bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

ABSTRACTAnalyses 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

ABSTRACTDunes 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

ABSTRACTLarge, 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

ABSTRACTFive 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

ABSTRACTHerring‐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

ABSTRACTAs 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

ABSTRACTAn 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

ABSTRACTThe 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

ABSTRACTA 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

ABSTRACTFibrous 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

ABSTRACTLarge 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

ABSTRACTAs 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

ABSTRACTPennsylvanian 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

ABSTRACTPrimary 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.