bluuebunny/crossref_metadata_2025_split huggingface dataset:
Giant flute‐like scour and other erosional features formed by the 1929 Grand Banks turbidity current
Authors: SHOR ALEXANDER N. , PIPER DAVID J. W. , CLARKE JOHN E. HUGHES , MAYER LARRY A. 
Published: June 2006

ABSTRACTErosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side‐scan sonar images and Seabeam multi‐beam echo‐soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100‐m‐deep flute‐shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic‐reflection profiles show that the flute‐shaped scour cuts through this conglomerate and into Plio‐Pleistocene valley‐floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel‐floor erosion was inhibited by the conglomerate veneer.Valley‐floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel‐floor erosion, increasing the volume of sediment transported by large turbidity currents.
Synsedimentary dolospar cementation: a possible Devonian example in the Camsell Formation, Northwest Territories, Canada1
Authors: MORROW DAVID W. 
Published: June 2006

ABSTRACTA unique association of beds containing fenestral pore‐filling medium crystalline dolomite with beds of dolospar and quartz sandstone occurs in the Lower Devonian Camsell Formation. Many of these sandstones consist almost entirely of dolospar sand. Sedimentological, petrographic, cathodoluminescent and chemical data indicate that the dolospar sands consist of grains that have been eroded and reworked during Camsell deposition from fenestral dolostone beds that directly underlie these sandstones. Erosional truncation of individual dolomite crystals and of intracrystalline cathodoluminescent zonations along the edges of sand grains indicates that either dolomitization, or dolomite cementation of the pore‐filling carbonate cements within fenestrae of the fenestral fabrics, occurred before erosion and deposition of dolospar sand. The fine details of the cathodoluminescent zonations suggest that the pore‐filling fenestral dolomite originated as primary cements that underwent some annealing recrystallization under the influence of high subsurface temperatures. This unique occurrence may document synsedimentary Devonian cementation by medium to coarsely crystalline dolomite.
Formation of siderite‐Mg‐calcite‐iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England
Authors: PYE K. , DICKSON J. A. D. , SCHIAVON N. , COLEMAN M. L. , COX M. 
Published: June 2006

ABSTRACTConcretions cemented mainly by siderite, Mg‐calcite and iron monosulphide are common in late Holocene marsh and sandflat sediments on parts of the north Norfolk coast. Field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate‐reducing bacteria are active. δ13C values ranging from −3 to −11·8% (mean −5·9%0) suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter. δ18O values ranged from −6·4% to + 0·8% (mean −1·0%) suggesting that carbonate precipitated in porewaters ranging from pure sea water to‐sea water diluted with meteoric water. Chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation. Some concretions have formed around fragments of wood or metal, but others contain no apparent nucleus. In field experiments siderite, FeS and Mg‐calcite were precipitated around several different nuclei within a period of six months. We suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction, such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron.
Particle over‐passing on depth‐limited gravel bars
Authors: CARLING P. A. 
Published: June 2006

ABSTRACTAn experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of ‘hump‐back’ bars and along the top of planar gravel sheets. Hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like‐sized and finer particles. For prescribed conditions, flat upper‐stage plane sand‐beds develop over the crestal location with pebbles rolling easily over the sandy bed. At the brinkpoint, flow separation ensures effective segregation of the gravel from the sand. Over the slip‐face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed‐form. Consequently, distinctive vertical assemblages of open‐work and closed contact framework gravels could be generated as another bar migrates over, and preserves, the initial structure.In respect to the observed mechanisms of sorting over the bars, a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper‐stage plane‐bed conditions on the crestal platform. The model then is used to ascertain whether the depositional environment ascribed to certain facies in the Bunter Pebble Beds, described in a recent publication, is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particle‐segregation process.
Thermoluminescence properties of fiord sediments from Engelskbukta, western Spitsbergen, Svalbard: a new tool for deciphering depositional environment?
Authors: FORMAN STEVEN L. 
Published: June 2006

ABSTRACTThis study explored the relationship between high‐Arctic fiord depositional environments and the natural thermoluminescence (TL) signal of sediments. The energy and duration of light exposure during transportation and deposition controls the TL level of silicate mineral grains in the sediment. The TL signal of sediments rapidly decreases within c. 0·5 km of a glacier sediment source. The highest TL levels are from tills and ice‐proximal glacial‐marine sediments, which receive little or no light exposure during transportation and deposition. Intermediate and consistent TL levels are recorded for ice‐distal glacialmarine muds, c. 0·5–5·0 km from the glacier front, reflecting slower sedimentation rates. The lowest TL levels are for littoral and sublittoral sediments which receive extended light exposure with shoaling. The granulometry of the sediments is fairly homogeneous and is not diagnostic of a sedimentary environment with most samples dominated by silt and clay; littoral and ice‐proximal samples exhibit peak abundances in sand. These results suggest that the relative TL signal of sediments is sensitive to a depositional environment, particularly for environments proximal (within 0·5 km) to a glacier terminus and in shallow water, less than 15 m deep.
Regressive and transgressive sequences in a raised Holocene gravelly beach, southwestern Crete
Authors: POSTMA GEORGE , NEMEC WOJCIECH 
Published: June 2006

ABSTRACTBoth a modern and a raised, late Holocene gravelly beach have been studied along the cliffed coast near the village of Chora Sfakion in southwestern Crete, Greece. The Holocene beach shows the record of regional, well‐documented relative sea‐level changes, in the form of regressive and transgressive sequences. The texture, fabric and depositional architecture of these sequences are described and compared with those of the modern, microtidal beach there. The depositional history of the raised beach is discussed in terms of the sediment transport processes and relative sea‐level changes.The regressive sequences of both the modern and the Holocene beach resemble those of the mesotidal beaches in the Sker district of southwest Wales, described by Bluck (1967). The transgressive sequence of the Holocene beach resembles the mesotidal beach sequence near Newton (southwest Wales), recognized by Bluck as another type of progradational (regressive) beach. Therefore, transgressive beach sequences in the stratigraphic record might easily be overlooked or misinterpreted as regressive. The stratigraphic depositional patterns of gravelly beach systems are, apparently, still poorly understood.
Reflected sediment gravity flows and their deposits in flysch of Middle Dalmatia, Yugoslavia
Authors: MARJANAC TIHOMIR 
Published: June 2006

ABSTRACTThe Eocene flysch of Middle Dalmatia comprises several beds that are interpreted to have been deposited from reflected sediment gravity flows. Their compositions are similar and two bed types are differentiated: complex beds that are debrite‐plus‐turbidite couplets, and turbidites.The sequence alternations in the turbidite part of the bed, opposing ripples within the same bed, and opposite flow directions indicated by flutes and ripples are indicative of flow reflections. The influence of seiches is suggested by the occurrence of symmetrical (oscillation) ripples.The palaeotransport directions of reflected flows show wide dispersal. A geometry of small, fault‐controlled sub‐basins with centripetal palaeotransport patterns is proposed.
Facies‐controlled shrinkage‐crack assemblages in Middle Proterozoic mudstones from Montana, USA
Authors: KIDDER DAVID L. 
Published: June 2006

ABSTRACTShrinkage‐crack morphotypes in the Libby Formation (upper Belt Supergroup) are confined to distinct environmental facies. The lower facies is characterized by flat rip‐up clasts, stromatolites, oolites, small‐scale symmetrical ripples, and fenestral fabric. These rocks were deposited above fair‐weather wave base on a periodically exposed mudflat. Shrinkage cracks in this facies are predominantly branching, incompletely connected features in plan view, except for local examples of completely connected polygonal cracks on purple argillite bed tops and rare, long spindle‐shaped cracks on bed tops of dark grey argillite. The upper facies was deposited below fair‐weather wave base and contains mainly unconnected, short spindle‐shaped shrinkage cracks, and rare slightly branching cracks.Restriction of some crack types to certain facies better constrains interpretation of the origin of these shrinkage cracks. The cracks in the upper facies were strongly influenced by sediment loading, and may have formed by compaction‐induced expulsion of water from pore space, resulting in synaeresis cracks. In the underlying shallower facies, polygonal cracks formed by desiccation. Elsewhere in this facies, incomplete, partially connected cracks and long spindle‐shaped cracks on the same bedding plane are interpreted as having formed by desiccation.Shrinkage cracks are an under‐used source of environmental information, but confusion as to their origin sometimes restricts their potential. More intensive analysis of properties of host sediment and crack fills may further our understanding of depositional and diagenetic influence on crack morphology. Crack cross‐sections, which are often more commonly exposed than bedding plane cracks, may provide critical additional information on crack genesis. Better understanding of crack genesis will strengthen our ability to interpret unfossiliferous muddy sequences common in Precambrian and lacustrine settings.
Sandstone‐hosted thorium‐bitumen mineralization in the Northwest Irish Basin
Authors: PARNELL JOHN , MONSON BRYAN 
Published: June 2006

ABSTRACTLow‐grade sandstone‐hosted thorium mineralization is recorded in the Carboniferous Northwest Irish Basin in the Republic of Ireland and Northern Ireland. The thorium is concentrated within bitumen nodules which are replacive within the sandstones. A source of thorium was available in a Caledonian granite in the basin watershed, and the bitumen was a product of hydrocarbons generated within the basin. The thorium occurs as thorite inclusions in the bitumen, and the development of the nodules was enhanced by the polymerization of fluid hydrocarbons by irradiation from the thorium. Thorium mobility may have been enhanced by the formation of organometallic complexes. Mineralization probably therefore occurred during hydrocarbon migration, dated as Late Carboniferous to Permian.
Rates of aerodynamic entrainment in a developing boundary layer
Authors: WILLIAMS JON J. , BUTTERFIELD GRAEME R. , CLARK DAVID G. 
Published: June 2006

ABSTRACTDespite its significance for inception of grain transport by wind, the initial dislodgement of grains from a static surface by aerodynamic forces of drag and lift in the absence of grain collision has received little attention. This paper describes a series of wind‐tunnel experiments in which the erosion of narrow strips of loose grains from the roughened surface of a flat plate exposed to a range of wind speeds was examined.The progressive downwind development of the boundary layer over the plate provided a range of airflow conditions which permitted systematic evaluation of grain entrainment rates arising from purely aerodynamic forces. Use of closely graded size fractions in flat, single grain layers resting on identical, fixed grain support eliminated the effects of surface irregularities and impacts from saltation. Results show that erosion of strips of loose grains develops with time according to an inverse exponential function in which the entrainment rate time constant relates to Shields dimensionless shear stress function. An empirical expression defining aerodynamic entrainment rate in terms of rate of strip erosion is derived and comparisons are made between present and published data. The need for additional data to resolve several questions raised by the present investigation is stressed.In addition, a simple, objective technique for accurate determination of the aerodynamic entrainment threshold of any loose, granular sediment is proposed.
A wind tunnel simulation and field verification of desert dust deposition (Avdat Experimental Station, Negev Desert)
Authors: GOOSSENS D. , OFFER Z. I. 
Published: June 2006

ABSTRACTThe deposition of natural dust in an area of 53 ha, situated in the northern Negev desert, is investigated in detail both in the wind tunnel (dust storm simulations over a topographic scale model) and in the field. The wind tunnel results and the field results show a high degree of agreement, indicating that scale‐model simulation may be considered an important technique for future loess and desert research. More dust settles on windward slopes than on leeward slopes, which is in contradistinction with the widespread wind shadow concept. Air‐flow separation zones immediately downwind of steep windward slopes have an important impact on dust deposition too.In the case of dust deposition on topographic scale models, a restricted height distortion of the model will not necessarily lead to serious problems. In addition, wind tunnel blockage percentages up to 13% may be allowed in order to obtain acceptable dust deposition patterns for the scale model.A mean gross dust deposition of about 200–250 g m−2 year−1 is calculated for the northern Negev desert for 1987. Thus, if the settled dust can be protected against erosion in the cultivated areas in the Negev, the dust content of the top soil will markedly increase with time. However, it has to be borne in mind that cultivation activities themselves may also contribute to a higher soil erosion and, hence, to a higher dust content in the atmosphere. At any rate, a higher dust content in the top soil will unquestionably have an important positive effect on agricultural yields.From the air dust concentration data and the dust deposition data, a deposition velocity of 4.7cms−1 can be calculated for Avdat dust.
A tidal‐inlet complex in the Cretaceous epeiric sea of North America: Virgelle Member, Milk River Formation, southern Alberta, Canada
Authors: CHEEL R. J. , LECKIE D. A. 
Published: June 2006

ABSTRACTThe upper portion of the Virgelle Member (Upper Cretaceous Milk River Formation) at Writing‐on‐Stone Provincial Park of southern Alberta preserves evidence of tidal processes along an otherwise wave‐dominated, progradational shoreline in the Cretaceous Interior Seaway of North America. The upper Virgelle Member is underlain by offshore transition to lower shoreface deposits of the Telegraph Creek Member and the lower Virgelle Member, respectively, and is overlain by the non‐marine shales and sandstones of the Deadhorse Coulee Member. The sediments of the upper Virgelle Member were deposited along a prograding shoreline and are interpreted here as those of a tidal‐inlet complex. Most inlet sections consist of an erosional base overlain by a shale‐pebble conglomerate, followed by cross‐bedded sandstones which become finer‐grained and decrease in scale upwards. Indicators of tidal processes include palaeocurrent distributions, mud couplets, tidal bundles, re‐activation surfaces and herringbone cross‐beds. The sequence through the tidal‐inlet complex can be differentiated, according to prevalent palaeoflow directions and sedimentary structures, as ebb‐dominated, flood‐dominated, or mixed‐tidal influence. Ebb‐dominated sections commonly contain lateral accretion surfaces whereas flood‐dominated sections contain tidal‐ramp deposits. Back‐barrier lagoon deposits are dominated by sandstones of an extensive flood‐tidal delta with only thin shales preserved locally at the top of the inlet complex. Deposits of ebb‐tidal deltas are absent, presumably due to the effective sediment dispersal by waves and wave‐induced longshore currents acting on the regionally wave‐dominated shoreline.
Sedimentology and tectonic implications of Cretaceous fan‐delta conglomerates, Queen Charlotte Islands, Canada
Authors: HIGGS ROGER 
Published: June 2006

ABSTRACTThe Honna Formation, of Coniacian age, consists of several hundred metres of polymictic clast‐supported conglomerate associated with sandstone and mudstone. Five conglomerate facies are recognized: ungraded beds; inverse graded beds; normal graded beds; inverse‐to‐normal graded beds; and parallel‐stratified beds. These facies are interpreted as the deposits of subaqueous cohesionless debris flows and/or high‐density turbidity currents.The depositional environment was a deep‐water, gravelly fan that draped a fault‐controlled, basin‐margin slope. The fan is inferred to have passed upslope directly into an alluvial fan (unpreserved); hence, the name fan delta can be applied to the overall depositional system. This type of fan delta, of which the Brae oilfield in the North Sea is an example, is defined here as a deep‐water fan delta. The lack of a shelf is in marked contrast to other types of fan delta.Three facies associations are recognized in the Honna Formation: subaqueous proximal‐fan conglomerates, distal‐fan turbiditic sandstones, and pro‐fan/interfan mudstones with thin sandy turbidites. The proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km; such a long subaqueous gravel belt lacks a known modern analogue. The distal fan was an unchannelled sandy extension of the proximal gravel belt.It is postulated that the Honna Formation accumulated in a foreland basin which migrated westwards from the Coast Mountains where the Wrangellia‐Alexander terrane was colliding with North America. In this model, the Honna fan delta was sourced by a (west‐verging) thrust sheet whose sole‐thrust was the Sandspit Fault immediately to the east.Deep‐water fan deltas appear to develop preferentially when eustatic sea‐level is relatively high, so that the‘feeder’ alluvial fan is small, and gravelly throughout. In petroleum exploration and field development, care should be taken to distinguish deep‐water fan deltas from base‐of‐slope (canyon‐fed) submarine fans, because the two systems differ significantly in terms of coarse‐sediment distribution.
Origin of dolomitization on the Barbwire Terrace, Canning Basin, Western Australia
Authors: WALLACE MALCOLM W. 
Published: June 2006

ABSTRACTA thick, areally extensive subsurface sequence of Upper Devonian carbonates occurs on the Barbwire Terrace in the Canning Basin of Western Australia. It is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized. Slightly deeper water marls have remained as limestones. The major, regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite (up to 600 m) with no primary calcite. A small volume of evaporitic, supratidal dolomite is present at one location. This dolomite is derived from highly saline fluids developed in an arid supratidal environment.Replacement dolomite of the regional dolomite type has a xenotopic form, with undulose extinction, and irregular crystal boundaries. In addition, saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite. This suggests the regional dolomite type was precipitated at slightly elevated temperatures. Dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution.Geochemically, the synsedimentary supratidal and regional dolomite types are quite distinctive. Supratidal dolomites have δ18O values which are significantly higher (δ18O=−2 to +1‰ (PDB)) than the regional dolomite type (δ18O=−9 to −2‰ (PDB)). Assuming the lowest δ18O values for the sabkha dolomite represent replacement in marine waters, the oxygen isotopic composition for Upper Devonian Canning Basin marine dolomite would be around δ18O=−2‰ (PDB).The petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin. However, sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the Barbwire Terrace. Therefore, it is suggested that dolomitization may have taken place in a near‐surface environment with a major recrystallization event superimposed during burial diagenesis.