bluuebunny/crossref_metadata_2025 huggingface dataset:
The estimation of palaeohydrodynamic processes from subtidal deposits using time series analysis methods
Authors: YANG CHANG‐SHU , NIO SWIE‐DJIN
Published: June 2006

Tidal bundle sequences are the characteristic large‐scale cross‐bedded sets with mud drapes deposited by strongly asymmetrical bidirectional tidal currents. By means of time series analysis of the bundle thickness of such sequences, the palaeohydrodynamic processes can be reconstructed. This technique involves: (1) Fourier analysis to test the periodicity of the bundle‐thickness sequence and to estimate the periods and the phases of the most important periodic components; (2) filtering analysis to resolve the bundle‐thickness sequence into different components (i.e. diurnal components, random variations, neap/spring components and longer period variations) and also to estimate the relative importance (amplitude) of each component.

From these analyses, useful information can be derived as to the palaeotidal regime (whether semidiurnal or mixed) and also about the possible influence of non‐tidal processes, such as storms (their strength, duration, frequency and direction). Results from a subrecent and an ancient example show that tidal currents and storm‐induced currents are the most important hydrodynamic processes in the transport and deposition of sediment in these shallow marine environments.
Quartz sand grain shape and other criteria used to distinguish glacial and non‐glacial events in a marine core from Frobisher Bay, Baffin Island, N.W.T., Canada
Authors: DOWDESWELL JULIAN A. , OSTERMAN LISA E. , ANDREWS JOHN T.
Published: June 2006

Analyses of quartz sand grain shape, sediment influx rates and foraminifera define glacial and non‐glacial episodes in a 9.69 m core from Frobisher Bay, Arctic Canada. Five radiocarbon dates on organic matter provide a preliminary core chronology, with a basal date of 11,910 yr BP. Quartz sand grain morphology is measured for samples at seven core levels using: (1) Fourier shape analysis; (2) percentage of grain surface conchoidally fractured. Samples at 2.0 and 7.5 m are most fractured and have Fourier roughness coefficients similar to particles sampled directly from glacier ice. These two samples probably represent glacial events in the core. Major intervals of non‐carbonate sand influx occur at 9.0–4.5 m and 3.5–1.5 m, separated by several thousand years of slower sedimentation. Detrital carbonate influx rates are relatively high prior to 4.3 m, then decline rapidly indicating a shift in sediment provenance from limestones flooring Frobisher Bay to rafting from far‐travelled icebergs. Bio‐ and lithostratigraphic analysis allows definition of five core units: (1) an environment similar to today below 8.5 m; (2) glacial conditions from 8.5–6.8 m, associated with ice proximal to the core site; (3) ameliorating conditions from 6.8 to 3.2 m; (4) cooler conditions from 3.2 to 0.5 m, probably related to increased iceberg flux rather than neoglacial advances of nearby ice caps; (5) an environment similar to today from 0.5 m. Sand grains sampled at 7.5 and 2.0 m, whose shapes indicate they are derived from glacier ice, lie within the cooler or glacial units defined from foraminiferal analysis. This indicates that quantitative measurement of particle surface morphology can provide useful environmental information in studies of marine cores.
Why deposits of longitudinal dunes are rarely recognized in the geologic record
Authors: RUBIN DAVID M. , HUNTER RALPH E.
Published: June 2006

Dunes that are morphologically of linear type, many of which are probably of longitudinal type in a morphodynamic sense, are common in modern deserts, but their deposits are rarely identified in aeolian sandstones. One reason for non‐recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long‐term sand‐transport direction, thereby depositing cross‐strata that have unimodal cross‐bed dip directions and consequently resemble deposits of transverse dunes. Dune‐parallel components of sand transport can be recognized in ancient aeolian sands by examining compound cross‐bedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along‐crest direction over the large dunes.
Correlations between Holocene flood tidal delta and barrier island inlet fill sequences: Back Sound‐Shackleford Banks, North Carolina
Authors: BERELSON WILLIAM M. , HERON S. DUNCAN
Published: June 2006

Large, well‐developed flood tidal deltas on a barrier island coastline generally indicate a wave‐dominated, microtidal sedimentary regime. Vibracores in a lagoon behind the barrier island Shackleford Banks, North Carolina contain an upward fining sequence of coarse‐medium, very shelly sand, medium‐fine laminated sand, fine‐very fine cross‐laminated sand and marsh mud. This sequence is interpreted as being a flood tidal delta deposit based on analogy with modern flood tidal delta sediments and represents lagoonal deposition in response to a migrating or closing inlet. The sand facies defined in lagoonal vibracores is found to be continuous beneath a lagoonal marsh and correlative with inlet sections identified in Shackleford Banks drill holes. The correlation of flood tidal delta deposits with inlet sequences in this microtidal environment indicates a close relationship between barrier and backbarrier inlet controlled sedimentation.
The Ventersdorp Contact placer: a gold‐pyrite placer of stream and debris‐flow origins from the Archaean Witwatersrand Basin of South Africa
Authors: KRAPEZ BRYAN
Published: June 2006

Five local varieties of the Ventersdorp Contact placer at the East Driefontein Gold Mine, Carletonville, are distinguished by clast‐type assemblage, reflecting discrete provenances. The placer further comprises six lithofacies which are not restricted to specific provenance‐varieties. The six lithofacies are: massive, matrix‐supported conglomerate (facies Gms): massive, or crudely bedded, clast‐supported conglomerate (facies Gm); channel‐based conglomerate (facies Gt); single clast layers (facies Glag); horizontally stratified sandstone (facies Sh); and trough cross‐stratified sandstone (facies St). Facies Gms represents debris‐flow deposits, whereas the other facies are stream deposits. Lithofacies and provenance‐varieties are complexly interrelated in local environments, which are themselves complexly related to geomorphic elements of a degradation surface. The maximum thickness of the placer is 7 m. The placer was deposited on a hard, i.e. non‐alluvial, bedrock surface, suggesting that it is a pediment mantle. It owes its preservation to a capping of conformably overlying basalts.

Economic gold mineralization is present in all the conglomerate facies and all the provenance‐varieties. Gold concentrations are high in gravel‐bar deposits and channel fills but are not restricted to these environments. A major feature of gold concentration is its variability between provenance‐controlled varieties. The gold concentration of a specific primary source is interpreted to be a major control on gold concentration in the final deposit. The origin of the placer as a pediment mantle, and its subsequent preservation by lava‐flows, suggests that there was little potential for selective reworking of heavy minerals. Nevertheless, the Ventersdorp Contact placer on the Carletonville Goldfield is one of the richest gold placers in the Witwatersrand Basin.
Fluvial herring‐bone cross‐stratification in a modern tributary mouth bar, Coonamble, New South Wales, Australia
Authors: ALAM M. MUSTAFA , CROOK KEITH A. W. , TAYLOR GRAHAM
Published: June 2006

Herring‐bone cross‐stratification occurs in tributary mouth bar sediments less than 150 yr old in Warrena Creek near its confluence with the Castlereagh River some 2000 river kilometres from the sea in northern New South Wales. These streams have low gradients, with straight to anastomosing channels which become sinuous and distributive downstream. Channel beds are sand but banks are almost exclusively mud which is burrowed and extensively penetrated by roots. Herring‐bone cross‐stratification results from flow reversals in Warrena Creek during flood events. Flow direction depends upon discharge and stage in the creek relative to that in the adjacent river. The lithofacies resemble inter‐tidal deposits and could easily be misidentified on the basis of herring‐bone cross‐stratification in an ancient sedimentary sequence. Herringbone cross‐stratification should be regarded as diagnostic of depositional environments in which current directions are principally determined by reversals of water surface gradient, rather than by regional slope. Flow reversal phenomena may be a characteristic of very low gradient fluvial systems.
Successive aeolian saltation: studies of idealized collisions
Authors: RUMPEL DIETER A.
Published: June 2006

As observed by Bagnold and experimentally reconfirmed by other workers, the impact angles of saltating grains are remarkably constant over a wide range of conditions, lying between 10° and 16°. It can be shown that successive saltation contains a mechanism which very effectively confines impact angles to that range. This control mechanism is most effective at windspeeds less than about 15–30 m s‐1, depending on grain diameter and mass.

The control mechanism is evaluated from model calculations of grain populations saltating over a level bed consisting of a layer of loose grains. The grains are assumed to be spherical and uniform in size and mass, also rigid and perfectly elastic. The model also describes distributions of maximum height of grain paths and of lift‐off‐angles.

Compared to other processes involved in aeolian saltation, successive saltation is the only process with a high probability of transferring energy from horizontal into vertical grain movement.

This fact, together with the calculations presented, strongly suggests that successive saltation plays a major role in saltation in air.

Successive saltation of uniform grains is theoretically impossible if the ground over which saltation occurs is tilted by about 15° against wind direction. Values of tilt angles in this range are observed in nature as stoss‐side angles of dunes and ripples, leading to the concept that stoss‐sides are tilted up by deposition until successive saltation is subdued.
Chattermarked garnets found in soil profiles and beach environments
Authors: GRAVENOR C. P.
Published: June 2006

An examination of the surface features on garnets from soil profiles developed on glacial sediments shows that the percentage of chattermarked garnets either remains unchanged throughout the profile or may decrease in the A and upper part of the B horizons. It is possible that a decrease in chattermarked garnets up profile may result from dissolution of the garnets by humic and carbonic acids. Most etch features on garnets in soil profiles are inherited but fresh pits are found in the A and B horizon of a Sangamon soil profile.

An analysis of the surface features of garnets from 17 beach environments shows that the degree of rounding varies markedly, but on average, they are better rounded than those found in glacial deposits. Less than 1% of the garnets in beach deposits have chattermark trails. The results of these studies on garnets found in soil and beach environments confirm the usefulness of chattermarked garnets to authenticate and determine the nature of ancient glaciations.
Re‐evaluation of coquinoid sandstone depositional model, Upper Jurassic of central Wyoming and south‐central Montana
Authors: BRENNER ROBERT L. , SWIFT DONALD J. P. , GAYNOR G. C.
Published: June 2006

The most extensive Jurassic marine transgression in North America reached its maximum limits during the Oxfordian Age. At this time, siliciclastic sediments were being brought into the North American seaway from an uplifted zone to the west. Within this setting, complexes of sand ridges and coquinoid sands layers were deposited. Coquinoid sandstones appear to fill erosional scours and were interpreted as channel fills. Re‐evaluation of these features in the light of recently discovered attributes of modern shelf sediments and processes has produced a revised model of coquinoid sand deposition in this setting. Coquinoid sandstones which fill ‘channel‐like’ scours in the Oxfordian (Upper Jurassic) rocks of central Wyoming and south‐central Montana, appear to have formed through the migration of sand waves across the crests of inner shelf sand ridges during periods of storm and tidal flow. Erosion in the zone of flow reattachment in the troughs between sand waves resulted in the development of shell lags. Migration of these scour zones as the sand waves advanced resulted in the deposition of sheet‐like coquinoid sandstone bodies. Sand waves crossing the ridge crest tended to migrate more slowly and to be overstepped by later sand waves. Sand wave troughs thus buried have channel‐like geometries with apparent epsilon bedding.
A kinematic model for the structure of lee‐side deposits
Authors: HUNTER RALPH E.
Published: June 2006

A kinematic model for the structure of the lee‐side deposit of a dune‐like bedform, Gilbert‐type delta, or similar step is developed, based on the assumptions that initial deposition is entirely by grainfall, that the rate of deposition decreases as a power function of distance downflow from the brink of the slipface, and that the resulting steepening of the slipface is periodically interrupted by avalanching. The parameters used in the model are: (1) the deposition rate at a given distance from the brink, (2) the exponent in the equation relating the deposition rate to distance from the brink, (3) the bedform migration rate, (4) the bedform height, (5) the avalanche speed, (6) the angle of initial yield, and (7) the residual angle after avalanching. From these parameters can be calculated structural characteristics such as the proportions of bottomset and foreset deposits, the proportions of avalanche and grainfall deposits in the foreset deposit, and the spacing of avalanche‐grainfall couplets.

The model correctly predicts the trends of changing avalanche activity and changing structural character with changes in flow character, grain size, and bedform height in both air and water. Moreover, the model correctly predicts certain consistent structural differences between aeolian and subaqueous lee‐side deposits. Quantitative evaluation of the model requires more accurate data on the values of the input parameters than are presently available.
Fibrous gypsum and fibrous anhydrite in veins
Authors: MACHEL HANS‐G.
Published: June 2006

Fibrous calcium sulphate occurs in nature mainly as gypsum (satinspar) in extensional veins. Several mechanisms have been put forward to account for these veins and their fillings, but a number of features remain unexplained or are not in accord with the interpretation(s) given. This paper describes anhydrite as a fibrous vein precipitate, which shows most features typical of satinspar: subhorizontal attitude of the vein(s), vertical fibres, a parting separating a lower from an upper seam and incorporated wall rock fragments. New observations include the stacked nature of the fibres, pressure shadows next to wall rock fragments and parabolic alignment of the latter. These features suggest that the veins opened due to vertically‐oriented tensile stress. The vein‐filling crystals grew centrifugally outwards from the initial plane of rupture, which now forms the parting, keeping pace with the dilation. The mode of incorporation of wall rock fragments in the fibres requires repeated differential opening of the fissure. Thereby, either wall rock/fibre interface ruptured at a time, and statistically the ruptures alternated so that trace element trends across the vein(s) are roughly symmetrical to the parting. The source of the calcium and sulphate ions may have been adjacent evaporite beds, but an ‘external’ source is indicated where no evaporite beds are present. In contrast to previous interpretations, hydration of anhydrite to gypsum in the host rocks is not a prerequisite for the formation of satinspar veins. The exact causes for the vertical dilation remain somewhat speculative and may have been different in various locations. The vertically‐oriented tensile stress may have resulted from lateral (tectonic) compression, or hydraulic overpressure, or a combination of the two.
Gravity‐induced soft‐sediment deformation in glaciomarine sequences of the Upper Proterozoic Port Askaig Formation, Scotland
Authors: EYLES NICHOLAS , CLARK BRYAN M.
Published: June 2006

Large numbers of post‐depositional deformation structures in the form of downward penetrating sandstone bodies are identified on well‐exposed diamictite surfaces of the glaciogenic Upper Proterozoic Port Askaig Formation, Scotland. On the Garvellach Islands, downfolds, irregular dykes and a polygonal network of wedges are composed predominantly of massive, fine‐ to medium‐grained sandstone. These structures occur towards the top of crude coarsening‐upward glaciomarine sequences of massive diamictite, stratified diamictite, variably cross‐stratified sandstone and conglomeratic lags. Massive diamictites record the dominance of pelagic sedimentation and ice‐rafting; succeeding lithofacies indicate the increasing importance of marine traction currents. These sequences are repeated in the Port Askaig Formation and by comparison with Late Cenozoic glaciomarine sequences may have formed in response to changing water depths during basin subsidence. Downfolds, dykes and a polygonal wedge network appear to be genetically related expressions of subaqueous gravitational loading and intrusion of sand into low‐strength diamict acting in response to reverse density gradients created by coarsening‐upward glaciomarine sedimentation. Analogues are provided by published laboratory investigations. Analysis of the regional tectonic setting of the formation suggests the importance of seismic shock as a triggering agent.

The subaqueous deformation model presented in this paper is of considerable significance for reconstruction of Late Proterozoic palaeoenvironments because the downward penetrating sandstone structures of the Port Askaig Formation are widely reported to be indicative of the former presence of subaerial permafrost. This paper stresses the importance of identifying the lithofacies sequence in which structures occur as a guide to ‘deformational environment’.
The record of density‐induced underflows in a glacial lake
Authors: WEIRICH FRANK
Published: June 2006

As part of an overall study of sedimentation processes in a proglacial lake an effort was made to compare field results with some of the general equations for density flows. The results suggest that in relatively small glacial lakes the occurrence of underflows with lower sediment loads involves a complex interplay between thermal and sediment effects which is extremely sensitive to varying hydrologic and climatic conditions. In terms of actual transport mechanics the results: (i) indicate that a higher α value of 0·6 or 0·7 gives a closer agreement between the measured velocity values and the established equations on moderately shallow slopes; (ii) provide field support for the experimentally derived relationship of Britter & Linden (1980) for the velocity of underflows and suggest the equation may be applicable in situations below 5° slopes; and (iii) support the relationship between velocity of the front and body of a continuous underflow for moderate slope situations suggested by Middleton (1966b). Finally the velocity values measured by electromagnetic current meters stationed in the lake, the grain‐size data obtained from mapping core data, and the application of other criteria support the concept that in this environment the underflows are capable of erosion.
Slump blocks, intraformational conglomerates and associated erosional structures in Pennsylvanian fluvial strata of eastern Canada
Authors: PLINT A. G.
Published: June 2006

Pennsylvanian fluvial channel sandstones in New Brunswick and Nova Scotia contain numerous examples of eroded mudstone surfaces, including in situ mudstone beds, boulders and slumped blocks. The eroded surfaces bear a variety of structures including linear scours, flutes, longitudinal furrows and rill marks. A block of interchannel mudstone up to 40 m in extent, displays a basal slip‐plane, slump‐related deformation and evidence of intense corrasion on a channel floor. Mudstone clasts from small pebbles to boulders over 4 m long are common immediately above channel‐base erosion surfaces and represent a lag. Clasts over 20 cm diameter are commonly fluted, occasionally on all sides, suggesting clast rotation. Rill marks occur on large mudclasts and in situ mudstone surfaces and indicate emergence and erosion by surging water or surface runoff. Preservation of the delicate erosional structures depended on a highly cohesive mud substrate and subsequent rapid burial. A previous interpretation of the mud blocks and their surficial features as the result of mud intrusion is inconsistent with the field evidence.
Hummocky cross‐stratification in the surf zone: flow parameters and bedding genesis
Authors: GREENWOOD BRIAN , SHERMAN DOUGLAS J.
Published: June 2006

Primary sedimentary structures exhibiting the diagnostic criteria for single sets of hummocky cross‐stratification (Harms et al.) have been found in the surf zone of a storm‐wave dominated coastline in the Canadian Great Lakes. Epoxy peels of box cores (0.45 m × 0.30 m) reveal hummocky stratification in well‐sorted, fine‐grained sands in water depths less than 2 m under conditions of wave breaking and strong longshore currents. The wavelengths of the hummocks (0.3–0.6 m) are somewhat smaller than the norm for their ancient analogues, but the ratios of length to height (8–12) are comparable. Depth of activity rods have been used to identify those hummocks that formed during sediment transport events when the near‐bed currents were recorded directly using electromagnetic flowmeters. Results from such experiments clearly identify the hummocky stratification as being produced by an actively growing bedform with little or no lateral migration. Hummocks occur under conditions close to that expected for the upper flat bed. In one vertical sequence, the hummocky cross‐stratification is underlain by subhorizontal, planar lamination and overlain by undulatory lamination which grades upward into small‐scale, trough cross‐lamination of wave ripple origin. This sequence was associated with a single storm and would appear to represent a combined‐flow regime sequence with the hummocky structure representing a post‐vortex (?) ripple bedform. At the inferred time of hummock formation, near‐bed oscillatory flows were dominant and reached maxima of 1.1 m s −1 with a superimposed longshore current of 0.27 m s−1. Rapid sedimentation associated with vertical growth of the hummocky bedform was triggered by a significant reduction in the orbital currents (by 19%) and'steady'currents (by 67%) while the total bed shear remained high.
Fluvial channel and overbank deposits from the Westphalian of the Durham coalfield, NE England
Authors: FIELDING C. R.
Published: June 2006

The Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England were deposited in lower and upper delta plain environments. Distributary channels crossed the plain and were separated by shallow, interdistributary lakes and bays. Detailed observation of three‐dimensional (3‐D) opencast (surface) mine exposures, in collaboration with subsurface borehole analysis, has revealed the existence of five varieties of channel deposits and two associated overbank facies within the Durham Coal Measures.

Major distributary channels were the major avenues of sediment transport across the Coal Measures plain, were variably sinuous, mostly 1–2 km wide and deposited elongate belts of sand mostly up to 5 km wide. Proximal, major crevasse splay channels formed by the breaching of major channel banks during flood events, were straight, sand‐filled and up to 400 m wide. Minor distributary channels formed by the sustained operation of such crevasses, varied from straight to highly sinuous, and deposited ‘shoestring’ sand/mud belts up to a few hundred metres wide. Minor crevasse channels, generally straight and up to 50 m wide, were formed through bank breaching of minor, and in a few cases major, distributaries. Distal feeder channels formed down‐palaeocurrent extensions of minor distributaries which supplied interdistributary minor delta subsystems, were generally straight and up to 200 m wide.

Of the two types of channel overbank (levee) deposits recognized, one, comprising thinly interbedded fine‐grained sandstone and siltstone/claystone, is mostly, though not exclusively, associated with major distributary channels. The other, consisting of ‘massive’ siltstones with regularly spaced, thin claystone bands, is uniquely developed at the margins of minor distributary channels.

The lower part of the Westphalian A succession in the northern Pennines records a change in the depositional environment upwards from a lower to upper delta plain. Through this transition, major channel deposits show evidence of having evolved from being of dominantly low sinuosity to being more variable in morphology. Channel sedimentation was profoundly influenced by regular, possibly seasonal, variations in flow stage and sediment load.
Facetted garnets formed by etching. Examples from sandstones of Late Triassic age, South Germany
Authors: BORG GREGOR
Published: June 2006

Detrital garnets from the Upper Triassic (Keuper) of southern Germany have well developed facets where they occur in calcite‐cemented sandstone. The grains belong to the pyralspite series and have a homogeneous chemical composition, without any zonation is revealed by microprobe analyses. It is concluded that the large facets are caused by etching during diagenesis and not by authigenic overgrowth. High calcium contents of the pore fluids may have been important in the formation of these garnet facets.
Numerical computation of co‐oscillating palaeotides in the Catskill epeiric Sea of eastern North America
Authors: SLINGERLAND RUDY
Published: June 2006

A numerical model describing two‐dimensional, shallow water, long wave propagation has provided estimates of the co‐oscillating palaeotides in the upper Devonian Catskill Sea. This sea was open to the ocean along the present Gulf Coast and, moving clockwise, was circumscribed by the Transcontinental Arch, the Old Red Sandstone Continent, and the rising Appalachian Orogen. Using the present best estimates of basin bathymetry and open‐ocean tidal range, the model indicates high mesotidal to low macrotidal ranges for the Catskill Shelf along the ancestral Appalachians. The Mid‐Continent Shelf, centred on Iowa, is considered to have been microtidal. These conclusions are not sensitive to reasonable variations in shelf geometry and support the hypothesis that at least some epicontinental seas could have been tide‐dominated.
The role of fungal biomineralization in the formation of Early Carboniferous soil fabrics
Authors: WRIGHT V. PAUL
Published: June 2006

Paleosols in the Lower Carboniferous limestones of South Wales commonly contain needle‐fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils. The needle‐fibre calcite occurs in two associations in the paleosols: as coatings on sediment grains and as rhizocretions. The former can be compared with the microbial grain coatings of Quaternary calcretes. The latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae, a symbiotic fungal sheath‐root association. These findings suggest that biomineralization was important in the formation of soil fabrics during the Carboniferous as it is in present day soils.
Push‐moraines and glacier‐contact fans in marine and terrestrial environments
Authors: BOULTON G. S.
Published: June 2006

ABSTRACT The local climatic regime and the mass balance state are important determinants of the dynamics of terrestrial and marine glacier fronts, which in turn determine the sediments and landforms produced at the glacier front. Many modern glaciers undergoing overall retreat in areas of‘maritime’climate produce winter push moraines during a late winter readvance, followed by a summer retreat, whilst in more‘continental’regions no significant winter readvance occurs and annual push‐moraines are absent. The frontal dynamics which lead to these changes are analysed and the form, structure, sequence and field relations of both terrestrial and marine push‐moraines are described from Iceland, Spitsbergen and Baffin Island.

Long‐term changes in mass balance leading to major glacier advances or readvances also generate large push‐moraines. In terrestrial environments push‐moraine formation is accompanied by uplift, rejuvenation and down‐cutting of outwash systems whose sediments become closely associated with glaciotectonic structures, which permit pre‐, syn‐ and post‐tectonic sequences to be identified.

The development of ice marginal fan/moraine complexes is modelled as a function of the relative magnitude of two parameters: the velocity of ice movement and the calving rate. A high ice velocity just exceeded by the calving rate gives closely spaced push‐moraines and confluent ice marginal fans. A high velocity far exceeded by the calving rate produces closely spaced moraines but separate ice marginal fans. A low ice velocity in combination with a high calving rate results in well separated and feebly developed push‐moraines, while a low ice velocity and a low calving rate produces feeble push‐moraines and coalescent fans.
Diagenesis of Tertiary playa sandstones of Northern Chile; implications for Andean uplift and metallogeny
Authors: FLINT S.
Published: June 2006

The Oligo‐Miocene Pacencia Group of Antofagasta Province, northern Chile consists of over 2 km of molassic sediments deposited in a series of alluvial fan and playa sub‐environments. Sandstones of the Artolla Member, exposed around San Bartolo, represent playa‐marginal sandflat deposits and locally host stratiform copper mineralization. A detailed study of these sediments has revealed a complex sequence of diagenetic modifications to the arkosic detrital mineral assemblage. Early dissolution of unstable ferro‐magnesian minerals and calcic feldspar resulted in the release of ions into intra‐stratal solution and the subsequent formation of a suite of eogenetic authigenic minerals.

These include early hematite coatings on framework grains, pore‐lining zeolite and pore‐filling calcite, gypsum and celestite cements. Syntaxial overgrowths of albite, quartz and less common K‐feldspar are well developed. Following early oxidative intrastratal conditions, the influx of acidic fluids derived from mudrocks during mesogenesis resulted in an important secondary porosity generation and the establishment of an acidic, reducing intrastratal environment. This facilitated the subsequent introduction of low temperature mineralizing solutions and formation of the San Bartolo copper deposit.

A return to oxidative diagenetic conditions is recorded by late stage overgrowths of K‐feldspar and quartz and the oxidation of native copper/chalcopyrite cements to cuprite/tenorite and chalcocite/covellite assemblages. This final telogenetic modification to the mineral assemblage is related to flushing by meteoric waters and provides independent evidence for an important phase of Andean deformation and uplift in the middle to upper Miocene.
Proterozoic ministromatolites with radial‐fibrous fabric
Authors: HOFMANN HANS J. , JACKSON GARTH D.
Published: June 2006

Small digitate stromatolites with diameters in the range of 0·2–20 mm (ministromatolites) are common in Early and Middle Proterozoic carbonate sequences, and extend stratigraphically from the Archaean to the Holocene. An occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial‐fibrous fabric and microscopically crinkled (microcrenate) lamination is described from the ˜ 1·9 Gyr old Belcher Supergroup (McLeary Formation) in southeastern Hudson Bay, Canada. The structures, which can be considered to be a variety of tufa, are unusually well preserved because of early diagenetic silicification. Columnar types are referable to Pseudogymnosolen (Asperia), and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern Canada, where the fabric is normally preserved by a secondary mosaic of dolomite. The textural evidence of angulate cross‐sections and rectilinear divergent patterns indicates that the radial‐fibrous fabric represents primary or very early diagenetic precipitation, and that pseudogymnosolenids with mosaic dolomite originally also had radial‐fibrous structure. The precipitation may have been within, or on, microbial mats.
Calcium‐poor dolomite from the sabkhas of Kuwait
Authors: GUNATILAKA A. , SALEH A. , AL‐TEMEEMI A. , NASSAR N.
Published: June 2006

Small quantities of a rare Ca‐deficient dolomite have been found coexisting in thin layers and alternating with the normal Ca‐rich variety in the Holocene sabkhas of southern Kuwait. The Ca‐poor dolomite has a molar composition of Ca46–49, whereas the Ca‐rich type is Ca51–56. The former type has been found only in two localities of the supratidal zone that are not subjected to tidal flooding today, and mostly within the fluctuating ground‐water table in these zones. The thin layers are either carbonate mud‐rich or pellet‐rich, and the amount of Ca‐poor dolomite is never more than 4 to 6 per cent by weight of the total carbonate fraction. The dolomites vary in size between 2 μm and 5 μm and exhibit characteristic rhombic crystal morphologies.
Initiation of motion in overland flow
Authors: GOVERS G.
Published: June 2006

This article reports the results of an experimental study concerning the initial movement of sediment in overland flow. Experiments were carried out in a small flume using water and quartz materials of various grain sizes.

The comparison of our experimental results with classic criteria showed that the Shields‐curve and the criterion of Yang can also be used for turbulent overland flow. Generally, higher shear stresses are necessary to provoke bed‐instability under laminar flow conditions, especially at higher shear‐velocity particle Reynolds numbers. A relative roughness effect also exists in laminar flow. Contrary to the observations by Savat on fixed beds, laminar flow does not show an ‘uncommon selectivity’, when loose, homogeneous beds are considered.
Coarse‐grained sediment gravity flow facies in a large supraglacial lake
Authors: EYLES NICHOLAS , CLARK BRYAN M. , CLAGUE JOHN J.
Published: June 2006

Near Williams Lake, in the central interior of British Columbia, the Fraser River exposes long sections of late Pleistocene glaciolacustrine sediments selectively preserved within a bedrock trough. The dominant facies types are thick, normally graded gravels and sands that occupy steeply dipping multistorey channels up to 300 m wide and several tens of metres deep. Channels appear to have been simultaneously cut and filled by high density turbidity currents in a glacial lake floored by stagnant ice. Fining upward sediment gravity flow sequences up to 50 m thick may be the product of quasi‐continuous ‘surging’ turbidity flows triggered by catastrophic meltwater discharges into the trough or retrogressive failure of ice‐cored sediments. Large‐scale post‐depositional deformation structures, such as synclinal folds, normal faults, sedimentary dyke swarms and dewatering structures, record gravitational foundering of sediment and pore‐water expulsion caused by the melt of underlying glacier ice. Melting of buried ice masses along the floor of the trough appears to have controlled the flow paths of turbidity currents by producing sub‐basins within the overlying sediment pile. An idealized model of ‘supraglacial’ lacustrine sedimentation is developed that may be applicable to other glaciated areas with similar bedrock topography.
Subtidal occurrences of ladder‐back ripples: their significance in palaeo‐environmental reconstruction
Authors: REDDERING J. S. V.
Published: June 2006

Ladder‐back ripples are common on modern intertidal flats and have been thought to be diagnostic of deposition in this environment. Recognition of superimposed, asymmetric ladder‐back ripples in a subtidal back‐barrier estuary and superimposed, symmetric ladder‐back ripples in an embayed, shallow marine environment indicates that these features are not diagnostic of deposition in the intertidal zone only, but can occur in other depositional environments, produced by different mechanisms and on different scales. The presence of ladder‐back ripples in the subtidal zones challenges aspects of a model used to determine the palaeotidal range of ancient tidal sequences.
Hummocky cross‐stratification, tropical hurricanes and intense winter storms
Authors: SWIFT DONALD J. P. , NUMMEDAL DAG
Published: June 2006

Duke (1985b) argues that ‘most examples (of hummocky cross‐stratification) were formed by tropical hurricanes.’ His statement is based on the assumption that ‘hurricane‐generated surface gravity waves form powerful oscillating or multidirectional flows at the water‐sediment interface which do not possess a significant unidirectional component.’ It is true, as one of us has previously stated, that hurricanes are rapidly‐moving, short‐lived, localized, and infrequent systems as compared with mid‐latitude storms; midlatitude storms are consequently more efficient in coupling with the shelf water‐column than are hurricanes. However, Duke's argument that hummocky cross‐stratification may be the result of purely oscillatory flow is untenable. His reasoning contradicts established theory about oscillatory bedforms, and his numerous examples of hummocky cross‐stratification come largely from continental shelf settings where the storms (tropical or otherwise) would have created concurrent alongshelf undirectional flow as well as wave oscillatory motion. There is no theoretical or observational basis for the belief that water movement on the sea‐floor during hurricanes is qualitatively different from water movement during mid‐latitude storms. Consequently, hummocks are no more liable to form beneath hurricanes than they are beneath mid‐latitude storms.
The carbonate compaction law: a new tool
Authors: RICKEN WERNER
Published: June 2006

Carbonate content, compaction, and porosity are evaluated from a large number of samples using micritic marl‐limestone alternations from Germany, France, and Italy. Compaction is measured directly by utilizing both deformed, originally cylindrical bioturbation tubes and steinkerns of ammonites. Additionally, a method is developed to indirectly evaluate the total compaction of the rock matrix by using early, selectively cemented burrows and fossils. Plots representing measurements of compaction versus carbonate content display distinctly non‐linear relationships which show increasing compaction with decreasing carbonate content. These relationships are found to clearly correspond with the carbonate compaction law. The compaction law is introduced as a theoretical derivation for sediments and rocks which calculates the carbonate and non‐carbonate content, compaction, and porosity. It is based on the assumption that the non‐carbonate fraction standardized to the primary sediment‐volume remains constant during carbonate diagenesis. The compaction law is used to calculate the most commonly found sediment/rock transformations (e.g. mechanical compaction, cementation, and chemical compaction) and to simulate the diagenetic processes of given examples.

Decompaction formulas are developed to evaluate the porosity and carbonate content of the primary sediment. An example of calculating decompaction and determining the original composition of the sediment is given utilizing carbonates spanning the Cretaceous‐Tertiary boundary at Gubbio (Italy).
Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)
Authors: BUSTILLO M. A. , RUIZ‐ORTIZ P. A.
Published: June 2006

In Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro‐ and cryptocrystalline quartz, with local chalcedonic quartz (both length‐fast and length‐slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro‐ and cryptocrystalline quartz, length‐slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg‐rich environment.

Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica‐rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.
Authigenic quartz and albite in Devonian limestones: origin and significance
Authors: MOLENAAR N. , de JONG A. F. M.
Published: June 2006

Euhedral quartz and albite crystals are common in Devonian (Givetian‐Frasnian) shallow‐marine shelf carbonates from the Belgian Ardennes. Several features such as morphology, the presence of carbonate inclusions, inversion temperatures and occurrence in the insoluble residues of stylolitic surfaces indicate that these crystals have developed authigenically. Oxygen isotope ratios point to an intermediate deep burial realm of origin at temperatures of 60–90°C. The predominance of illite and the almost total absence of smectite clay minerals is interpreted as an indication that illitization produced the silica needed for authigenesis. The mineral composition of inclusions indicates that the carbonate host rock must have consisted of low‐Mg calcite already at the time of authigenesis. These inclusions represent an earlier diagenetic stage than the present carbonate rock, since they were protected from further diagenetic alteration by the surrounding quartz. The calcite inclusions display a higher Sr/Ca and Mg/Ca ratio than the carbonate host‐rock. Because neomorphic diagenesis of the carbonate continued after silicate authigenesis, the contents of Mg and Sr in the calcite of the host carbonate are even lower. The authigenic feldspar mineralogy seems to be determined by the composition of the host‐sediment.
Sedimentology of an ancient erg margin: the Lower Jurassic Aztec Sandstone, southern Nevada and southern California
Authors: PORTER M. L.
Published: June 2006

The Lower Jurassic Aztec Sandstone is an aeolian‐deposited quartzose sandstone that represents the western margin of the southerly‐migrating Navajo‐Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments.

In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue‐shaped aeolian sand bodies interstratified with fluvially‐deposited coarse sandstone and mudstone. Above, evaporite‐rich fine sandstone and mudstone are overlain by thick, cross‐stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west.

Thin aeolian sequences are interbedded with volcanic flow rocks, ash‐flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian‐deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc‐derived sediment.

A two‐part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross‐strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed the fore‐erg, is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian‐deposited sediment encased in fluvial and sabkha deposits. Continued sand‐sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thick central‐erg sediments buried the fore‐erg deposits. The trailing, upwind margin of the erg is represented by back‐erg deposits in northern Utah and Wyoming.
Sedimentology of a marine intermontane Pleistocene Gilbert‐type fan‐delta complex in the Crati Basin, Calabria, southern Italy
Authors: COLELLA A. , DE BOER P. L. , NIO S. D.
Published: June 2006

The Crati Basin is a Pliocene‐Holocene extensional basin filled by the progradation of different types of marine fan‐delta systems. Coarse‐grained Gilbert‐type fan‐deltas developed during the Pleistocene. They represent the sedimentary response to a strong differential uplift involving the basin margins and the basin itself. The differential uplift was responsible for the fragmentation of the basin into several sub‐basins, into which these fan‐deltas prograded. The protection and lateral confinement by structural highs, steep coastlines and the absence of strong tidal action in the adjacent Ionian Sea, allowed the regular progradation of these fan‐deltas in the restricted gulfs and narrow embayments of the Crati Basin. For the classical Gilbert‐type (fan) deltas in lacustrine settings, homopycnal inflow favours a rapid mixing of water masses and deposition of sediment close to the river mouth. In the case of the example described here, the density contrast between the sea water and the inflowing river water caused the separation of the muddy fraction from the coarse sandy and conglomeratic part of the sediment. This allowed the development of steep mud‐poor coarse‐grained delta foresets.

Slope instability features (slump scars, conglomeratic flow slides) are fairly scarce in the proximal parts of the San Lorenzo del Vallo system. Towards the north, where protection from the Ionian Sea was less, they increase in importance.