bluuebunny/crossref_metadata_2025 huggingface dataset:
Differences in Chicken Major Histocompatibility Complex (MHC) Class Iα Gene Expression between Marek's Disease‐Resistant and ‐Susceptible MHC Haplotypes
Authors: Dalgaard T. S. , Højsgaard S. , Skjødt K. , Juul‐Madsen H. R.
Published: March 2003

The expression of chicken major histocompatibility complex (MHC) class Iα genes was investigated in spleen cells from a panel of chickens with well‐defined MHC haplotypes, and two class Iα transcripts of 1.9 and 1.5 kb were detected in various amounts. In BW1, B130 and B21, the two transcripts were almost equally expressed. In B2, B6, B12 and B19, the ratio between the two transcripts was 4 : 1, with the 1.9 kb transcript having the strongest expression. In B14 and B15, the 1.5 kb transcript was undetectable and the 1.9 kb transcript appeared to be exclusively expressed. Thus, haplotypes considered to have an MHC‐determined resistance to Marek's disease (MD) had the highest relative amount of the 1.5 kb transcript, whereas haplotypes considered to be MD‐susceptible had the lowest.

In order to address a possible correlation between MHC‐Iα transcriptional patterns and MD resistance, a larger animal material experimentally infected with MD virus (MDV) was examined. The expression of MHC class Iα genes was investigated in spleens as well as in other organs, 9 weeks post‐infection (p.i.), from animals of the two MD‐resistant haplotypes B21 and BW1 as well as from the MD‐susceptible haplotype B19. In the spleen cells of infected animals, the relative amount of the 1.5 kb transcript in the haplotypes BW1 and B21 was shown to be significantly higher than that in B19. Interestingly, in infected BW1 and B21 animals, the relative amount of the 1.5 kb transcript was also significantly higher than that in healthy MHC‐matched controls. In B19, no differences were detected between uninfected and infected animals.

Furthermore, it was shown in BW1 and B21 that the two classical MHC‐Iα genes located in the MHC region were both able to produce both mRNA transcripts. Hybridization experiments, using specific probes upstream and downstream of the polyadenylation signals in the 3′ end of the MHC‐Iα genes, demonstrated that alternate use of these signals is probably involved in the production of the two mRNA transcripts.
Grain susceptibility to the effects of microboring: implications for the preservation of skeletal carbonates
Authors: Perry Christopher T.
Published: March 2003

The boring activity of microendolithic organisms such as cyanobacteria, chlorophytes, rhodophytes and fungi, represents a major destructive process affecting sediment preservation within reef environments. This study demonstrates the presence of two distinct assemblages of microborers within sediments collected from the fringing reefs of north Jamaica, correlating to the upper (<18 m depth) and lower (>18 m depth) photic zones. The upper photic zone assemblage is dominated by cyanobacteria and chlorophytes, whilst rhodophytes and fungi become more abundant with increasing water depth. Most significant from a grain preservation perspective is the variable nature of grain infestation observed between different reef sites and different carbonate grain types. The highest degree of grain infestation occurs within shallow, low‐energy back‐reef environments and the most susceptible grains (at all sites) are corals, molluscs and foraminifera. Coralline algae,Halimedaand echinoid fragments are rarely heavily infested. High rates of infestation at back‐reef sites result in rapid diminution of the most susceptible grains, especially coral, which are either underrepresented, or contribute only to the finer sediment fractions, in the subsurface. Fore‐reef grain assemblages undergo relatively little alteration. Microboring therefore has potential to bias the fossil record by removing the most susceptible skeletal grains. The impact of microboring upon back‐reef grain assemblages must be considered when attempting to model depositional processes within both modern and ancient reef environments.
Upper Ordovician facies in the Lac Saint‐Jean outlier, Québec (eastern Canada): palaeoenvironmental significance for Late Ordovician oceanography
Authors: LAVOIE  , ASSELIN
Published: March 2003

Upper Ordovician (Caradocian) carbonates of eastern North America were deposited along the Iapetus continental margin and record a transition from warm‐ to cool‐water settings despite this margin having been within the southern hemisphere tropical belt. This event has been documented from Virginia (USA) to southern Québec (Canada) although, not previously from areas close to the palaeoequator.

Field, petrographic and major element geochemistry data have been gathered from the poorly‐known Upper Ordovician carbonate succession outcropping in the Lac Saint‐Jean outlier in central Québec. The succession consists of a lower siliciclastic formation (Tremblay) overlain by three limestone formations (Simard, Shipshaw and Galets) and capped by shales (Pointe‐Bleue Shale). From macro‐ and microfaunal evidence, carbonate sedimentation occurred during the late Caradoc and is younger than the early‐ to mid‐Caradoc carbonate succession present farther south. Relative sea level fluctuations recorded in the sediments suggest an overall sea level rise briefly halted by a minor end‐Caradocian sea level fall.

The lower limestone formation (Simard) consists of muddy sediments with algal‐coral‐stromatoporoid boundstones; green algae are abundant. This unit reflects low energy sedimentation on a shallow warm‐water carbonate ramp colonized by a diverse chlorozoan fauna. The upper limestone formation (Galets) is typified by coarse‐grained bioclastic sediments punctuated by numerous phosphate‐rich hardgrounds with evidence for high energy shallow marine conditions. Faunas were dominated by crinoids and bryozoans. This unit represents high energy sedimentation on a cool shallow water carbonate ramp colonized by a brynoderm faunal association. Between both units, a deeper marine (outer shelf) limestone formation (Shipshaw) was developed.

In the Lac Saint‐Jean area, a transition from warm‐ to cool‐water carbonate ramps occurred in latest Caradoc times and is litho‐ and biofacies‐wise, similar to what is documented for lower Caradocian limestones present farther south. Upwelling of nutrient‐rich cool bottom oceanic waters was a probable cause for this transition.
Large‐scale structure of Calamus River deposits (Nebraska, USA) revealed using ground‐penetrating radar
Authors: BRIDGE JOHN , COLLIER RICHARD , ALEXANDER JAN
Published: March 2003

The large‐scale (i.e. bar‐scale) structure of channel deposits of the braided, low‐sinuosity Calamus River, Nebraska, is described using ground‐penetrating radar (GPR) profiles combined with vibracores. Basal erosion surfaces are generally overlain by medium‐scale, trough‐cross‐stratified (sets 3–25 cm thick), very coarse to medium sands, that are associated with relatively high amplitude, discontinuous GPR reflectors. Overlying deposits are bioturbated, small‐scale cross‐stratified (sets <3 cm thick) and vegetation‐rich, fine to very‐fine sands, that are associated with low‐amplitude discontinuous reflectors. Near‐surface peat and turf have no associated GPR reflectors. In along‐stream profiles through braid and point bars, most GPR reflectors dip downstream at up to 2° relative to the basal erosion surface, but some reflectors in the upstream parts of bars are parallel to the basal erosion surface or dip upstream. In cross‐stream profiles through bars, GPR reflectors are either approximately parallel to bar surfaces or have low‐angle inclinations (up to 6°) towards cut banks of adjacent curved channels. Basal erosion surfaces become deeper towards cut banks of curved channels. These structures can be explained by lateral and downstream growth of bars combined with vertical accretion. Convex upwards forms up to 0·5 m high, several metres across and tens of metres long represent episodic accretion of unit bars (scroll bars and bar heads). Stratal patterns in channel fills record a complicated history of erosion and deposition during filling, including migration of relatively small bars. A revised facies model for this type of sandy, braided river has been constructed based on this new information on large‐scale bedding structure.
Sedimentary organic matter in condensed sections from distal oxic environments: examples from the Mesozoic of SE France
Authors: BOMBARDIERE LUCA , GORIN GEORGES E.
Published: March 2003

A detailed analysis of sedimentary organic matter (or palynofacies) was carried out on thermally immature to early mature Upper Jurassic and Hauterivian condensed intervals in deep‐sea carbonate–marl alternations outcropping in the Vocontian Basin (SE France). All the condensed sections studied are characterized by intense bioturbation and very low organic carbon content (< 0·25 wt.%), indicative of oxic depositional conditions.

Oxic condensed sections display variable palynofacies signatures, which are best illustrated by: (1) the ratio of continental to marine constituents; (2) the ratio of opaque to translucent phytoclasts (i.e. woody debris) and (3) the preservation of palynomorphs (based on fluorescence intensity and morphological preservation state in transmitted light microscopy). Both of the ratios increase with the degree of palynomorph degradation, which shows that phytoclasts, especially the opaque ones, become relatively concentrated in the most degraded facies. These observations lead to the classification of oxic condensed sections into three organic facies types showing different degrees of preservation and palynofacies signatures. Type 1 organic facies display intense degradation and are characterized by high values of the ratio of continental to marine fraction. They record unfavourable depositional environments for preservation of organic matter. Type 2 organic facies are most common and are characterized by a decreasing value of the ratio of continental to marine fraction. Type 3 organic facies display the same trend of the ratio of continental to marine fraction as type 2, but the palynomorph assemblage is better preserved. Type 1 and type 3 organic facies are relatively rare.

Recognizing these organic facies types is important when analysing the relationship between sedimentary organic matter and sequence stratigraphy, because it allows the use of the appropriate palynofacies parameters. In particular, the use of the ratio of continental to marine constituents, usually a very good indicator of regressive–transgressive trends, becomes questionable in highly degraded intervals. Moreover, distinguishing between well‐preserved or highly degraded palynofacies in condensed intervals provides valuable information on the oxicity of the depositional environment.
Geochemical, mineralogical and sedimentological studies on the Taklimakan Desert sands
Authors: HONDA MASATOSHI , SHIMIZU HIROSHI
Published: March 2003

Major‐element composition, mineral composition and grain‐size distribution have been studied for Quaternary aeolian sediments from the Taklimakan Desert, north‐western China, together with the variation of chemical and mineralogical compositions of different grain‐size fractions. Aeolian sediments from the Taklimakan Desert have higher ratios of feldspar/quartz and calcite/quartz, finer grain size, poorer roundness of quartz and feldspar grains and lower abundances of frosted quartz, than found in aeolian sediments from other deserts such as the Saudi Arabian Desert. In spite of these immature mineralogical and sedimentological features, the aeolian sediments from the Taklimakan Desert show low regional variations in major‐element and mineral compositions and are homogenized. These observations confirm that two processes, glacial activity within surrounding mountains and aeolian activity at the Tarim Basin, are important in the homogenization of the Taklimakan Desert sands. Taklimakan Desert sediments are constantly and effectively supplied from basement rocks in the surrounding mountains by glacial erosion. The supplied sediments are further homogenized by aeolian activity in the desert and are partly blown away, serving as the source of Chinese aeolian loess. Compositional differences are observed between loess (mainly 10–40 μm particles) and the <45 μm fraction of the Taklimakan Desert sediments, as well as between loess and whole rock of the Taklimakan Desert sediments. These observations provide constraints for precise modelling of loess formation, and for assessment of the chemical composition of the upper continental crust based on the chemical composition of aeolian loess.
Controls on sedimentation in distal margin palaeovalleys in the Early Tertiary Alpine foreland basin, south‐eastern France
Authors: GUPTA SANJEEV
Published: March 2003

The influence of palaeodrainage characteristics, palaeogeography and tectonic setting are rarely considered as controls on stratigraphic organization in palaeovalley or incised valley systems. This study is an examination of the influence of source region vs. downstream base level controls on the sedimentary architecture of a set of bedrock‐confined palaeovalleys developed along the distal margin of the Alpine foreland basin in south‐eastern France. Three distinct facies associations are observed within the palaeovalley fills. Fluvial facies association A is mainly dominated by poorly sorted, highly disorganized, clast‐to‐matrix‐supported cobble‐to‐boulder conglomerates that are interpreted as streamflood deposits. Facies association B comprises mainly yellow siltstones and is interpreted as recording deposition in an estuarine basin environment. Estuarine marine facies association C comprises interstratified estuarine siltstones and clean, well‐sorted washover sandstones. The sedimentary characteristics of the valley fill successions are related to the proximity of depositional sites to sediment source areas. Palaeovalleys located proximal to structurally controlled basement palaeohighs are entirely dominated by coarse fluvial streamflood deposits. In contrast, distal palaeovalley segments, which are located several kilometres downstream, contain successions showing upward transition from coarse fluvial facies into estuarine central basin fines, and finally into estuarine‐marginal marine facies. Facies distributions suggest that the fluvial deposits form wedge‐shaped, downstream‐thinning sediment bodies, whereas the estuarine deposits form an upstream‐thinning wedge. The vertical stacking of fluvial to estuarine to marginal marine depositional environments records the fluvial aggradation and subsequent transgression of relatively small bedrock‐confined river valleys, which drained a rugged, upland terrain. Facies geometries suggest that a fluvial sediment wedge initially prograded downvalley, in response to high bed load sediment yields. Subsequently, palaeovalleys became drowned during the passage of a marine transgression, with the establishment of estuarine conditions. Initial fluvial aggradation and subsequent marine flooding of the palaeovalleys is a consequence of the interaction of high local rates of sediment supply and relative sea‐level rise driven by flexural subsidence of the basin.
Eustatic and hydrodynamic controls on the architecture of a deep shelf sand bank (Celtic Sea)
Authors: Reynaud Jean‐Yves , Tessier Bernadette , Proust Jean‐Noël , Dalrymple Robert , Marsset Tania , De Batist Marc , Bourillet Jean‐François , Lericolais Gilles
Published: March 2003

The architecture of a tidal sand bank in the south‐eastern Celtic Sea was examined using very high‐resolution seismic surveys. The bank comprises four depositional units. The lowest unit 1 is characterized by gently dipping (1–8°) strata that strike parallel to the length of the bank. Unit 1 is erosionally overlain by unit 2, which forms the bulk of the bank. This unit consists of stacked sets of downcurrent‐dipping (7–12°) master bedding formed by climbing, sinuous‐crested tidal dunes that are up to 20 m high. These deposits are locally incised by an anastomosed channel network (unit 3) that may represent a buried swatchway system. The upper part of the bank comprises wave‐related deposits that are mainly preserved on the bank flanks (unit 4). The outer bank surface is erosional. The bank is believed to have formed during the last post‐glacial sea‐level rise. The facies evolution from unit 1 to unit 3 indicates an upward increase in tidal energy, mainly characterized by the thickening of dune cross‐bed sets in unit 2. The majority of bank growth is inferred to have occurred in water depths of the order of 60 m. This evolution was controlled by relative sea‐level rise, which is likely to have caused an episode of tidal resonance with associated strong tidal currents that were responsible for the incision of the deep, cross‐cutting channels of unit 3. The transition to wave‐dominated sedimentation in unit 4 is related to the decay of resonance with continued sea‐level rise.
Quantitative models of sedimentary basin filling
Authors: Paola Chris
Published: March 2003

Quantitative modelling of the filling of sedimentary basins was begun in earnest in the 1960s. Dozens of themes and variations have been proposed since then, and have yielded an abundance of idealized stratigraphic patterns as functions of both imposed changes and basin properties. Post‐plate‐tectonic modelling began with ‘rigid‐lid' models, which show the stratigraphic signature of subsidence variation. This work introduced the connection between stratigraphy and the rheology of the lithosphere. Rigid‐lid models are the simplest type of geometric model, in which the sediment surface is assigned prescribed geometries, usually corresponding to different depositional environments. These can reproduce many aspects of overall stratal geometry but are formally restricted to relatively long timescales, for which quasi‐steady surface topography can be assumed. So‐called dynamic models attempt to represent the morphodynamics of the sediment surface by abstracting and averaging short‐term transport processes. Most of the dynamic models proposed to date can be seen as special cases of a single general morpho‐dynamic equation.

The most important result of the first wave of quantitative basin‐filling models is that even relatively simple models can produce reasonable stratal patterns. We now have a wide array of tools for exploring scenarios, searching for general behaviours and effects, and making initial quantitative predictions. We have also learned that basin response to external forcing as recorded in stratigraphy can be as sensitive to the characteristics of the basin as to the forcing.

The main brake on the development of basin modelling is not computing power but lack of methods and data for testing the models we have already developed. Physical experiments, which are only just beginning, are one means of doing this. Experimental stratigraphy is a bridge to quantitative field tests, which will require collaboration among academic researchers from a wide range of areas, and between academia and industry, on projects of greater scale and degree of integration than we have seen so far. The advancement of quantitative sedimentary geology will also require significant changes in the way the subject is taught, at all levels.
Surface and subsurface sedimentary structures produced by salt crusts
Authors: Goodall Timothy M. , North Colin P. , Glennie Kenneth W.
Published: March 2003

The growth and subsequent dissolution of salts on or within sediment may alter sedimentary structures and textures to such an extent that it is difficult to identify the depositional origin of that sediment and, as a result, the sediment may be misinterpreted. To help to overcome such problems with investigating ancient successions, results are presented from a comprehensive study of the morphology and fabrics of three large areas of modern salt flats in SE Arabia: the Sabkhat Matti inland region and the At Taf coastal region, both in the Emirate of Abu Dhabi, and the Umm as Samim region in Oman. These salt flats are affected by tidal‐marine, alluvial and aeolian depositional processes and include both clastic‐ and carbonate‐dominated surficial sediments. The efflorescent and precipitated salt crusts in these areas can be grouped into two main types: thick crusts, with high relief (>10 cm) and a polygonal or blocky morphology; or thin crusts, with low relief (<10 cm) and a polygonal or blister‐like appearance. The thin crusts may assume the surface morphology of underlying features, such as ripples or biogenic mats. A variety of small‐scale textures were observed: pustular growths, hair‐like spikes and irregular wrinkles. Evolution of these crusts over time results in a variety of distinctive sedimentary fabrics produced by salt‐growth sediment deformation, salt‐solution sediment collapse, sediment aggradation and compound mixtures of these processes. Salt‐crust processes produce features that may be confused with aeolian adhesion structures. An example from the Lower Triassic Ormskirk Sandstone Formation of the Irish Sea Basin demonstrates how this knowledge of modern environments improves the interpretation of the rock record. A distinctive wavy‐laminated facies in this formation had previously been interpreted as the product of fluvial sheetfloods modified by soft‐sediment deformation and bioturbation. Close inspection of laminations seen in core reveals many of the same sedimentary fabrics seen in SE Arabia associated with salt crusts. This facies is the product of salt growth on aeolian sediment and is not of fluvial origin.
Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan
Authors: Hiroki Yoshihisa , Masuda Fujio
Published: March 2003

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.
The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology
Authors: Carling  , Gölz  , Orr  , Radecki‐Pawlik
Published: March 2003

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D50 of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well‐sorted medium to coarse sand (D50 of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D50 < 0·6 mm), and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0·7–1 m and 5–10 m. Ripples and small dunes may have sinuous crestlines but primarily exhibit two‐dimensional planforms. In contrast, large dunes are primarily three‐dimensional barchanoid forms. Ripples on the backs of small dunes rarely develop to maximum steepness. Small dunes may achieve an equilibrium geometry, either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes. Secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they traverse the crestal region. However, secondary dunes more than 5 m in length are rare. The dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rarely exceed 1 m high in water depths of ≈4 m. Dune celerity increases as a function of the mean flow velocity squared, but this general relationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows downstream progression and steepens the dune form. During steady or falling stage, an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height. These diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six‐stage model of dune growth and diminution is proposed to explain variations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload‐dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation. This finding is significant, as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport.
Characterization of alluvial bajada facies distribution using TM imagery
Authors: Milana Juan Pablo
Published: March 2003

Automatic mapping techniques using multiband satellite image information have been used to study sediment grain‐size variations on an alluvial bajada. A previous study of sparsely vegetated alluvial surfaces in central‐western Argentina showed that the reflectivity recorded in seven‐band Landsat TM images is controlled by sediment composition, age and grain size. At diameters >4 mm, clast composition influences image information, while at grain sizes smaller than sand, clay mineralogy begins to influence spectral characteristics. The progressive increase in desert varnish and the loss of fine grain‐size fractions as a result of deflation cause the age influence. The bajada sedimentary environment is well suited for testing the influence of grain‐size variability on image recovery, as most drainage basins are small and sourced by a single geological unit, which produces compositionally homogeneous alluvial surfaces. Additionally, most drainage lines are active within intervals of 10–50 years, reducing the effect of surface ageing. Larger than average drainage basins produce oversized drainage lines that disrupt the bajada, generate individual alluvial fans and have slightly different compositions. Two typical bajadas were selected to map grain‐size characteristics using automatic classification techniques. The obtained classes were checked in the field, and grain‐size and compositional counts were completed. Although both bajadas are very different in composition, most grain‐size curves showed similar shapes, suggesting that deposition took place by the same process (hyperconcentrated flash floods). The values of the median and mean were consistent across the same class between both bajadas. Thus, unsupervised classification techniques are useful for mapping sediment grain size, although minor field control is needed. Image classes represent areas of similar grain size, which are elongated parallel to the mountain front in an active alluvial bajada, indicating a homogeneous distribution of sedimentary processes along strike. Changes in the width of image classes indicate different downstream fining rates closely related to the topographic gap and the slope change rate. In contrast to bajadas, alluvial fans have semi‐circular belts and a pie‐piece‐shaped area in which most active streams are located. Thus, unlike fans, bajadas lack autocyclic mechanisms for producing heterogeneous sedimentary sequences. The sedimentary log of an ancient bajada was measured in order to show the influence of allocyclic factors in the absence of autocyclicity.
Identifying causes of ground‐penetrating radar reflections using time‐domain reflectometry and sedimentological analyses
Authors: Van Dam Remke L. , Schlager Wolfgang
Published: March 2003

Ground‐penetrating radar (GPR) is a geophysical technique widely used to study the shallow subsurface and identify various sediment features that reflect electromagnetic waves. However, little is known about the exact cause of GPR reflections because few studies have coupled wave theory to petrophysical data. In this study, a 100‐ and 200‐MHz GPR survey was conducted on aeolian deposits in a quarry. Time‐domain reflectometry (TDR) was used to obtain detailed information on the product of relative permittivity (ɛr) and relative magnetic permeability (μr), which mainly controls the GPR contrast parameter in the subsurface. Combining TDR data and lacquer peels from the quarry wall allowed the identification of various relationships between sediment characteristics and ɛrμr. Synthetic radar traces, constructed using the TDR logs and sedimentological data from the lacquer peels, were compared with the actual GPR sections. Numerous peaks in ɛrμr, which are superimposed on a baseline value of 4 for dry sand, are caused by potential GPR reflectors. These increases in ɛrμr coincide with the presence of either organic material, having a higher water content and relative permittivity than the surrounding sediment, or iron oxide bands, enhancing relative magnetic permeability and causing water to stagnate on top of them. Sedimentary structures, as reflected in textural change, only result in possible GPR reflections when the volumetric water content exceeds 0·055. The synthetic radar traces provide an improved insight into the behaviour of radar waves and show that GPR results may be ambiguous because of multiples and interference.
Aeolian architecture, bedform climbing and preservation space in the Cretaceous Etjo Formation, NW Namibia
Authors: Mountney  , Howell
Published: March 2003

Sets of aeolian cross‐strata within the Cretaceous Etjo Formation of NW Namibia are bounded by a hierarchy of surfaces, the origin of which are ascribed to one of four processes related to aeolian bedform and erg behaviour. The base of the main aeolian succession is characterized by a basin‐wide erosional supersurface that formed in response to a period of aeolian deflation before the onset of the main phase of erg building. Interdune migration surfaces formed by draa migration are planar in sections parallel to the palaeowind and are inclined at up to 5° in an upwind direction (SW). Perpendicular to the palaeowind, interdune surfaces form 500‐m‐wide troughs, signifying crestline sinuosity within the original bedforms. Superimposition surfaces are inclined at 5–10° in a downwind direction and indicate the migration of crescentic oblique dunes over larger, slipfaceless transverse draa. Reactivation surfaces associated with minor changes in dune slipface orientation are distinct from other bounding surface types because overlying cross‐strata lie parallel to them, rather than downlap onto them. Analysis of the geometry of these bounding surfaces, together with the orientation of the cross‐strata within the sets that they bound, has enabled the detailed morphology of the original bedforms to be reconstructed. The maximum preserved thickness of individual aeolian sets varies systematically across the basin, from 52 m in the basin depocentre to only 8 m at the basin margin. The set architecture indicates that this spatial variation is primarily the result of decreased angles of bedform climb at the basin margin, rather than the presence of smaller bedforms. Similarly, a temporal reduction in the angle‐of‐climb, rather than a reduction in bedform size, is considered to be responsible for an upward decrease in preserved set thickness. Reductions in bedform climb angle reflect progressive loss of accommodation space as the accumulating erg filled the basin.
The structure and development of foredunes on a locally prograding coast: insights from ground‐penetrating radar surveys, Norfolk, UK
Authors: Bristow Charlie S. , Chroston P. Neil , Bailey Simon D.
Published: March 2003

The internal structure of coastal foredunes from three sites along the north Norfolk coast has been investigated using ground‐penetrating radar (GPR), which provides a unique insight into the internal structure of these dunes that cannot be achieved by any other non‐destructive or geophysical technique. Combining geomorphological and geophysical investigations into the structure and morphology of these coastal foredunes has enabled a more accurate determination of their development and evolution. The radar profiles show the internal structures, which include foreslope accretion, trough cut and fill, roll‐over and beach deposits. Foredune ridges contain large sets of low‐angle cross‐stratification from dune foreslope accretion with trough‐shaped structures from cut and fill on the crest and rearslope. Foreslope accretion indicates sand supply from the beach to the foreslope, while troughs on the dune crest and rearslope are attributed to reworking by offshore winds. Bounding surfaces between dunes are clearly resolved and reveal the relative chronology of dune emplacement. Radar sequence boundaries within dunes have been traced below the water‐table passing into beach erosion surfaces. These are believed to result from storm activity, which erodes the upper beach and dunes. In one example, at Brancaster, a dune scarp and erosion surface may be correlated with erosion in the 1950s, possibly the 1953 storm. Results suggest that dune ridge development is intimately linked to changes in the shoreline, with dune development associated with coastal progradation while dunes are eroded during storms and, where beaches are eroding, a stable coast provides more time for dune development, resulting in higher foredune ridges. A model for coastal dune evolution is presented, which illustrates stages of dune development in response to beach evolution and sand supply. In contrast to many other coastal dune fields where the prevailing wind is onshore, on the north Norfolk coast, the prevailing wind is directed along the coast and offshore, which reduces the landward migration of sand dunes.
Diamictic sediments within high Arctic lake sediment cores: evidence for lake ice rafting along the lateral glacial margin
Authors: Smith I. Rod
Published: March 2003

Sediment cores from six small lake basins in the Canadian high Arctic reveal a gravel‐rich (≤30% by weight) to gravel‐poor (≥2%) diamict facies underlying massive, post‐glacial, clayey silt. Ten other lakes contain a second diamict facies within what are interpreted to be glaciolacustrine sedimentary assemblages. The sedimentology, clast fabrics and fossil remains (diatoms, ostracodes and chironomid head capsules) within both diamict facies suggest that these deposits are not tills. Clast fabrics yielded low S1 (0·41–0·57) and high S3 (0·09–0·22) eigenvalues, placing them within the range of ice‐rafted diamictons and glacigenic sediment flows. The high percentage of clast dip angles >45° (15–61%), random clast azimuth and lower diamict contacts conformable to underlying current‐bedded sediment favours an origin as a rain‐out or settling deposit. Samples of the matrix and scrapings of clasts from the diamicts revealed a diatom assemblage dominated by littoral and planktonic forms, such as are found in the littoral regions of the lakes today. This contrasts sharply with the assemblages within the overlying clayey silt, in which benthic forms predominate. Clasts are thus interpreted to have been rafted from the littoral areas of the lake. The process proposed to explain this is rafting by the lake ice cover in a glacial‐marginal environment. Early season meltwater, impounded along the lateral margin of retreating cold‐based glaciers, would buoyantly lift the lake ice cover and any adfrozen lake sediment. Higher lake levels and increased areal extent of seasonal freeze‐on between the lake ice cover and the lake bed would allow the redeposition of littoral sediments to the benthic regions through greater lateral shifting of the ice cover as it broke up. Incision by meltwater streams into the lateral glacial margins would later isolate the lake, allowing seasonal warming of lake water, enough to support the growth and maturation of the ostracode and chironomid species found as fossils within the diamicts.
Lithofacies associations and stable isotopes of palustrine and calcrete carbonates: examples from an Indian Maastrichtian regolith
Authors: Tandon S. K. , Andrews J. E.
Published: March 2003

Detailed information on semi‐arid, palustrine carbonate–calcrete lithofacies associations in a sheetwash‐dominated regolith setting is sparse. This is addressed by studying the Lower Limestone of the Lameta Beds, a well‐exposed Maastrichtian regolith in central India. The general vertical lithofacies assemblage for this unit comprises: (a) basal calcareous siltstones and marls with charophytes, ostracods and gastropods; (b) buff micritic limestones associated in their upper parts with calcretized fissure‐fill sandstones; (c) sheetwash as fissure‐fill diamictites and thin pebbly sheets, locally developed over a few metres; and (d) sandy, nodular, brecciated and pisolitic calcretes at the top. The sequence is ‘regressive’, with upsection filling of topographic lows by increased sheetwash. Lateral lithofacies change is marked, but there are no permanent open‐water lake deposits. In topographic lows close to the water table, marshy palustrine or groundwater calcretes formed, whereas on better drained highs, brecciation and calcretization occurred. Prolonged exposure is implied, suggesting that shrinkage was the main cause of brecciation. Evidence for rhizobrecciation and other biological calcrete fabrics is sparse, contrasting with the emphasis on root‐related brecciation in many studies of palustrine lithofacies. Stable isotope (δ18O and δ13C) values are consistent with the palustrine limestones being fed from meteoric‐derived groundwater with a strong input of soil‐zone carbon. There is overlap of both δ18O and δ13C values from the various palustrine and calcrete fabrics co‐occurring at outcrop. This suggests that, in groundwater‐supported wetlands, conversion from palustrine carbonate to calcrete need not show isotopic expression, as the groundwater source and input of soil‐zone carbon are essentially unchanged. Cretaceous–Tertiary δ18O and δ13C values from palustrine lithofacies and associated calcretes appear to be strongly influenced by the inherited values from lakes and wetlands. Hydrologically closed lakes and marine‐influenced water bodies tend to result in low negative palustrine δ18O and δ13C values. During brecciation and calcretization, the degree of isotopic inheritance depends on whether or not alteration occurs in waters that are different from those of the original water body or wetland. Marked biological activity (e.g. rhizobrecciation or root mat development) during calcretization may lower δ13C values where C3plants are abundant but, in shrinkage‐dominated systems, δ13C values will be largely inherited from the palustrine limestones.
Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments
Authors: Melezhik Victor A. , Fallick Anthony E. , Medvedev Pavel V. , Makarikhin Vladimir V.
Published: March 2003

Magnesite forms a series of 1‐ to 15‐m‐thick beds within the ≈2·0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680‐m‐thick formation is composed of a stromatolite–dolomite–‘red bed’ sequence formed in a complex combination of shallow‐marine and non‐marine, evaporitic environments. Dolomite‐collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by δ13C values from +7·1‰ to +11·6‰ (V‐PDB) and δ18O ranging from 17·4‰ to 26·3‰ (V‐SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high δ13C values ranging from +9·0‰ to +11·6‰ and δ18O values of 20·0–25·7‰. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water‐derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high δ13C values reflect a combined contribution from both global and local carbon reservoirs. A13C‐rich global carbon reservoir (δ13C at around +5‰) is related to the perturbation of the carbon cycle at 2·0 Ga, whereas the local enhancement in13C (up to +12‰) is associated with evaporative and restricted environments with high bioproductivity.
The crystallographic fabric and texture of siderite in concretions: implications for siderite nucleation and growth processes
Authors: Hounslow Mark W.
Published: March 2003

The crystallographic fabric of siderite in siderite concretions has been determined for upper Carboniferous (Westphalian‐A) non‐marine concretions and lower Jurassic (Pliensbachian) marine concretions. Compositional zoning indicates that individual siderite crystals grew over a period of changing pore water chemistry, consistent with the concretions being initially a diffuse patch of cement, which grew progressively. The siderite crystallographic fabric was analysed using the anisotropy of magnetic susceptibility, which is carried by paramagnetic siderite. The siderite concretions from marine and non‐marine formations exhibit differences in fabric style, although both display increases in the degree of preferred siderite c‐axis orientation towards the concretion margins. The Westphalian non‐marine siderites show a preferred orientation of siderite c‐axes in the bedding plane, whereas the Pliensbachian marine siderites have a preferred orientation of c‐axes perpendicular to the bedding. In addition, a single marine concretion shows evidence of earlier formed, inclined girdle‐type fabrics, which are intergrown with later formed vertical c‐axis siderite fabrics. The marine and non‐marine fabrics are both apparently controlled by substrate processes at the site of nucleation, which was probably clay mineral surfaces. Siderite nucleation processes on the substrate were most probably controlled by the (bio?) chemistry of the pore waters, which altered the morphology and crystallographic orientation of the forming carbonate. The preferred crystallographic orientation of siderite results from the orientation of the nucleation substrate. Fabric changes across the concretions partially mimic the progressive compaction‐induced alignment of the clay substrates, while the concretion grew during burial.
Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda ‘white earth’ (Pliocene, central Spain)
Authors: García Del Cura M. Angeles , Calvo Jose P. , Ordóñez Salvador , Jones Blair F. , Cañaveras Juan C.
Published: March 2003

Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. δ18O‐ and δ13C‐values in dolomites range from −3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from −6·34‰ to −0·39‰ PDB (mean=−3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between δ18O and δ13C for dolomite is extremely low and negative (r=−0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3−/CO32− groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.