bluuebunny/crossref_metadata_2025 huggingface dataset:
The application of cathodoluminescence to interpreting the diagenesis of an ancient calcrete profile
Authors: SOLOMON STEPHEN T. , WALKDEN GORDON M.
Published: June 2006

A laterally extensive calcrete profile has been identified in the Late Asbian (Lower Carboniferous) shallow marine shelf limestones of the Llangollen area, North Wales. The upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone.

The profile contains a unique series of early pore‐filling vadose cements which only occur down to 1 m below the palaeokarstic surface. Cathodoluminescence reveals that these cements pre‐date the late pore‐filling meteoric phreatic cements which occur throughout local Asbian lithologies. A spar cement stratigraphy has been established for the calcrete profile. Subaerial vadose cements comprise two generations of non‐luminescent cement, followed by a brightly luminescent generation which occasionally shows an acicular habit. This needle‐fibre calcite represents the final stage of vadose cementation. Precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone.

Textures, visible only with cathodoluminescence, provide evidence of recurrent periods of fabric dissolution. The most extensive phase of dissolution occurred immediately after the precipitation of the non‐luminescent subaerial vadose cements. Several different textures have been recorded, each reflecting the morphology of a partially dissolved substrate. Dissolution textures are generally confined to the walls of the larger pores and to early brecciation fractures. These probably acted as fluid pathways in the calcrete during early subaerial diagenesis.

Much of the non‐marine micrite in the calcrete profile appears as needle‐fibre calcite under cathodoluminescence. This acicular calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near‐surface evaporation. Since needle‐fibre luminescence is strongly variable, these ambient conditions are not believed to have directly controlled the activator ion concentrations of cementing pore waters. Needle‐fibre calcite is considered to be a cement precipitate which has almost completely recrystallized to micrite, probably during the late stages of subaerial diagenesis. Two generations of subaerial micrite which define a ‘micrite stratigraphy’, have been distinguished under cathodoluminescence.

Reconstructing the diagenetic history of this ancient calcrete profile has revealed that subaerial alteration was multistaged, with many diagenetic processes acting simultaneously during a single phase of emergence.
A comparative study of the Rare Earth Element (REE) distributions within the Lower Cretaceous dolomites and limestones of Central Tunisia
Authors: TLIG SAĪMD , M'RABET ALI
Published: June 2006

REE distribution, strontium content and oxygen isotopic composition have been studied in the Lower Cretaceous dolomites and their interbedded and/or parental limestones of Central Tunisia. Dolomites, as indicated by previous sedimentologic, geochemical and isotopic study, are different in origin. Environments of dolomite formation include: evaporitic sabkha, deep phreatic, karst and lacustrine. With the exception of the sabkha environment, dolomitizing fluids presumably have been essentially meteoric. The comparison between REE distributions in dolomites and limestones clearly suggests that the general shapes of the REE patterns are preserved during dolomitization. Nevertheless, the total REE amounts are somewhat lowered in dolomites comparatively to parental limestones. Moreover, within the dolomites, the total REE contents are positively correlated to the Sr and δ18O contents; thus indicating that the decrease in REE, Sr, and δ18O contents is related to a decrease in the salinity of dolomitizing fluids. In addition, dolomites associated with emergence surfaces (karst) are the most impoverished in REE. Furthermore, they are relatively enriched in light REE, indicating a more or less important fractionation with the REE distributions. This enrichment has been related to the influence of the inorganic complexes which favours the greatest solubility of the heavy REE in basic solution with respect to the lightest members of the REE group.
Proximal to distal sedimentological variation in a linear turbidite trough: implications for the fan model
Authors: MACDONALD DAVID I. M.
Published: June 2006

Determination of geographically dependent sedimentological variation (‘proximality’) in ancient flysch deposits formed an important part of early turbidite studies. Attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models. However, there are many turbidite formations, such as the Lower Cretaceous Cumberland Bay Formation (CBF) of South Georgia, which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes. The CBF is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back‐arc basin, principally by currents flowing WNW, parallel to the basin margin. Four lithofacies associations are recognized on the basis of sandstone/shale ratio. The two finergrained associations are constant in character across the CBF outcrop. In the coarse‐grained associations there is a change in character WNW, down the palaeocurrent direction. This is brought out by decreasing sandstone bed thickness and percentage amalgamation, but these changes are not always marked or consistent. In contrast, the internal character of the sandstone beds changes strongly, with a marked proportional increase in Tband Tcdivisions within the bed downcurrent. The evidence suggests that the system was aggradational rather than progradational: tectonic control of the basin margins prevented major migration of the depositional system, and most areas remained in the same position relative to source through time. Comparison of the CBF with other turbidite formations suggests two end‐member states which will produce radically different vertical sequences. Progradational systems will produce strong vertical facies changes, where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source, however there will be no lateral change in the character of any particular facies type. In contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical.
Skin friction behind isolated hemispheres and the formation of obstacle marks
Authors: PAOLA CHRIS , GUST GISELHER , SOUTHARD JOHN B.
Published: June 2006

We have measured the mean magnitude and direction, and the rms fluctuation intensity, of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long. Downstream of reattachment, the mean skin‐friction magnitude is about 20% greater than its free‐stream value along the centre‐line and comparably reduced to either side of it. The horizontal divergence of the skin‐friction vector field is positive (divergent) along the centre‐line and negative (convergent) to either side of it. Neither of these conditions favours development of a ridge longer than the separation length along the centre line. The development of ridges or tails many obstacle heights long, commonly observed in nature, requires considerable modification of the simple sediment‐free wakes we have studied.
The St George Group (Lower Ordovician) of western Newfoundland: tidal flat island model for carbonate sedimentation in shallow epeiric seas
Authors: PRATT BRIAN R. , JAMES NOEL P.
Published: June 2006

The St George Group consists of peritidal carbonate rocks deposited on the continental shelf of North America bordering the ancient Iapetus Ocean. These Lower Ordovician rocks are similar to other lower Palaeozoic limestones and dolostones that accumulated in epeiric seas and veneer cratonic areas worldwide. A wide variety of facies in the St George is grouped into seven lithotopes, interpreted to represent supratidal, intertidal and shallow, high‐ and low‐energy subtidal environments. Rapid lateral facies changes can be observed in some field exposures, and demonstrated by correlation of closely spaced sections. The stratigraphic array of these lithotopes, although too irregular to be simplified into shallowing‐upward cycles, suggests that they were deposited as small tidal flat islands and banks. Shallow subtidal areas around islands generated sediment and permitted tidal exchange. Tidal flat islands were somewhat variable in character at any one time, and evolved with changing regional hydrographic conditions.

The St George rocks suggest an alternative theory of carbonate sedimentation in large, shallow epeiric seas, namely as small islands and banks built by processes that operated in a tidal regime. Furthermore, this island model provides a framework for a mechanism of cyclic carbonate sedimentation, by which small‐scale, peritidal cycles represent tidal flat islands that accreted vertically and migrated laterally as local sediment supply from neighbouring subtidal areas waxed and waned during relatively constant subsidence.
Aeolian to marine transition in Cambro—Ordovician cratonic sheet sandstones of the northern Mississippi valley, U.S.A.
Authors: JR R. H. DOTT , BYERS C. W. , FIELDER G. W. , STENZEL S. R. , WINFREE K. E.
Published: June 2006

Cratonic quartz sandstones have presented several intractable problems. Besides their extreme textural and compositional maturity and paucity of shale, their sheet‐like geometry is particularly notable. If the sandstones were entirely marine, as long supposed, such geometry is difficult to explain in terms of modern shelf sediments, which are generally held to be either relict or only slightly reworked by the Holocene transgression (palimpsest). Re‐study of two quartz sandstones in the northern Mississippi Valley region reveals evidence for significant non‐marine deposition followed by varying degrees of marine reworking during transgressions.

Facies patterns are similar in the Cambrian Wonewoc and Ordovician St Peter sandstones, both of which overlie unconformities. In both, a large‐scale cross‐stratified facies believed to represent aeolian ergs passes laterally into a planar‐and‐channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits. Criteria of aeolian deposition in both facies include adhesion structures, large ripple index, fine climbing translatent lamination, grainfall and grainflow stratification. Criteria of braided fluvial deposition include shallow channels containing sequences of thinning‐upward sets of trough cross‐stratification, reactivation surfaces, low‐index ripples, and polygonal cracks. Probable aeolian sand sheets contain flat bedding punctuated by small channels, adhesion structures, and coarse‐sand ripples with large index. There is a conspicuous absence of trace and body fossils from these inferred non‐marine deposits. In contrast is a burrowed and trough cross‐stratified facies characterized by medium‐scale cross‐bedding alternating with bioturbated intervals and rare brachiopod or trilobite‐mould coquinas, which is interpreted as shallow marine. In both formations, this last facies replaces laterally and overlaps the other two, reflecting transgression and variable reworking. The main areas of non‐marine deposits in both formations are capped by a thin, burrowed subfacies that represents the culmination of each transgression; that is, a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense.

It is suggested that the sheet‐like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes. That such processes must have been orders‐of‐magnitude more important on pre‐Devonian, vegetation‐free landscapes than they have been since, not only helps to explain the sheet‐like character but also the exceptional maturity of the older cratonic sandstones.
A model to estimate the depositional brine depths of ancient halite rocks: Implications for ancient subaqueous evaporite depositional environments
Authors: TUCKER ROGER M. , CANN J. R.
Published: June 2006

A model is derived which allows a more general approach to the interpretation of bromine profiles in halite rocks than is possible with the models of Kuhn or Holser. Allowing for an open system, with influx and reflux, reduces the depth of brine necessary to generate a given salt deposit. Even very small and regular bromine gradients, which historically have been interpreted as the result of deposition from deep brine bodies, can be generated in an open system from brine no more than a few tens of metres deep. The bromine gradient produced from a given depth of brine is strongly dependent on the composition and amount of influx, but less so on the composition of brine already in the basin.

A Zechstein profile is analysed which in a closed basin would require brine 2665 m deep. It can equally be modelled by influx of normal seawater which fully replenishes a basin no more than 140 m deep. A somewhat irregular profile from the Paradox Basin, instead of requiring a basin 100–400 m deep, can be modelled as being produced by deposition from brines only 10 m deep. Reflux in both cases can only be a few per cent of influx at a maximum.
The possible role of benthic microbial mats during the formation of carbonaceous shales in shallow Mid‐Proterozoic basins
Authors: SCHIEBER JÜRGEN
Published: June 2006

A considerable portion of the upper member of the Mid‐Proterozoic Newland Formation, Meagher County, Montana, consists of carbonaceous silty shales (striped shale facies). This type of shale facies is common in Proterozoic basins and is host to several major base metal deposits. The striped shales were deposited in a subtidal setting, basinward of carbonates characterized by cryptalgal laminites, mudcracks and flat pebble conglomerates. The carbonaceous silty shales are considered remnants of benthic microbial mats. Irregular internal laminae, patterns of particle trapping, mechanical deformation during penecontemporaneous soft‐sediment deformation and filamentous microbiota provide evidence for this interpretation. The dolomitic clayey shale contains graded silt‐clay couplets, and these are interpreted as storm layers. Modern subtidal microbial mats can only survive under special conditions, but in the Proterozoic, it is suggested that benthic microbial mats colonized the shallow seafloor during periods of low sediment input, leading to the formation of carbonaceous shales.
Thin section and S.E.M. textural criteria for the recognition of cement‐dissolution porosity in sandstones
Authors: BURLEY S. D. , KANTOROWICZ J. D.
Published: June 2006

Cement‐porosity relationships are described from the Lower Triassic Sherwood Sandstone Group and the Middle Jurassic Ravenscar Group in the United Kingdom. Calcite cemented sandstones display a variety of replacement textures, with preferential replacement of grains and of overgrowth faces with high free‐surface energy. Dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates. Examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains. These range from small ‘v’‐shaped notches and pits, through regular and irregular shaped embayments, into large depressions. These textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones, and are interpreted to have been formed by the dissolution of pore‐filling and grain replacive authigenic carbonates. This is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones. These textures indicate that part of the intergranular porosity in these sediments is secondary in origin, and has been generated by the dissolution of carbonate cements. The identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface.
Synsedimentary faulting and palaeocurrent patterns in the Triassic sandstones of the High Atlas (Morocco)
Authors: PETIT JEAN‐PIERRE , BEAUCHAMP JACQUES
Published: June 2006

The influence of syndepositional fault patterns on palaeocurrents is demonstrated in fluvial to shallow‐marine sandstones of Upper Triassic basins in the High Atlas. The synsedimentary nature of faults is deduced from hydroplastic slickensides, thickness variations due to block tilting and dislocation of layers next to the fault scarp. On a regional scale, it is shown that the major normal fault trend of N050–070° controlled the overall palaeocurrent pattern which was directed towards the west‐southwest, i.e. in the direction of the future Atlantic ocean. Some anomalies in the palaeocurrent pattern could be related to an increase in subsidence which induced a general coarsening of sediment towards the top of the Triassic.
Sediment pathways and transport rates through a tide‐dominated entrance, Rangaunu Harbour, New Zealand
Authors: PICKRILL R.A.
Published: June 2006

Bed load sediment traps were deployed at two sections across channels in Rangaunu Harbour entrance. Traps were inspected and emptied by divers at hourly intervals through both spring and neap tidal cycles for a total of 292 trap deployments. Current velocities were measured simultaneously with the trap inspections. Transport is concentrated in sandy megaripple fields on the channel banks and sub‐tidal platforms flanking the channels. There, transport is almost continuous throughout the tidal cycle, increasing with flow velocity but lagging by approximately one hour. The channel floors are lined with shell‐gravel lag across which bedload transport rates are low and discontinuous. Tidal asymmetry produces a net seaward transport through the channel troughs and a net landward transport across the channel banks and flanking sub‐tidal platforms. Sediment leaving the harbour recirculates in anticlockwise gyres across the ebb‐tide delta to re‐enter the harbou and maintain the supply of sand to the megaripple field. Transport during spring tides is typically 25–30 times that during neaps. Predictions of transport rates, from a method developed by Black & Healy utilizing the Yalin bedload equation, produced transport rates similar to the traps over sand beds. Transport over shell lag surfaces appears independent of near‐bed velocity and more dependent on the passage of ribbons of sand across the lag surface.
Diagenesis of sandstones in the back‐arc basins of the western Pacific Ocean
Authors: LEE YONG IL , KLEIN GEORGE DeVRIES
Published: June 2006

ABSTRACT Sandstones occur in back‐arc basins of the western Pacific at DSDP sites 299 (Sea of Japan), 297 (northern Shikoku Basin), 445 and 446 (Daito‐Ridge‐and‐Basin Province), 453 (Mariana Trough), 286 (New Hebrides Basin) and 285 (South Fiji Basin). These sandstones are dominantly volcaniclastic arenites derived from andesitic island arcs. The degree of sandstone diagenesis is dependent on original composition, burial rate, heat flow history of the basin, and timing of sandstone deposition with respect to rifting processes and associated high heat flow.

Sandstones containing a larger proportion of volcaniclastic components showed more diagenetic effects than sandstones containing a significant volume of other rock fragments and mineral components. Sandstones deposited during early stages of rifting (sites 445, 446) with a slow burial rate and high crustal heat flow showed the greatest degree of downhole diagenetic change. These diagenetic changes include early pore‐space reduction and rim cementation by clay minerals followed later by calcite, and subsequent pore‐fill cementation by clinoptilolite, heulandite, analcite and later calcite. Replacement of recognizable volcanic rock fragments by chert, calcite and zeolites was observed in the deepest part of the hole. Sandstones deposited after rifting under conditions of associated lower heat flow showed considerably less diagenetic changes, particularly if burial was rapid.

The high heat flow associated with earliest rifting, associated fluid circulation driven by thermal convection, and slow burial rate controlled the diagenetic history of these sandstones. Thus, timing of sandstone deposition with rifting stage and associated burial rates were key factors in controlling sandstone diagenesis in back‐arc basins.
Characterization of pebble fabrics in modern terrestrial glacigenic sediments
Authors: DOWDESWELL JULIAN A. , SHARP MARTIN J.
Published: June 2006

ABSTRACT Pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers, where sedimentary processes can be observed or inferred with relatively little ambiguity. Over 100 samples from contemporary environments illustrate fabrics characterizing melt‐out till, deformed and undeformed lodgement till, sediment flow deposits and ice slope colluvium. Lodgement till fabric variability is related to the two‐layer structure of these sediments; a structureless, friable upper layer with low shear strength and high consolidation coefficient, overlying a very compact material of horizontal platy structure. Fabric strength (assessed by eigenvalue analysis) is weaker and pebble dip is more dispersed in the upper structureless horizon. Stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon. Alternatively, upper horizon fabrics may be characteristic of all recently‐deposited lodgement tills, with stronger fabrics developing at depth by dewatering and consolidation. There is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt‐out tills, through undeformed and deformed lodgement tills, to sediment flow deposits and ice slope colluvium. There is, however, considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills. Fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some Quaternary glacial sediments. Relatively strong fabrics characteristic of melt‐out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments.
The mineralogy and origin of some Silurian bentonites, Welsh Borderland, U.K.
Authors: TEALE C. TARQUIN , SPEARS D. ALAN
Published: June 2006

ABSTRACT Thirty clay‐rich horizons from the Wenlock and Ludlow Series of the Welsh Borderland were analysed to investigate their mineralogy and possible origins. Two mineralogical assemblages were determined: an assemblage of illite‐smectite and kaolinite of volcanic origin which was associated in many samples with an assemblage of illite and chlorite of detrital origin. Immobile chemical discriminants (Zr/TiO2 versus Nb/Y) infer an originally rhyolitic ash composition with a possible temporal progression, from the Wenlock to Ludlow, to more differentiated compositions. The thin nature of the bentonites and lack of juxtaposed subduction related calc‐alkaline magmatism suggests that the tuffs were derived from distant explosive eruptions from volcanic centres, possibly in NE Europe.
Cross‐strata and bedforms of probable transitional dune to upper‐stage plane‐bed origin from a Late Precambrian fluvial sandstone, northern Norway.
Authors: RØE SIGNE‐LINE
Published: June 2006

A Late Precambrian fluvial sandstone sequence in northern Norway is dominated by large‐scale cross‐sets that show either lenticular or tabular geometries in the streamwise sections. The lenticular sets interdigitate and in places show nearly symmetrical formsets. The tabular sets are in places solitary, but are mainly grouped in cosets. In both cross‐set types, the cross‐strata range from concave‐up to sigmoidal in shape, with the latter variety comprising subhorizontal to gently inclined topset strata (with parting lineation) that merge uninterruptedly downflow into the steeper (10–2°) foresets. Within the cross‐sets the geometry and dip azimuths of the foresets are conspicuously consistent, although the concave‐up and sigmoidal strata commonly alternate downcurrent.

The cross‐strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand. Both cross‐set types are interpreted as representing bedforms generated by flow in the dune to upper‐stage plane‐bed transition. The lenticular cross‐sets probably represent periodic dunes, but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes, or rather as solitary to quasi‐periodic bars.
A recent turbidity current event, Hueneme Fan, California: reconstruction of flow properties
Authors: REYNOLDS SUZANNE
Published: June 2006

A recent (100 yr old) turbidite is described from Hueneme Fan, California Continental Borderland. Dense sampling over the fan surface has allowed excellent delineation of the characteristics of this deposit. It exhibits Bouna DE sequences and has a distinctly bimodal, sandy silt grain size distribution. Through the use of generalized fluid dynamics equations, it is possible to reconstruct original flow properties of the current which deposited this material. The calculated velocities ranged from 10–90 cm s‐1 and excess density (above ambient seawater) from 0·001–0·005 g cm‐3 in the lower midfan and upper fan channel regions, respectively. Height of the current ranged from 5–15 m, on slopes from 1·5 to 0·15°. A total of 107 m3 of sediment was deposited during 10 days. The turbidity current is conjectured to have originated from direct river input during the floods of 1884.

An older event is also described, which has distinctly different properties and origins. The grain sizes of this older deposit are much coarser, and sedimentary structures suggest higher flow regimes. This turbidite is conjectured to have been deposited from a higher density, faster current thought to have been generated by slumping. The need for a better understanding of the controls on the characteristics of turbidity currents and their effect on fan morphology is emphasized.
Dust transport and the question of desert loess formation
Authors: TSOAR H. , PYE K.
Published: June 2006

Although contemporary dust storms are frequent in arid and semi‐arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid‐latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub‐tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re‐examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short‐term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long‐term suspension over a greater height range and may be widely dispersed.

The availability of silt and the frequency, magnitude and direction of dust‐transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid‐latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi‐arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert‐marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over‐estimate the longer‐term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi‐arid areas and is finer than typical loess.
Sedimentation at the margins of a late Pleistocene ice‐lobe terminating in shallow marine environments, Dundalk Bay, eastern Ireland
Authors: McCABE A. MARSHALL , DARDIS GEORGE F. , HANVEY PATRICIA M.
Published: June 2006

Late Pleistocene morainic sequences around Dundalk Bay, eastern Ireland, were deposited in a variety of shallow, glaciomarine environments at the margins of a grounded ice lobe. The deposits are essentially ice‐proximal delta‐fan and ‐apron sequences and are divided into two lithofacies associations. Lithofacies association 1 occurs as a series of morainal banks formed at the southern margin of the ice lobe in a body of water open to influences from the Irish Sea. The morainal banks consist mainly of diamictic muds deposited from turbid plumes and by ice‐rafting with minor occurrences of turbidites, cross‐bedded gravels (subaqueous outwash) and massive boulder gravels (high‐density debris flows). Lithofacies association 2 was deposited in a narrow arm of the sea at the north‐eastern margin of the ice lobe. The deposits consist mainly of a series of coalescing, ice‐proximal Gilbert‐type fan deltas which are interbedded distally with tabular and lens‐shaped subaqueous deposits. The latter are mainly ice‐rafted diamictons, debris‐flow deposits and subaqueous sands and gravels. Both lithofacies associations are draped by diamictons formed by a combination of rain‐out, debris flow and traction‐current activity. At a few localities the upper parts of the sequence have been sheared by minor oscillations of the ice sheet margin.

These sequences form part of an extensive belt of glaciomarine deposits which border the drumlin swarms of east‐central Ireland. Lithostratigraphic variability is partially related to the arrival of large volumes of debris at the ice lobe margin when the main lowland ice sheet surged during drumlin formation. Complex depositional continua of this type lack any major erosional breaks and should not be used either as climatic proxies or for stratigraphic correlations.
Petrological and isotopic implications of some contrasting Late Precambrian carbonates, NE Spitsbergen
Authors: FAIRCHILD IAN J. , SPIRO BARUCH
Published: June 2006

The dominantly shallow‐marine Vendian succession of NE Spitsbergen contains distinctive types of carbonate rock. Limestones deposited before Vendian glaciation resemble those described from other Upper Proterozoic successions, being high in Sr and inferred to have been originally aragonitic, including the distinctive 5–10 Jim equant polygonal calcite of cemented shrinkage cracks. In contrast, manganoan stromatolitic limestones within marginal‐marine glacial‐outwash deposits, and consisting of micrite, microspar and fascicular‐optic calcite are interpreted as originally calcitic. The restriction of primary marine calcite to cold seawater is comparable with Recent and Permian carbonates, although the Precambrian example formed in a sea diluted with meltwater.

There is good textural preservation of relatively 18O‐rich oolitic dolostones which were cemented in a supratidal environment by artesian fluids. Nevertheless, early diagenetic replacement is inferred, immediately prior to a glacial episode. Post‐glacial dolostones are either replacive marine, or evaporative lacustrine, but share rather more negative δ18O values, closer to the mean of Late Precambrian dolostones.

The heaviest oxygen isotope values constrain seawater δ18O to no more negative than — 2 to — 4SMOW. The main reason for the pronounced oxygen isotopic depletion of most Late Precambrian carbonates is their initial metastable mineralogy. The possibility of determining palaeolatitudes of the enigmatic widespread Late Proterozoic glaciations by isotopic analysis of freshwater periglacial calcareous precipitates is raised. Significant carbon isotope variations reflect changes in depositional water chemistry: some of these could be global in extent.
Petrographic and geochemical analysis of caliche profiles in a Bahamian Pleistocene dune
Authors: BEIER J. A.
Published: June 2006

Two caliche profiles from a Pleistocene carbonate dune on San Salvador Island, Bahamas, were examined by petrographic and geochemical analysis. Profile A is an immature buried caliche profile characterized by caliche pisolites, a friable crust and abundant Cerion. Profile B is a more well‐developed caliche profile at the top of the dune which contains abundant pisolites, rhizomorphs, laminated calcrete, a breccia and abundant Cerion.

Geochemical changes in caliche profiles relative to the host rock are an increase in Al2O3, Fe2O3 and total organic carbon, a decrease in Mg and Sr, and a decrease in δ13Ccarb, δ18Ocarb and δ13Corg. The magnitude of these changes is probably a function of the duration of subaerial exposure and resultant colonization by dune plants and associated microflora.

Abundance of calcified filaments and needle‐fibre crystals in profile A attests to the importance of microbial processes in the early development of caliche profiles. Biogenic structures are largely destroyed in profile B due to recrystallization, but indirect evidence of biological activity is retained in the form of carbon isotope values.
Sedimentation in ice‐covered Lake Hoare, Antarctica
Authors: NEDELL SUSAN S. , ANDERSEN DAVID W. , SQUYRES STEVEN W. , LOVE F. GORDON
Published: June 2006

The sedimentation mechanisms that occur in ice‐covered Lake Hoare, Antarctica are examined, to determine how sediment enters the lake, and how the sedimentation pattern affects blue‐green algal growth at the lake bottom. The 3 m‐thick ice cover contains pebbly sand as much as 2 m below the surface. Sediment with similar texture and mineralogy is found at the lake bottom. This evidence, together with the lack of sediment in the inflowing stream and the markedly different texture of sediment from the other terrains around the lake suggest that most of the sediment at the lake bottom comes in through the ice cover. Sand grains intermittently migrate through porous ice on the surface, water‐filled vertical gas‐channels penetrating two‐thirds of the ice cover, and possibly through cracks in the ice that act as conduits. The algae at the lake bottom are able to survive in part because sediment that comes through the ice cover does not obliterate them.
Selective gravel entrainment and the empirical evaluation of flow competence
Authors: KOMAR PAUL D.
Published: June 2006

The concept of flow competence is generally employed to evaluate velocities and bed stresses of river floods from the sizes of the largest sediment particles transported. For the most part, this evaluation has been empirical, combining data from a number of separate flood events in different river systems. Those data are re‐examined and compared with empirical equations for the selective entrainment of gravel from deposits of mixed sizes. It is found that the competence relationships trend counter to those obtained for selective entrainment, indicating that the competence evaluations are affected by varying degrees of selective size entrainment. Individual data sets which have been employed to establish the flow‐competence relationships either show no trend on their own or yield a trend which runs counter to the competence equation, instead being more compatible with the selective‐entrainment relationships. In most instances, the empirical competence equations greatly overestimate the hydraulics of flood flows, and it is suggested that the better established selective entrainment equations be used for competence evaluations as well. Empirical equations are available for this purpose, relating the dimensionless Shields entrainment function or the bed shear stress to the diameter of the largest grain moved and to the median diameter of the deposit as a whole.
The origin and tectonic significance of asymmetrical meander‐belts
Authors: LEEDER MIKE R. , ALEXANDER JAN
Published: June 2006

In the Hebgen lake area of SW Montana, the Madison and South Fork Rivers occupy the northeast sides of their respective meander‐belts. The form of the abandoned meander‐loops within these asymmetric meander‐belts indicates that they were produced by gradual northeast migration of the active channels. This progressive migration can be related to tectonic tilting produced by regional extensional faulting at a high angle to the original channel‐belt axis which ran down a gently sloping outwash cone.

Asymmetric channel sandstone bodies in the rock record can be recognized by the preferred dip of successive lateral accretion units in meander loops. Together with soft sediment deformation, facies and thickness variations they may indicate tectonic deformation of the sedimentary surface.
A Terra Rossa‐like paleosol complex from the Upper Jurassic of Portugal
Authors: WRIGHT V. PAUL , WILSON R. C. L.
Published: June 2006

The basal Upper Jurassic unconformity in the Lusitanian Basin of Portugal commonly exhibits an irregular palaeokarstic surface developed in underlying Middle Jurassic carbonates. In the Serra da Arrabida area south of Lisbon, wire cut quarry faces expose a thick Upper Jurassic paleosol complex overlying fissured and brecciated limestone veneered with a calcrete crust and associated with colluvial deposits. The paleosol complex is a red mudstone with calcrete stringers which superficially resembles present‐day red Mediterranean Terra Rossa soils. A detailed micromorphological study indicates the absence of any clay illuviation in the paleosol unit, which suggests that it is not comparable to the true Terra Rossa Alfisols, but more closely resembles present‐day Aridisols. This difference from true Terra Rossa soils probably reflects formation under a drier climate, which is confirmed by the occurrence elsewhere in Portugal of evaporitic lake deposits of the same age. The paper stresses the role of soil petrography (micromorphology) in interpreting pedogenesis in paleosols.
Sediment diffusion during overbank flows
Authors: PIZZUTO JAMES E.
Published: June 2006

Distinctive overbank sediments deposited since European settlement on the floodplain of the Brandywine Creek, Pennsylvania, are used to calibrate and test a diffusion model of overbank deposition. The predictions of the model can be calibrated to reproduce the topography of the post‐settlement lithosome with an average error of 7%. The model also correctly predicts the decrease in mean grain size away from the channel. The model greatly underestimates the ability of floodwaters to transport sand away from the channel. Apparently, sand is transported across the floodplain by bedload transport and by advective suspended sediment transport as well as by diffusion.

If flow duration data for 1912–1981 and the present rating curve for the Brandywine Creek at Chadds Ford, Pennsylvania, are assumed to apply throughout the post‐settlement period, the model may be used to estimate palaeohydraulic characteristics of post‐settlement floods. Calculations indicate that 212 post‐settlement floods covered the floodplain to an average depth of 1.6 m, transported an average excess suspended sediment concentration of 6200 ppm, and deposited an average thickness of 1.4 cm of sediment on levees next to the channel.
The importance of double diffusion to the settling of suspended material
Authors: GREEN THEODORE
Published: June 2006

The vertical fingering motions which can occur when warm, salty water overlies colder, fresher water are important to the vertical flux of salt in the ocean. An analogous and not uncommon situation is that where turbid water overlies clearer and denser water. For the case of an originally sharp horizontal interface between the two water layers (associated, as an example with an interflow due to a warm, muddy river entering a stratified lake), the resulting downward sediment flux is compared with that occurring by way of gravitational settling. Rules are proposed to ascertain when the fingering effect could be important to sedimentation. For example, it seems that fingering could be of consequence in the situation where the sediment has a diameter of 2 μm and the upper layer is about 1°C warmer than the (clear) lower layer, when the upper‐layer concentration is 1 ppm. The effect of fingering on the residence time of a gravitationally stable turbid layer is calculated. Such layers may be more short‐lived than commonly thought.
Burial dolomitization and porosity development in a mixed carbonate‐clastic sequence: an example from the Bowland Basin, northern England
Authors: GAWTHORPE ROBERT L.
Published: June 2006

Widespread dolomitization and leaching occur in the Asbian to Brigantian (Dinantian) sequence of the Bowland Basin. Within this mudrock‐dominated succession, dolomite is developed in calcarenites and limestone breccia/conglomerates deposited in a carbonate slope environment (Pendleside Limestone) and also within graded quartz wackes deposited by density currents in a generally ‘starved’ basin environment (Pendleside Sandstone). The dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature.

All stages of calcite cement pre‐date dolomitization and calcite veins are dolomitized. Dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites. Thus dolomitization was a late stage process within the carbonate diagenetic sequence. A late‐stage diagenetic origin is also indicated within the sandstones, with dolomite post‐dating the development of quartz overgrowths.

Six main textural styles of dolomite are observed: (1) scattered; (2) mosaic; (3) subhedral to euhedral rhombic; (4) microcrystalline; (5) single crystal and (6) saddle. The style of dolomite developed is dependent on the host rock mineralogy, on whether it is space‐filling or replacive and also on temperature. Chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole % FeCO3. Generally the dolomites have isotopic compositions depleted in δ18O compared to the host limestone, with similar or lighter δ13C values.

Initial dolomite was of the scattered type, but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced. There was a general increase in the Fe and Mn content and reduction in δ18O ratio of the crystals during dolomitization. Leaching is restricted to partly dolomitized horizons, where calcite, feldspars, micas, clays and, to some extent, dolomite have been leached. This has produced biomouldic and vuggy secondary porosity within the carbonates, whereas in the sandstones honeycombed, corroded and floating grains associated with oversized pores occur. Porosity within both carbonates and sandstones is reduced by ferroan dolomite/ankerite cements.

Field, petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks (Bowland Shales) during intermediate/deep burial. Fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure, the fluids migrating along the most permeable horizons—the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity.
Improved sampling of soft bottom sediments by combined box and piston coring
Authors: BLOMQVIST SVEN , BOSTRÖM KURT
Published: June 2006

Field tests demonstrate that sampling of soft bottom sediments can be significantly improved by combined box coring and shipboard sampling using hand‐operated piston coring. A convenient piston equipment devised for such subsampling is presented.
Glauconitization of detrital silica substrates in the Barton Formation (upper Eocene) of the Hampshire Basin, southern England
Authors: HUGHES ANDREW DAVID , WHITEHEAD DAVID
Published: June 2006

Upper Eocene detrital silica grains (chert and quartz) of the Hampshire Basin display alteration and replacement fabrics by glauconite. Silica grains have etched surfaces due to glauconitization which appear green in reflected light and thin section.

Quartz grains were glauconitized by surface nucleation and replacement, which spreads from the margin with progressive glauconitization, replacing the quartz grain interior. Chert grains were glauconitized by surface replacement and nucleation internally along cracks and in pores. Different forms of glauconite are associated with the two minerals; glauconite associated with quartz is generally highly‐evolved whereas glauconite associated with chert is of the evolved variety. This is interpreted as being due to different surface‐reaction control mechanisms associated with the two forms of silica. There is no evidence to suggest that glauconite evolved in stages from a nascent form.

Two crystalline morphological forms of glauconite are found associated with both quartz and chert. Glauconite growing within a confined space has a laminated morphology whilst glauconite occurring on the surface has a rosette morphology.
Geometries of Gilbert‐type deltas and large channels in the Meteora Conglomerate, Meso‐Hellenic basin (Oligo‐Miocene), central Greece
Authors: ORI GIAN GABRIELE , ROVERI MARCO
Published: June 2006

The Meteora conglomerate is a thick, coarse‐grained unit of Oligo‐Miocene age in the Meso‐Hellenic basin consisting of Gilbert‐type delta deposits (wedge‐shaped bodies) and deep channel fills. Gilbert‐type deltas prograded from the eastern margin basinward, perpendicular to the NW‐SE basin axis. Delta bodies are thicker in the distal portion, where the dip of the foresets is lower than in the proximal area. Organized and disorganized conglomerates coexist, suggesting a mixture of mass‐ and turbulent‐flow processes. The mass flow deposits seem to prevail in the uppermost part of the foresets. Channels (up to 25 m deep) are cut into the Gilbert‐type delta deposits, and have axes perpendicular to the progradational trend of the deltas. Giant bars displaying accretion of the downstream side formed inside the channels. The channels were probably cut into Gilbert‐delta deposits due to uplift of the source area or to a fall in sea‐level. Although large cross‐sets of both Gilbert delta and channels have been formed by progradation, they can be distinguished by: (i) angle of foreset bed dips, (ii) facies and textures and (iii) vertical sequences. Differences in these features aid in distinguishing Gilbert‐type deltas from in‐channel bars.