole 801B) in non-recovered intervals;

(4)

correct location of recovered portions within the cored interval, through matching of specific sedimentary features.

Examples of these applications of the FMS images to the poorly recovered ODP Leg 129 cores are presented. The use of FMS logging has widespread application in any borehole where conventional cores have not been taken or where core recovery is limited. However, log calibration with core samples is essential.
Wireline log signatures of resedimented volcaniclastic facies, ODP Leg 129, West Central Pacific
Authors: Salimullah A. R. M. , Stow D. A. V.
Published: April 2008

During Ocean Drilling Program Leg 129, two sites (800, 801) were drilled in Pigafetta Basin and one site (802) in East Mariana Basin, West Central Pacific. At all three sites, a thick (192–211 m) Cretaceous succession of dominantly volcaniclastic sediments was encountered, and at site 802 a further 222 m of Miocene-Pliocene volcaniclastics were drilled. These sediments are composed mainly of volcaniclastic material that has been resedimented downslope by various mass-flow processes including slumps, debris flows and turbidity currents. Sedimentary structures and textural variations observed in the recovered section can be used to calibrate the corresponding high-resolution Formation Micro-Scanner (FMS*) images and Dipmeter Micro-resistivity readings. Gamma ray log shapes show grain size evolution within a facies. Fining-upwards and coarsening-upwards grain size trends are observed for some sequences of turbidites and debrites. No obvious change in grain size is observed through massive structureless sandstones. CCA (calcium yield in decimal fraction) and CSI (silicon yield in decimal fraction) readings from the geochemical combination logs, correlated with visual compositional data, are useful to define calcareous and siliceous turbidite facies. This combination of wireline logs has been used in the poorly recovered intervals of volcaniclastic section in the ODP Leg 129 sites to improve significantly our understanding of basinal sedimentation and tectonics in the area.
High-frequency pseudo-Rayleigh waves as a new indicator of shear velocity
Authors: Raaen Arne Marius
Published: April 2008

In slow formations, where the shear velocity is lower than the mud compressional velocity, several indirect methods have been used to estimate shear velocity from full waveform acoustic logs. These methods include Stoneley wave velocity and inversion of refracted P-wave amplitude. In this paper we show that in addition to the methods previously reported, the amplitude of the leaky pseudo-Rayleigh waves may in some cases be strongly dependent on the shear wave velocity of the formation. Numerical analysis has shown that there is also some dependence on the velocity and attenuation of the mud, and the borehole radius, whereas the amplitude of the leaky pseudo-Rayleigh waves is quite insensitive to the compressional velocity of the formation and the intrinsic anelasticity of the formation. The predictions have been confirmed by model experiments in a slow formation. It is felt that the effect may be a useful tool, for example, in the study of stress dependent velocities around a borehole.
Downhole mineralogy logs: mineral inversion methods and the problem of compositional colinearity
Authors: Harvey P. K. , Lovell M. A.
Published: April 2008

The inversion of chemical data derived from geochemical and related nuclear spectroscopy tools to provide mineralogy logs is now a widespread approach to the interpretation of these data. The most valuable of such transforms attempt to provide the percentages of actual phases present (minerals and fluids) at each depth interval rather than the ideal minerals occurring in simple theoretical models. Of the numerous problems involved in the inversion for a particular phase assemblage, the most serious is probably that of compositional colinearity in which three or more of the phases sought lie on, or close to, the same compositional plane. Depending upon the algorithm used for the inversion the effects of such compositional constraints may vary between a failure to find any (numerical) solution, failure to find a unique solution, or a solution which may be significantly in error for only a very small deviation in the chemical log or phase compositions. These effects are illustrated using geochemical logs from sedimentary environments together with examples from laboratory derived and numerically simulated geochemical data.
The third age of wireline log analysis: application to reservoir diagenesis
Authors: Selley R. C.
Published: April 2008

Geophysical well logs were first developed to identify the porosity and fluid composition of potential petroleum reservoirs. Subsequently their use was extended to include diagnosing the depositional environment of reservoirs, as an aid to predicting their geometry. Both these techniques are now routine.

A new age of the application of geophysical logs now dawns due to the enhanced understanding of the diagenetic processes that affect petroleum reservoirs, including cementation which destroys porosity and solution which enhances it. These ideas can be used to interpret both conventional porosity logs and the new types of log that, when carefully interpreted, allow vertical variations in mineralogy to be measured.

Three main diagenetic effects can be recognized in geophysical well logs: facies-related diagenetic phenomena; cemented and leached zones related to migrating diagenetic fronts of acid meteoric or deep connate fluids; and diagenetic effects related to petroleum migration and entrapment.
Groundwater movement and water chemistry associated with weathering profiles of the African surface in parts of Malawi
Authors: McFarlane M. J.
Published: April 2008

This paper summarizes the main results of recent geomorphological research on the hydrogeology of weathering profiles on the African erosion surface in Malawi. Deep regolithic profiles have developed by protracted and aggressive weathering and differential leaching. In the advanced stages of weathering, congruent kaolinite ‘dissolution’ causes the saprolite to collapse, forming a thick residuum of the most resistant materials, dominated by silica in the form of quartz and iron as goethite. Aluminium has been extensively leached. Such profiles pertain to a mechanism of land surface formation dominated by the activity of infiltrating water rather than direct surface runoff and it results in terrain with a generally basined configuration. Low-lying areas are occupied by dambos. These are clay-filled bottomlands which are seasonally waterlogged and are located where lithology favours leaching and saprolite collapse. The contemporary fluviatile-like configuration of the dambos is attributed to post-incision modification of the ancient land surface. As a result, their area is reduced and they become inset into it, following geological controls which express integrated groundwater movement within the saprolite. The infill is shown to be essentially an alumino-silicate evaporite rather than of alluvial origin.

Analysis of dambo and dambo-peripheral profiles supports this genetic model, identifying the continuity of the contemporary dambo infill with a ‘palaeodambo’ clay wedge below the surficial sands in peripheral situations. The very low permeability of the clay is held to be responsible for the marginal seepage zone, fed by shallow throughflow from topographically higher profiles and augmented by upward discharge of deep water.

Elements lost from the interfluves were identified in evaporites which occur on the dambo floors in the dry season. These include aluminium. Analysis of water discharging into the dambos failed to identify this element. There is a need for reassessment of technique to determines element mobilization in tropical groundwater, where organic binding is implicated.
The hydrogeology of crystalline aquifers in northern Nigeria and geophysical techniques used in their exploration
Authors: Hazell J. R. T. , Cratchley C. R. , Jones C. R. C.
Published: April 2008

The large number of rural water supply boreholes sited and drilled in northern Nigeria during the ground water decade of the eighties has provided much data on the hydrogeology of the Basement regolith and has led to the development of new techniques in locating aquifers. Regolith aquifer characteristics and their dependence on geology and environment are reviewed with particular emphasis on lithology, weathering pattern and fracturing in Bauchi, Kano and Sokoto States. Specific capacity is shown to be related to both lithology and grade of weathering; dry season water levels and frequency of occurrence of successful holes are related to lithology. Water quality data for Kano State show some high nitrate concentrations.

The process of borehole siting begins with desk study (records and photointerpretation), then field reconnaissance of lithology, structure, water points, topography and soils. EM traverses and resistivity soundings follow in targeted areas. Maximum regolith thicknesses are estimated from the vertical to horizontal coil (EM) response using computed model graphs for 40 m and 20 m coil spacings. VES measurements at these sites give regolith resistivities, hence some estimate of its suitability as an aquifer. In addition EM traverses have located steeply-dipping fracture zones, dykes and pegmatites and interpretation of these in terms of strike and dip has been aided by results from a computer-controlled modelling system.
Borehole siting in an African accelerated drought relief project
Authors: Barker R. D. , White C. C. , Houston J. F. T.
Published: April 2008

In response to recent droughts in central Africa, an accelerated well drilling programme was commissioned in Victoria Province, Zimbabwe. Geophysical techniques involved in the siting of 370 boreholes included electromagnetic profiling and resistivity sounding, the latter using the offset sounding system. An analysis of the field data showed that although observed errors were low, a significant proportion of the soundings were affected by strong lateral changes in resistivity, this probably indicating the presence of a very irregular bedrock surface. Nevertheless, mean regolith thickness and lithology were reliably estimated.

Geomorphological features which were found to have the greatest potential for water supply boreholes include valleys and zones of weathering around bornhardts. In these situations geophysical surveys proved useful in locating areas of thickest regolith. Borehole yield could then be estimated from the measured formation resistivity although a precise relationship was somewhat masked by the clay mineralogy of the regolith and the type of bedrock which must be taken into account when siting boreholes.
Rural water supplies: comparative case histories from Nigeria and Zimbabwe
Authors: Houston John
Published: April 2008

Two rural water supply projects based on groundwater sources are described and compared. One in Nigeria aimed to provide yields of greater than 1 l/s using electric submersible pumps to villages and towns with populations from 2000 to 30 000; the other in Zimbabwe aimed to supply yields greater than 0.2 l/s using hand pumps to small communities and villages from 100 to 2200 people. The higher yields in Nigeria are examined in relation to geology and aquifer characteristics, aquifer recharge, siting methods and borehole design. It is found that the higher yields are partly due to the criterion used to define a ‘successful’ borehole and partly due to other factors. Geology and aquifer characteristics appear to be similar in both places but recharge is much higher in Kwara State, leading to higher water levels and higher potential yields. A greater use of geophysics in Kwara State leads to a higher success rate and greater drilled depths into the bedrock in Kwara State also contribute to higher yields.
Detection and prediction of Lower Cretaceous sandstone distribution in the Scapa Field, North Sea
Authors: Harker Stuart D. , Chermak Andrew
Published: April 2008

The Scapa Field is a combination structural/stratigraphic trap located in UK North Sea Block 14/19, 112 miles northeast of Aberdeen. The productive intervals consist of turbidite sandstones in the lower part of the early Cretaceous Valhall Formation. Three discrete sandstone bodies have been identified during the development of the Scapa Field, ranging in age from early Valanginian to late Hauterivian. The sandstones are interbedded with marls, limestones and conglomerates that filled the early Cretaceous northwest-southeast trending syncline between the Halibut Shelf and Claymore tilt block, on the southwest flank of the Witch Ground Graben. The combination of lithofacies, wireline log correlation, palaeontological, reservoir production and pressure studies led to the recognition of a five-fold subdivision (‘SA’ to ‘SF’) of the Scapa Sandstone Member. Detailed seismic studies, in particular amplitude extraction mapping using the 3D data set, have enhanced the interpretation of the distribution of the sandstone bodies. Tight conglomerate facies dominate the Lower Valhall sequence to the south in a narrow fringe bordering the Halibut Shelf. Reservoir sandstones interfinger with these conglomerates and thin to the north, changing facies to sandy marls onto the Claymore tilt block. Within the main Scapa syncline, sandstone distribution was controlled by clastic input associated with subtle structural control. The gravity-driven turbidite system concentrated deposition in the bathymetric lows. Lime muds were deposited in a constant hemipelagic rain, both over the intervening highs and interbedded with the sands.
Quantitative analysis of pore structure and its effect on reservoir behaviour: Upper Jurassic Ribble Member sandstones, Fulmar Field, UK North Sea
Authors: Clelland W. D. , Kantorowicz J. D. , Fens T. W.
Published: April 2008

Image analysis has been used to quantify the pore structure of dolomite-cemented sandstones in the Ribble Member of the Fulmar Field, UK North Sea. This technique was used because the data obtained from conventional petrography did not enable quantification of the controls on permeability. Three-quarters of the sandstones in the Ribble Member have high measured porosity and permeability (ø17–29%,
K
h
40–1800mD). In these poorly cemented sand-stones, intergranular porosity predominates and permeability reflects (depositional) sediment texture. One quarter of the sandstones contain > 20% dolomite. The dolomite (mean δ
13
C PDB +0.8‰, mean δ
18
O PDB −2.3‰) formed from sea water trapped and buried within the sediments and occurs as patches or nodules that should not form reservoir scale barriers to fluid flow. These cemented sandstones are less permeable (ø15–25%,
K
h
0.4–600mD). They contain grain-dissolution pores created by the dissolution of sponge spicules and intercrystalline pores between the dolomite rhombs.

Backscattered electron imaging and automated image analysis have enabled quantification of the differences in pore structure between the poorly cemented and cemented sandstones. Permeability is not only related to the amount of porosity present, but also to the amount of dolomite cementation. The degree of cementation influences average pore area and pore connectivity, which in turn controls permeability. Permeability is lower in the cemented sandstones because the dolomite rhombs have occluded the intergranular pores, creating restricted flow paths through small intercrystalline pores. As a result of these differences in pore structure, hydrocarbon production from the cemented sandstones will be slower than from the poorly cemented sandstones.

The permeability of the cemented sandstones cannot be predicted from porosity logs alone but also requires knowledge of the amount of dolomite cement present. In uncored wells this information should be obtainable using modern wireline log evaluation techniques. This study demonstrates the importance of combining a detailed understanding of the rock’s pore structure with a sound geological model in order to predict reservoir behaviour.
Sedimentological and reservoir characteristics of a fluvial-dominated delta-front sequence: Ferron Sandstone Member (Turonian), East-central Utah, USA
Authors: Lowry Philip , Jacobsen Torgrim
Published: April 2008

Exposures of the Ferron Sandstone Member (Turonian) of the Mancos Shale Formation in east-central Utah offer an opportunity to examine the dimensions of delta-front reservoir elements and their associated heterogeneities. Field mapping within the Ferron Sandstone has demonstrated that distributary mouth-bar facies have maximum lengths of 8–9 km and maximum widths of 4–6 km. Distributary channel width-to-thickness ratios commonly have values between 10:1 and 20:1. Distal bar sandstone thicknesses and shale lengths are dependent upon their location within the mouth-bar. Distal bar shales are shortest within the proximal region of the mouth-bar, where amalgamation of distal bar sandstones is greatest.

850 field minipermeameter measurements were obtained from the various facies elements of a distributary mouth-bar sandbody. The data display significant correlation between facies elements and permeability structure. Trough cross-bedded upper mouth-bar facies exhibit the highest means and lowest coefficient of variation. Muddy and bioturbated distal bar facies exhibit the lowest means and highest coefficient of variation. The permeability distributions appear to be related to the degree of heterogeneity, and show increasing left-skewness with increasing heterogeneity.

Fluid flow simulations of two geological models of a distributary mouth-bar were conducted to assess the effect of permeability contrasts on predicted reservoir behaviour. One of the models depicts a lithologically homogeneous mouth-bar, while the other depicts a mouth-bar that is composed of two internal facies elements (lower and upper mouth-bar). Results of the simulations show that up to 30–45% difference in predicted water breakthrough can occur between the two models.
The late Cenozoic
Globorotalia truncatulinoides
datum-plane in the Atlantic, Pacific and Indian oceans
Authors: Jenkins D. Graham , Gamson Paul
Published: April 2008

Since 1967 it has been thought that the planktonic foraminiferal taxon
Globorotalia truncatulinoides
(d’Orbigny) evolved in the oceans at about 1.9 Ma, with an anomalous date of 2.5 Ma for its first appearance in the Southwest Pacific. A new model is presented which demonstrates that
G. truncatulinoides
evolved in the Southwest Pacific at 2.5 Ma and later spread, while still evolving, into the South Atlantic via Drake Passage and then into the North Atlantic arriving there at 1.9 Ma; it also migrated and arrived in the Central and North Pacific at this time. The diachronous nature of the first appearances of both
G. truncatulinoides
, and
Globorotalia inflata
(d’Orbigny) which also evolved in the Southwest Pacific and later spread to other oceans, has implications for first appearances of species in the Cenozoic that have been labelled ‘datum-planes’.
Geochemical correlation of marl bands in Turonian chalks of the Anglo-Paris Basin
Authors: Wray David S. , Gale A. S.
Published: April 2008

Geochemical analysis of a series of laterally correlative Middle and Upper Turonian marls collected from exposures in Kent, Sussex and on the French coast reveal that it is possible to chemically fingerprint individual marls. The fingerprint is derived from four elements (scandium, titanium, vanadium and yttrium) which are believed to be located within detrital clay minerals. The true potential of this method is demonstrated by applying it to a series of Turonian marls within and below the condensed succession overlying the London Platform and western edge of the Wessex Basin (the Chalk Rock and Spurious Chalk Rock). Previously, correlation of these marls with those in the main part of the Basin was difficult and tentative. Using the geochemical fingerprinting method it is possible to make accurate marl band correlations which demonstrate that, on the London Platform, much of the Middle and Upper Turonian is condensed into a series of closely spaced hardgrounds.
Gain function analysis of sand transport in a turbulent air flow
Authors: Hardisty Jack , Rouse Helen L. , Hart Samantha
Published: April 2008

Recent work on the modelling of sand transport processes in the wind is concentrating on the effects of turbulence and, in particular, on flow accelerations and particle inertia. An approach is developed here in which the problem is cast in the frequency domain using the particle momentum equation which balances grain inertia against resultant steady and acceleration related fluid forces. A transport gain function is defined which relates the normalized transport spectrum to the normalized, turbulent flow spectrum. Experiments are reported in which the gain function was determined using a hot wire anemometer and a new pin impact probe to make high-frequency measurements of the velocity and sand transport profiles within the saltation layer in a small laboratory wind tunnel. The results were subjected to frequency analysis and the corresponding wind and transport spectra were obtained. Preliminary calculations suggest that the transport gain function is reasonably flat over the range up to 0.4 Hz and, therefore, models based upon gain function analysis may more properly account for sand transport within a turbulent boundary layer.
Desert dune dynamics and the evaluation of shear velocity: an integrated approach
Authors: Wiggs Giles F. S.
Published: April 2008

Few empirical measurements have been undertaken of shear velocity, sand transport or dune morphology in the upwind basal regions of dunes. This study compares field measurements in this region from a barchan dune in Oman with calculations from a complex flow model using linearised equations (FLOWSTAR) and measurements in a wind tunnel.

The calculations of shear velocity from wind velocity observations in the field and those predicted by the FLOWSTAR model reflect the observations of previous studies in exhibiting the widely reported upwind reduction in shear velocity. Despite these observations, measurements of sand transport did not reduce at the base of the dune. The implications of these findings for the predictive power of typical saltation flux equations and the dynamics of the dune itself are discussed.

Wind tunnel modelling using a pulse wire probe suggests that the conventional methods of shear velocity derivation are inadequate. The pulse wire measurements exhibited no reduction in shear at the toe. An argument is presented to explain the maintenance of shear velocity in this position. It is suggested that the reduction in shear stress due to the fall in wind velocity is counteracted by an increase in shear stress due to streamline curvature. The adequacy of field methods for determining shear velocity in this respect is considered.
Aeolian processes and deposits in northwest Ireland
Authors: Carter R. W. G. , Wilson Peter
Published: April 2008

The exposed Atlantic coast of northwest Ireland supports numerous dunefields derived largely from glacigenic deposits in the area of the present continental shelf. The present-day dunes began to form about 5000 to 6000 years
bp
at, or just after, the mid-Holocene sea-level peak which reached a maximum of + 3 m OD on parts of the County Antrim coast. As the sea-level fell so sediment was transferred onshore accumulating as beach ridges and dunes. Some dunes are perched on gravel ridges of marine origin that were exposed long enough to aeolian processes to allow the formation of ventifacts. Since about 4000 years
bp
the primary sediment budget has become strongly negative, allowing morphological reworking of many dune systems. The processes of erosion may well have been triggered by climatic changes or human impacts or a mixture of both. Periods of dune instability are marked by the widespread engulfment of former soil surfaces by large-scale mobile bedforms, the transport of sand inland and the formation of extensive sand sheets. At Maghera, County Donegal and Downhill, County Londonderry there are ramp dunes against former sea cliffs, and between Portstewart, County Londonderry and Portrush, County Antrim extensive cliff top dunes formed in the late Holocene which are now isolated from their original sediment source. Non-coastal aeolian sands are of limited extent in northwest Ireland, but some local examples are described.
Use of cement types in the palaeoenvironmental interpretation of coastal aeolianite sequences
Authors: McLaren Susan J.
Published: April 2008

The interpretation of palaeodune sequences is an important component in palaeoenvironmental analyses. Studies of the diagenetic alteration of palaeodunes is one of the main ways of determining post-depositional environmental conditions. This paper questions the long-held and generally accepted ideas that cement types/fabrics are a ‘major method of recognising the original mineralogy of the cements and by inference their origin under marine, vadose or phreatic conditions’ (Arthur
et al.
1982). A detailed study of last interglacial aeolianites from Mallorca, Tunisia and the Bahamas has shown that although many cement types may be characteristic of the meteoric vadose zone, none are unique. No single cement type is diagnostic of the vadose zone.
Occurrence and palaeoenvironmental implications of the Late Pleistocene loess along the eastern coasts of the Bohai Sea, China
Authors: Li Pei-Ying , Zhou Li-Ping
Published: April 2008

Loess is found to blanket various landforms of different altitudes in the eastern coastal areas of the Bohai Sea, from Liaodong Peninsula to the Miaodao Islands and Jiaodong Peninsula in eastern China. Two major lithological units have been recognized: the Penglai Loess and the Dalian Loess. Recent studies reveal a weathered unit within the Dalian Loess and confirm the occurrence of Holocene loess. Random distribution of marine microfossils and concomitant occurrence of contrasting species in the loess rule out the possibility of the coastal loess being a marine sediment. The coastal loess is coarser and contains a higher content of unstable minerals compared to the inland equivalents. Regional variations in texture and heavy mineral assemblage within the coastal loess zone are also observed, suggesting that the primary source of the coastal loess is not northwestern China but is more likely to be from the coastal areas and the adjacent Bohai Sea basin. When the coastline was displaced a few hundred kilometres eastwards during the last glacial period, vast areas of the continental shelf of the Bohai Sea were exposed. Coastal dunefields may have been expanded and perhaps developed locally into a desert shelf environment. With intensified aeolian process, fine particles in the ancient fluvial and marine sediments were transported by small-scale low-level northwesterly and northerly winds and deposited along the eastern coasts of the Bohai Sea. The basal age of the Late Pleistocene Dalian Loess was previously estimated to be 11–25 ka, but preliminary thermoluminescence dating results suggest that it started to accumulate at least 30 ka earlier.
Palaeosols and sandbody prediction in alluvial sequences
Authors: Kraus Mary J. , Bown Thomas M.
Published: April 2008

Diverse palaeosols in the Eocene Willwood Formation illustrate how overbank deposits can help predict the location of fluvial sandbodies. Palaeosols of differing maturity formed in response to local fluvial processes and tectonically controlled sediment accumulation rates. The various palaeosols can be distinguished using criteria including colour, pedogenic nodules, and grain size trends, all of which should be observable in core.

Palaeosol maturity increases laterally with increasing distance from a coeval channel sandbody. Consequently, a specific type of Willwood palaeosol provides an estimate of the relative distance to a channel sandbody at a specific stratigraphic level. Vertical sequences of overbank deposits also show changes in palaeosol type as a result of channel avulsion. The vertical changes can thus help predict the position of channel sandbodies within a fluvial package. On a basin-wide scale, the rate of subsidence influenced both palaeosol maturity and the location of channel sandbodies. Areas of the basin that subsided rapidly (thus sediment accumulated rapidly) are commonly rich in channel sandbodies and also contain relatively immature palaeosols. In contrast, areas that subsided more slowly and in which channel sandbodies are not concentrated are characterized by relatively mature palaeosols.
Contrasting styles of fluvial-aeolian interaction at a downwind erg margin: Jurassic Kayenta-Navajo transition, northeastern Arizona, USA
Authors: Herries Robert D.
Published: April 2008

Fluvial-aeolian interactions result from both short-term interplay between an ergmargin and adjacent fluvial system, and much longer-term expansion and contraction of the erg in response to climatic, tectonic or eustatic effects.

Near Moenkopi, in northeastern Arizona, a zone of intertonguing characterizes the transition between the fluvial Kayenta Formation and the overlying aeolian Navajo Sandstone. Three fluvial to aeolian drying-up cycles form the largest scale interaction and represent the repeated advance of the Navajo erg across the Kayenta alluvial plain, probably in response to an increasingly arid climate. During the aeolian phase of each cycle, small-scale interactions resulted from fore-erg deposition on the alluvial plain, and fluviallyfed interdunal flooding, which both occurred during periods of erg activity.

The top of each cycle marked a return to wetter climatic conditions which terminated erg activity, forming a stabilization surface. As a consequence, prograding fluvial systems were able to transgress the denuded surface, covering the area formerly occupied by the erg to form the base of the next drying-up cycle.

From a hydrocarbon reservoir viewpoint, the two types of interaction have contrasting geometries. Fluvial deposits of the large-scale cycles have regionally extensive sheet-like geometries and are likely to form widespread barriers to vertical flow. Small-scale interdunal flooding is less significant and forms ribbon-like baffles to flow which extend roughly parallel to the strike of the aeolian cross-strata. The key to differentiating each type of interaction lies in determining whether the fluvial syste coexisted with the
active
erg, or prograded as a result of erg
stabilization
.
Palaeomagnetic study and magnetostratigraphy of the Triassic Skagerrak Formation, Crawford Field, UK North Sea
Authors: Rey D. , Turner P. , Yaliz A.
Published: April 2008

A detailed palaeomagnetic study has been made of the Skagerrak Formation (Triassic) in two wells from the Crawford Field in the northern North Sea. In 9/28a-9 (unorientated) a well-defined magnetostratigraphy is present comprising up to four reversed and three normal zones. There are secondary magnetizations (Tertiary 8 Recent in age) particularly in areas which are oil stained. In 9/28a-13 (orientated), three normal and four reverse magnetozones can be isolated. Despite the existence of orientation data it is more difficult to establish a reliable Triassic magnetostratigraphy. This is partly due to the fact that the sandier sequence in this location is impregnated with oil and associated diagenetic effects have resulted in pervasive magnetic overprints which are difficult to separate from the original Triassic magnetization. The effects of oil emplacement included widespread dissolution and the precipitation of a suite of secondary iron minerals including pyrite and possible pyrrhotite or magnetite. These later phases carry a secondary component of magnetization which overprints the original Triassic characteristic remanence. Directions of secondary magnetization are consistent with oil emplacement in Tertiary times.

In attempting to correlate the two wells a number of factors have to be taken into account, in particular variations in sedimentation rate and the lack of precision in the published Triassic polarity time scale. The results of this study strongly suggest that the cored sequences in the two wells are not time equivalent. However, the sequence in 9/28a-9 compares most closely with the Upper Triassic polarity time scale.
The Sapat mafic-ultramafic complex, Kohistan arc, North Pakistan
Authors: Jan M. Qasim , Khan M. Asif , Qazi M. Sufyan
Published: April 2008

The Sapat layered complex, occupying the tectonic base of the Cretaceous Kohistan island arc, occurs in the immediate hanging wall of the Indus suture zone north of the Kaghan valley. Previously undescribed, much of the complex consists of gabbroic rocks, but in the basal part there are ultramafic cumulates (15 × < 2 km) in direct tectonic contact with the Indian plate. In the central part, the complex can be divided into five zones. From the base these are: (1) homogeneous serpentinized dunites (350 m thick), (2) dunites with thin chromite layers (100 m), (3) thinly layered dunites-pyroxenites (30 m), (4) layered dunites-pyroxenites-gabbros (170 m) with chromite layers in the lower part, and (5) gabbros. The gabbroic rocks are amphibolitized and consist of a variety of lithologies ranging from troctolites to anorthosites, with local pyroxenite (± olivine) layers and horizons of olivine-rich ultramafics. These rocks are locally cut by pyroxene-rich pegmatites containing up to 45 cm long pyroxene crystals. Preliminary microprobe studies on the ultramafic rocks show that the clinopyroxene is Mg-Ca-rich diopside, and olivine has a forsterite content of 92 to 78, but in rare cases > 95. The chromite displays high Cr No. (75–67) and low Fe
3+
/R
3+
(<10). These data, especially the chromite analyses, are akin to those of ultramafic rocks of island arcs, including the Jijal complex, 80 km to the west. We propose that the Sapat complex formed the basal part of the Kohistan terrain and is related to arc magmatism. It has been metamorphosed under epidote amphibolite facies conditions.
Compression- and transpression-related deformation in the Kohat Plateau, NW Pakistan
Authors: Pivnik David A. , Sercombe William J.
Published: April 2008

Detailed surface mapping and recently acquired subsurface data from the Kohat Plateau of northwestern Pakistan show complex structural styles created by the overprinting by transpression-related structures of pre-existing compression-related structures. The Kohat Plateau is roughly divided into two structurally distinct regions; a northwest region (NW) and a southeast region (SE). Surface structures in the Plateau include doublyoverturned folds (both limbs overturned) which incorporate a lower Eocene to Pleistocene stratigraphic succession. Intensely folded with the stratigraphy in the NW region is a relict thrust belt, representing an earlier compressional episode of deformation. In the SE region many folds (related to wrench faulting at depth) have had either their southern or northern limbs faulted out by high-angle reverse faults which flatten near the surface along a Lower Eocene shale and evaporite horizon. In the NW region, lower Eocene evaporites and limestones are replaced by thick shales which served as a fault-flat forming horizon. This is believed to have hindered the surfaceward propagation of faults, leaving detachment folds exposed at the surface. Steeply dipping strata at depths of over 4 km, steeply dipping faults that become shallow near the surface forming positive flower structures, large vertical displacements across faults, and left-stepping folds suggest transpressional rather than purely compressional deformation. Regions adjacent to the Kohat Plateau also contain transpression-related structures, rather than thin-skinned imbricate thrusts. In view of the position of the Kohat Plateau near a major syntaxial bend in the Himalayan orogen, both north-south and east-west convergence played a dominant role in creating structural styles, and the combination of these two convergence directions influenced the development during the Plio-Pleistocene of transpression-related structures.
Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan
Authors: France-Lanord Christian , Derry Louis , Michard Annie
Published: April 2008

We report Sr, Nd, O, and H isotopic data and clay mineral abundances for turbidite sediments recovered in ODP Leg 116 cores from the Bengal Fan at 1°S. The samples studied cover the period betweenc.17 Ma and the present. We also present new and compiled data on the isotopic compositions of potential source regions for the Bengal Fan sediments. ɛNd(0) values in the Bengal Fan sediments (all samples) define a narrow range about − 16.0.87Sr/86Sr values (all samples) are also in a narrow range near 0.741. δ18O values in quartz separates define a narrow range at +12.8±0.5‰. Coarse biotite-chlorite separates give δ18O = 3.6−5.6‰. Combined δ18O values of quartz and biotites indicate a metamorphic source. Clay mineral abundances define two clay facies: an illite-chlorite-rich assemblage (IC) and a smectite-kaolinite-rich assemblage (SK). δ18O in the IC clay fractions is 11.5–15‰, while SK clays are 18.2–22.6‰. The narrow range of isotopic values throughout the deposition history implies that the source of the Bengal Fan sediments has not changed since the early Miocene, despite changes in sedimentation rate, sedimentary facies, tectonic history and climactic regime. The difference between δ18O in the IC and SK clay fractions represents different alteration histories of the same source material. The SK clays appear to have been altered at low T in the Indo-Gangetic Plain, while the IC clays and coarse fractions preserve metamorphic signatures. The narrow range of the Sr values, despite wide variation in Rb/Sr ratio, also argues for a source that underwent isotopic homogenization shortly before erosion and deposition of the sediment. The source that meets these criteria is the High Himalayan Crystalline series (HHC) or a close analogue, although subordinate contributions (probably <20%) from the Lesser Himalaya (LH) and Tibetan Sedimentary Series (TSS) are possible.

A model in which the HHC are exposed to erosion since the early Miocene on the south flank of the orogen by thrusting along the MCT, while the TSS is simultaneously removed by northward-directed normal faulting satisfies the constraints above. The results of this study require that the Himalaya have been a significant topographic feature since at least the early Miocene. Independent evidence supports this contention. Variations in the sedimentation style in the Bengal Fan since that time appear to represent a combination of factors, including tectonic activity and the coupled effects of climate and sea-level changes.
Downstream variation in bed material characteristics: a turbulence-controlled form-process feedback mechanism
Authors: Clifford N. J. , Hardisty J. , French J. R. , Hart S.
Published: April 2008

Downstream changes in bed material characteristics represent an element of the sediment transport process which is of immediate geomorphological and economic significance. Particle size trends represent the joint effect of abrasion and sorting, often complicated by in situ weathering and variable sediment supply. In principle, the combined effects of abrasion and shorting can be explained if attention is paid to the role and characteristics of near-bed turbulence, via the mechanisms of ‘abrasion in place’ and the size-dependent particle acceptance or rejection involved in Bluck’s ‘turbulence template’. Field data describing spatially- and stage-dependent differences in turbulence characteristics by which the significance of such processes can be quantified are, however, lacking.

In this paper, high-frequency three-dimensional velocity data obtained with an ultrasonic current meter in a braided section of Langden Brook, Lancashire, are used to demonstrate significant variations in turbulence parameters at the bar-pool scale. These are related to grain abrasion, sorting and the arrangement of grains in surface microtopographic bedforms using a force-balance model which incorporates turbulent velocity components in two dimensions. Following Bluck, it is suggested that bar forms act as, and result from, sediment trapping phenomena, whose fundamental controls lie in the characteristics of turbulence at the bar surface. Variations in turbulence provide a physical rationale for differing values of sorting/abrasion coefficients used in descriptive models of longitudinal size contrasts. In contrast to single-thread channels, however, spatially- and stage-dependent variation in turbulence characteristics is more marked in braided environments. Consequently, greater research effort is required to fully incorporate this into a spatially-realized form-process feedback mechanism applicable at the reach-scale and beyond.
Planform pattern and channel evolution of the Brahmaputra River, Bangladesh
Authors: Thorne Colin R. , Russell Andrew P. G. , Alam Muhammad K.
Published: April 2008

The Brahmaputra is one of the world’s greatest rivers, ranking fifth in terms of discharge and eleventh in terms of drainage area. It also has a very high sediment discharge, ranking third in the world. The river is braided with meta-stable islands and nodal reaches, mobile sand bars, shifting anabranches and severe bank erosion. The dominant discharge is about 38 000 cumecs, which is a high in-bank flow. Islands have top elevations that are adjusted to bankfull discharge, with a spacing scaled on the width of the primary channel. Bar top elevations are adjusted to the dominant discharge and bars are scaled on the width of the major anabranches. The dominant bedforms are large dunes scaled on the third order sub-channels of the major anabranches. Sustained right bank erosion has occurred during the last 35 years, with average rates of about 90 m per year. The left bank shows both erosion and accretion that together produce relatively low rates of net movement. Erosion is faster in island reaches and slower in nodal reaches. Recognition of the geomorphic controls of bank erosion allows its spatial distribution to be related to the development of the braided pattern of the river and a tentative hypothesis is proposed to explain recent trends in both bankline movement and the evolution of the channel planform pattern.